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\title{Photoplethysmography for non-invasive diagnosis of cold urticaria}



\author[1]{Daria Fomina}%
\author[1]{Sofia Serdotetskova}%
\author[2]{Alexander Machikhin}%
\author[2]{Mikhail Volkov}%
\author[2]{Anastasia Guryleva}%
\author[3]{Mojca Bizjak}%
\author[2]{Valeriya Bukova}%
\author[1]{Marina S. Lebedkina}%
\author[1]{Gerelma Andrenova}%
\author[1]{Olga Mukhina}%
\author[4]{Mukatova Irina}%
\author[5]{Inna Danilycheva}%
\author[6]{Marcus Maurer}%
\affil[1]{GBUZ Gorodskaa kliniceskaa bo'lnica No 52 Departamenta zdravoohranenia goroda Moskvy}%
\affil[2]{FGBUN Naucno-tehnologiceskij centr unikal'nogo priborostroenia Rossijskoj akademii nauk}%
\affil[3]{Klinika Golnik}%
\affil[4]{Astana Medical University}%
\affil[5]{NRC Institute of Immunology FMBA of Russia}%
\affil[6]{Charite Universitatsmedizin Berlin}%


\vspace{-1em}



  
  \date{May 26, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
\textbf{Background:} Cold urticaria (ColdU) is a common type of chronic
inducible urticaria. Patients with typical ColdU develop wheals in
response to standard cold stimulation tests (CSTs), an ice cube (ICT) or
TempTest \selectlanguage{ngerman}\textsuperscript{®}. As of now, the evaluation of CST response
is visual and subjective. Validated, robust, and objective test readouts
lacking today are needed. \textbf{Methods:} We subjected 63 patients (39
with typical ColdU and 24 with atypical ColdU) and 15 healthy controls
(HCs) to TempTest \textsuperscript{®} CSTs and critical temperature
threshold assessments. Blood microcirculation photoplethysmography (PPG)
measurements were performed 5 min before and 10 min after the ICT on the
volar forearm. \textbf{Results:} PPG amplitudes reflected normal
baseline skin blood perfusion in patients with typical or atypical
ColdU. Ice cube CSTs induced a marked increase in blood perfusion and
PPG amplitudes in typical but not atypical ColdU, with distinct pre-post
CST changes in PPG amplitudes in the former. The ratio of
post-provocation and baseline PPG amplitudes ( \emph{R}
\textsubscript{PPG}) in typical ColdU patients exceeded that in atypical
ColdU patients and HCs more than 3-fold. Almost all typical ColdU
patients (98\%), but only 13\% of atypical ColdU patients and 7\% of HCs
had \emph{R} \textsubscript{PPG} \textgreater{}3. PPG results matched
those of CSTs in 94\% of all tested individuals. \textbf{Conclusion:}
Photoplethysmographic assessments of CST responses appear accurate and
provide objective readouts. PPG may be of use in diagnosing ColdU,
distinguishing typical and atypical ColdU, and more precise threshold
testing.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


Photoplethysmography for non-invasive diagnosis of cold urticaria

Short title: Photoplethysmography in cold urticaria diagnosis

\textsuperscript{+}Sofia Serdotetskova\textsuperscript{1}, Alexander
Machikhin\textsuperscript{2}, Daria Fomina\textsuperscript{1,3,4},
Mikhail Volkov\textsuperscript{2,5}, Anastasia
Guryleva\textsuperscript{2}, Mojca Bizjak\textsuperscript{6}, Valeriya
Bukova\textsuperscript{2,7}, Marina Lebedkina\textsuperscript{1},
Gerelma Andrenova\textsuperscript{1}, Olga Mukhina\textsuperscript{1},
Mukatova Irina\textsuperscript{4}, Inna Danilycheva\textsuperscript{8},
Marcus Maurer\textsuperscript{9,10}

\textsuperscript{1}\emph{Urticaria Center of Reference and Excellence
(UCARE), Moscow Center of Allergy and Immunology, Clinical City Hospital
No. 52, Ministry of Healthcare of Moscow, Russia}

\emph{\textsuperscript{2} Scientific and Technological Center of Unique
Instrumentation, Russian Academy of Sciences, Moscow, Russia}

\emph{\textsuperscript{3} Sechenov University, Moscow, Russia}

\emph{\textsuperscript{4} Astana Medical University, Astana, Kazakhstan}

\emph{\textsuperscript{5} ITMO University, Saint-Petersburg, Russia}

\emph{\textsuperscript{6} Urticaria Center of Reference and Excellence
(UCARE), University Clinic of Respiratory and Allergic Diseases, Golnik,
Slovenia}

\emph{\textsuperscript{7} Bauman Moscow State Technical University,
Moscow, Russia}

\emph{\textsuperscript{8} NRC Institute of Immunology FMBA of Russia,
Moscow, Russia}

\selectlanguage{ngerman}\emph{\textsuperscript{9} Urticaria Center of Reference and Excellence
(UCARE), Charité -- Universitätsmedizin Berlin, corporate member of
Freie Universität Berlin, Humboldt-Universität zu Berlin, Dermatological
Allergology, Allergie-Centrum Charité, Department of Dermatology and
Allergy, Berlin, Germany}

\emph{\textsuperscript{10} Fraunhofer Institute for Translational
Medicine and Pharmacology ITMP, Allergology and Immunology, Berlin,
Germany}

\textbf{Correspondence:}

Daria Fomina

\emph{Urticaria Center of Reference and Excellence (UCARE), Moscow
Center of Allergy and Immunology, City Clinical Hospital No. 52,
Ministry of Healthcare of Moscow, Russia}

3 Pekhotnaya Street, Moscow 123182

Tel: +7-916-597-18-06

E-mail: daria.s.fomina@gmail.com

\textbf{Article Type:} Original Article

\textbf{Manuscript word count:} 2646

\textbf{Abstract word count:} 233

\textbf{Tables:} 2

\textbf{Figures:} 5

\textbf{Supplemental Tables:} 0

\textbf{Supplemental Images:} 2

\textbf{Conflict of interest:} The authors declare that they have no
conflicts of interest in relation to this work. Outside of the scope of
this work, MM is or has recently been a speaker and/or advisor for
and/or has received research funding from Allakos, Amgen, Aralez,
ArgenX, AstraZeneca, Celldex, Centogene, CSL Behring, FAES, Genentech,
GIInnovation, GSK, Innate Pharma, Kyowa Kirin, Leo Pharma, Lilly,
Menarini, Moxie, Novartis, Pfizer, Roche, Sanofi/Regeneron, Third
Harmonic Bio, UCB, and Uriach.

\textbf{Author contribution:}

Alexander Machikhin, Daria Fomina and Inna Danilycheva designed the
study. Valeriya Bukova and Sofia Serdotetskova performed the
experiments. Mikhail Volkov and Anastasia Guryleva processed the
experimental data. Sofia Serdotetskova, Alexander Machikhin, Mikhail
Volkov, Anastasia Guryleva, Marcus Maurer, Marina Lebedkina, Olga
Mukhina, Gerelma Andrenova and Irina Mukatova contributed to data
interpretation. Sofia Serdotetskova, Alexander Machikhin wrote the
manuscript with critical input from Mojca Bizjak and Marcus Maurer. The
project is managed by Daria Fomina. All authors reviewed the manuscript.

\textbf{Acknowledgements:} This study is supported by the local Research
Ethics Committee of City Clinical Hospital No. 52 (protocol \#06/0621,
dated 29.05.2021) and has benefited from the support of the
GA\textsuperscript{2}LEN Urticaria Centers of Reference and Excellence
(UCARE) network (www.ga2len-ucare.com).

\textbf{ABSTRACT (233/250)}

\textbf{Background:} Cold urticaria (ColdU) is a common type of chronic
inducible urticaria. Patients with typical ColdU develop wheals in
response to standard cold stimulation tests (CSTs), an ice cube (ICT) or
TempTest\textsuperscript{®}. As of now, the evaluation of CST response
is visual and subjective. Validated, robust, and objective test readouts
lacking today are needed.

\textbf{Methods:} We subjected 63 patients (39 with typical ColdU and 24
with atypical ColdU) and 15 healthy controls (HCs) to
TempTest\textsuperscript{®} CSTs and critical temperature threshold
assessments. Blood microcirculation photoplethysmography (PPG)
measurements were performed 5 min before and 10 min after the ICT on the
volar forearm.

\textbf{Results:} PPG amplitudes reflected normal baseline skin blood
perfusion in patients with typical or atypical ColdU. Ice cube CSTs
induced a marked increase in blood perfusion and PPG amplitudes in
typical but not atypical ColdU, with distinct pre-post CST changes in
PPG amplitudes in the former. The ratio of post-provocation and baseline
PPG amplitudes (\emph{R} \textsubscript{PPG}) in typical ColdU patients
exceeded that in atypical ColdU patients and HCs more than 3-fold.
Almost all typical ColdU patients (98\%), but only 13\% of atypical
ColdU patients and 7\% of HCs had \emph{R}
\textsubscript{PPG}\textgreater{}3. PPG results matched those of CSTs in
94\% of all tested individuals.

\textbf{Conclusion:} Photoplethysmographic assessments of CST responses
appear accurate and provide objective readouts. PPG may be of use in
diagnosing ColdU, distinguishing typical and atypical ColdU, and more
precise threshold testing.

\textbf{Keywords:} cold urticaria, image processing, non-invasive
diagnostics, photoplethysmography, skin inspection

\textbf{Abbreviations}\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
AU & Arbitrary units &\tabularnewline
\midrule
\endhead
CInd: & Chronic inducible urticaria &\tabularnewline
ColdU: & Cold urticaria &\tabularnewline
CST: & Cold stimulation test &\tabularnewline
CTT: & Critical temperature threshold &\tabularnewline
DLQI: & Dermatology Life Quality Index &\tabularnewline
ICT: & Ice cube test &\tabularnewline
HC: & Healthy control &\tabularnewline
PPG: & Photoplethysmography &\tabularnewline
\emph{R}\textsubscript{PPG}: & Ratio of post-provocation and baseline
PPG amplitudes & Ratio of post-provocation and baseline PPG
amplitudes\tabularnewline
UCT: & Urticaria Control Test & Urticaria Control Test\tabularnewline
\bottomrule
\end{longtable}

\textbf{GRAPHICAL ABSTRACT:}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\emph{ColdU} , cold urticaria, \emph{CST} , cold stimulation
test,\emph{FN} , false negative, \emph{FP} , false positive, \emph{HCs}
, healthy controls, \emph{ICT} , ice cube test, \emph{PPG} ,
photoplethysmogram, \emph{TN} , true negative, \emph{TP,} true positive.

\textbf{GRAPHICAL ABSTRACT HIGHLIGHTS:}

Microcirculation response to cold triggers varies between typical ColdU
patients and atypical ColdU patients / HCs.

Reflection-mode PPG enables non-contact quantitative estimate of the
perfusion.

Changes in perfusion can detect typical ColdU and indicate the disease
activity.

PPG-based measurements correlate with CSTs (ICT or
TempTest\selectlanguage{ngerman}\textsuperscript{®}) in 94\% of cases.

INTRODUCTION

Cold urticaria (ColdU) is the second most common subtype of chronic
inducible urticaria (CindU) after symptomatic dermographism. ColdU is
characterized by wheals and/or angioedema following skin contact with
various cold triggers (air, liquids, surfaces, objects). The main hazard
of ColdU is the development of potentially life-threatening cold-induced
anaphylaxis occurring in approximately one third of patients.

ColdU often starts in early adulthood (18--27 years) and lasts on
average for 4--5 years, with spontaneous remission or relief of symptoms
in 50\% of cases within 5 years. The incidence of ColdU in Central
Europe is estimated at 0.05\%. ColdU is more frequent in women.

ColdU is diagnosed based upon patient history and cold stimulation tests
(CSTs). For CSTs, cold is applied to the volar forearm for 5 min, and
the response is evaluated 10 min after the end of stimulation.
Appearance of whealing at the site of exposure represents a positive
test result (\textbf{Figure 1A and 1B} ). About 75\% of all patients
with ColdU with positive responses to standard CSTs have typical ColdU.
In atypical ColdU, CSTs are negative or induce atypical responses, such
as delayed whealing. About 25\% of all ColdU patients have atypical
ColdU, and CSTs other than the standard ones are needed to elicit
whealing and confirm the diagnosis\textsuperscript{4}. Due to its easy
accessibility, the most widely used CST is a melting ice cube test (ICT)
with an ice cube in a water-filled plastic bag. However, testing with
TempTest\textsuperscript{®}, a Peltier element producing a 4--44°C
temperature gradient, is preferred over ICT, because it can be used to
assess disease activity, i.e. the critical temperature threshold (CTT).

The evaluation of CST responses in routine clinical practice is done by
macroscopic inspection and is therefore subjective. In other words, CST
sites are visually evaluated for whealing by the testing physician. This
may lead to inter- and intra-observer variability, false positive and
false negative CST results, and unreliable CTT measurements. For CTT
determination with TempTest\textsuperscript{®}, the length of the wheal,
starting at the 4°C contact site, reflects the wheal-inducing
temperature range in individual patients; the longer the wheal, the
higher the CTT. Where the wheal ends is therefore of key importance for
informing patients about their CTTs and their risk of reacting to cold.
Since the end of TempTest\textsuperscript{®} induced wheals often only
measures 1 mm in width, it can be challenging to determine visually
where that end is and what the CTT is. These and other issues with
reading CSTs need to be addressed. Validated, robust, and objective test
readouts lacking today are needed. In the future, better methods for CST
response assessment in patients with typical ColdU should be easy to
use, reliable, cost-effective, and objective.

Upon skin exposure to cold, patients with ColdU develop wheal- and
flare-type skin reactions explained respectively by increased
extravasation and vasodilation. These processes are brought about by the
degranulation of mast cells. Mast cell mediators, including histamine,
induce vasodilation and extravasation, and they also activate sensory
skin nerves, which contributes to vasodilation and flare responses. This
response to cold is unique to ColdU and does not occur in people without
ColdU, in whom cold exposure first induces vasoconstriction and then,
upon rewarming, vasodilation. The differences in skin responses to the
cold between healthy subjects and ColdU patients are linked to certain
changes in skin microvasculature. Based on this, we hypothesized that
the assessment of ColdU patients and healthy subjects permits detection
of distinct microvasculatory responses and their differentiation.

\emph{In vivo} microvasculature responses can be assessed, mapped,
quantified, and monitored by several methods and techniques, including
laser Doppler perfusion and laser contrast speckle imaging, as well as
photoplethysmography (PPG). The latter makes use of a light source and a
photodetector to measure volumetric variations of cutaneous blood
circulation. PPG provides reliable, low-cost, and easy to perform
readings of local blood volume changes in the microvasculature of the
inspected skin area. By contrast to other techniques, PPG is less
sensitive to the patient's motion, more cost-effective, and easier to
implement.

Microcirculation imaging techniques are most commonly targeted at the
nailfold area, but for ColdU, the most representative data is acquired
by imaging affected skin. CSTs are known to produce ColdU-specific skin
circulatory responses. Meyer et al. studied vascular reactions caused by
mast cell degranulation after exposure to a cold stimulus and showed
that vascular reactions change under the influence of antihistamines.

Here, we used PPG to test our hypothesis that the development of
cold-induced skin lesions in patients with typical ColdU is linked to
distinct and detectable changes in skin microvasculature. Our long-term
aim is to improve ColdU diagnosis and management by providing an
objective, easy, reliable, and low-cost measure of typical ColdU CST.

METHODS

\emph{Study subjects and conduct}

The study was performed at the Center of Allergy and Immunology, City
Clinical Hospital No. 52 of the Ministry of Healthcare of Moscow
(Moscow, Russia) between May 2021 and February 2023 after approval by
the independent local ethics committee (protocol \#06/0621). All study
participants provided prior written informed consent.

We analyzed 63 patients with expert-confirmed ColdU. Of these, 39
patients had typical ColdU confirmed by positive ICT and
TempTest\textsuperscript{®} CSTs. The other 24 patients had atypical
ColdU with negative ICT and TempTest\textsuperscript{®} CSTs. In both
groups, we recorded gender, age, the incidence of life-threatening
reactions like angioedema and hypotensive reactions (weakness,
dizziness, loss of consciousness), comorbid CSU, and other types of
CIndUs, frequency of allergic, autoimmune and oncological diseases,
duration of ColdU, treatment, and results of standardized validated
questionnaires (Urticaria Control Test, UCT; Dermatology Life Quality
Index, DLQI). Patients with typical and atypical ColdU were similar in
their demographic and clinical features, except for the duration of
disease, which was 24 months (range 11 to 71 months) in typical ColdU
and 212 months (range 60 to 360 months) in atypical ColdU (Table 1). We
also investigated 15 healthy controls (HCs) who had no history of
pathological reactions associated with cold exposure, and all of them
had negative CSTs.

\emph{Assessment of disease control and impact}

The UCT was used to assess disease control during the past month. The
maximum UCT score, 16 points, reflects completely controlled disease,
and scores from 0 (the lowest score) to 11 points indicate poorly
controlled disease. The DLQI permits clinicians to assess the impact of
skin disease on the quality of life of patients over the past week. The
minimum score (0 points) corresponds to the absence of a negative effect
of urticaria symptoms on the patient's quality of life. The higher the
score, the more severely the quality of life is affected (the maximum
result is 30 points).

\emph{Cold stimulation testing and critical temperature measurements}

ICT was performed following a standard protocol. Briefly, a melting ice
cube in thin plastic bag was placed on the volar surface of the patient
forearm for 5 min, with macroscopic assessment of the test site 10 min
later. TempTest\textsuperscript{®} CSTs were done with
TempTest\textsuperscript{®} 4.0 (Courage \& Khazaka, Cologne, Germany),
which has a single Peltier element (length: 350 mm, width: 2 mm) that
provides a continuous temperature gradient from 4°C to 44°C. The use of
TempTest\textsuperscript{®} allows for reproducible and standardized
cold (and heat) provocation tests and the identification of CTTs.
TempTest\textsuperscript{®} CST results were also assessed 10 min after
the end of cold exposure.

\emph{Experimental setup for PPG measurements}

We developed a setup for high-magnification and high-resolution imaging
of the forearm area where ICTs were performed (\textbf{Figure 2} ). To
achieve a uniform glare-free illumination, we installed two 5 W
LED-based light sources with green (central wavelength \selectlanguage{greek}λ \selectlanguage{english}= 520 nm, FWHM
30 nm) illumination that corresponded to the hemoglobin absorption, thus
enabling the high contrast of blood vessels against the blood-free
imaging field. Our imaging system included a long-working-distance (70
mm) microscope tube with continuously adjustable
1.5--2.5\selectlanguage{ngerman}\textsuperscript{×} zoom and a monochrome CMOS camera (IDS uEye
UI-3060CP-M-GL Rev.2, 1/1.2'', 1936×1216 pixels, frame rate up to 166
fps). For uniformly sharp imaging, we flattened the skin surface with a
2.5 mm thick glass plate located on a separate stand. To minimize
residual distortions in the image edges, we limited the resolution to
1000×1000 pixels. Thus, our setup allowed imaging of skin areas in the
range of 1.5×1.5 mm\textsuperscript{2} to 2.5×2.5
mm\textsuperscript{2}with resolution high enough to detect single
capillaries of 5--10 \selectlanguage{greek}µ\selectlanguage{english}m in diameter.

\emph{PPG data acquisition and processing}

To obtain photoplethysmographic data, we acquired 12-bit skin images
within 40 s at a 50 Hz frame rate and then processed the whole stack of
2000 frames in MATLAB. The key stages of the data processing pipeline
have previously been described and are shown in \textbf{Figure 3} .
Briefly, we eliminated uneven illumination and sensor non-uniformity and
then enhanced the contrast of all images and carried out frame-to-frame
matching using the GeFolki algorithm to exclude image shifts caused by
the patient's movements. The change of pixel intensity in the areas
associated with the blood flow in the resulting image stack is a
pulsatile signal. For other regions, this change is insignificant and
aperiodic. To compute the blood flow signal, we subtracted slowly
time-varying background and frequency components out of the
cardiovascular-related range of 0.3--7 Hz. The resulting well-matched,
noise-free and intensity-corrected blood flow images were made suitable
for PPG calculation by averaging the intensity pixel values of each
frame. The PPG amplitude was proportional to the amount of arterial
blood that reaches the visualized skin area and thus characterized its
blood perfusion. It was measured in arbitrary units (AU). We performed
photoplethysmographic assessments of the volar forearm of all patients 5
min before and 10 min after the ice cube application.

\emph{Statistical analyses}

PPG values are presented as the mean \(\pm\) standard error
of mean (SEM). The criterion of statistical significance was
\(p<0.05\) for all PPG characteristics. We applied Origin
(OriginLab Corp.) for the ROC-analysis and for the two-way ANOVA at the
\(\alpha\)=0.05 to test for differences between PPG amplitudes
and the age and sex of study subjects. Statistical characteristics
including correlation coefficient, sensitivity, specificity, positive
and negative predictive value (PPV and NPV, respectively), and accuracy
were calculated using MATLAB.

RESULTS

\emph{Patients with ColdU exhibit normal baseline skin blood perfusion}

Before ICTs, at baseline, patients with typical ColdU, patients with
atypical ColdU, and HCs exhibited similar skin blood perfusion with an
average PPG amplitude of 6.2\selectlanguage{ngerman}·10\textsuperscript{-4} ±
5.7·10\textsuperscript{-6} AU, 6.3 10\textsuperscript{-4} ±
4.6·10\textsuperscript{-5} AU, and 5.4·10\textsuperscript{-4} ±
3.6·10\textsuperscript{-5} AU, respectively. Across all study subjects
and groups, age and sex were not linked to differences in PPG
amplitudes.

\emph{Cold-induced whealing in patients with typical ColdU is linked to
markedly increased blood perfusion and PPG amplitudes}

Patients with typical ColdU had high blood perfusion at skin sites that
developed wheals in response to ICT, with an average PPG amplitude of
41.4·10\textsuperscript{-4} ± 5.8·10\textsuperscript{-5} AU. This was
markedly higher than their blood perfusion at baseline (PPG amplitude:
6.2·10\textsuperscript{-4} ± 5.7·10\textsuperscript{-6} AU) as well as
that of cold-exposed skin sites in patients with atypical ColdU (PPG
amplitude: 9.9·10\textsuperscript{-4} ± 1.4·10\textsuperscript{-4} AU)
and HCs (PPG amplitude: 9.5·10\textsuperscript{-4} ±
1.5·10\textsuperscript{-4} AU). Patients with atypical ColdU and HCs did
not develop wheals at CST skin sites.

\emph{Patients with typical ColdU exhibit distinct pre-post CST changes
in PPG amplitudes} The ratio of post-provocation and baseline PPG
amplitudes (\emph{R} \textsubscript{PPG}) was markedly higher in typical
ColdU patients (6.7 ± 0.3) than in atypical ColdU patients (1.9 ± 0.4)
and HCs (1.9 ± 0.5). Of 39 typical ColdU patients, 38 (97.4\%) had
an\emph{R} \textsubscript{PPG} \textgreater{}3, but only 3 of 24
patients with atypical ColdU (12.5\%) and one of 15 (6.7\%) HCs
(\textbf{Figure 4} ).

\emph{Tripling of PPG amplitude predicts positive CST response}

In 93.6\% of all tested individuals, CST results matched those of PPG,
categorized as negative and positive based on\emph{R}
\textsubscript{PPG} \selectlanguage{english}[?]3 and \textgreater{}3, respectively. With this
cutoff, PPG correctly predicted 93.7\% of typical and atypical ColdU
patients (\textbf{Table 2} ). As for the HCs, 93.3\% demonstrated
negative PPG responses, i.e. insignificant microcirculation change due
to the exposure to cold, and were thus correctly identified in line with
their CST results.

4 DISCUSSION

Our study demonstrates that the results of cold provocation testing in
patients with typical ColdU can be accurately and objectively assessed
by PPG. This suggests that the implementation of photoplethysmographic
measurements in everyday clinical practice may improve the diagnostic
work-up of patients with ColdU. PPG may be especially helpful for
differentiating typical and atypical ColdU and for testing patients with
typical ColdU for their temperature thresholds. Unlike the current
practice of visual evaluation, PPG makes it possible to perform a
quantitative and objective assessment of the skin response of ColdU
patients to cold exposure.

PPG measurements before and after ICT showed that microcirculation
responses to ICT are different in typical ColdU patients and HCs. After
exposure to cold, the relative increase in PPG amplitude was
significantly higher in ColdU patients than in HCs. These results
indicate a high sensitivity of PPG-based measurements in detecting
perfusion changes in patients with a positive CST, as well as a high
negative prognostic accuracy. Our proposed non-invasive and
easy-to-implement approach may thus become the basis for new
technologies and devices for ColdU diagnosis, complementing current ICTs
and improving the assessment of ColdU disease activity and treatment
effectiveness.\textsuperscript{23} The identified\emph{R}
\textsubscript{PPG} threshold value may further be used in clinical
practice to distinguish typical ColdU from atypical ColdU. The ability
of PPG to detect and quantify even small increases in skin blood flow is
underscored by the distinct pre-post CST changes in PPG amplitudes,
where tripling of PPG amplitudes is predictive of positive CST
responses, in patients with typical ColdU.

Our finding that patients with ColdU demonstrate normal baseline skin
blood perfusion is not surprising. In ColdU, urticarial lesions appear
only when the skin is exposed to sub-threshold temperatures. It was
therefore expected that the PPG-based assessment of skin blood flow at
baseline would not show differences in blood perfusion between ColdU
patients and HCs. This also supports the notion that there are no major
long-term changes in the skin microvasculature of patients with ColdU.
It would be interesting to compare baseline PPG signatures of ColdU
patients and patients with CSU, where chronic inflammatory changes such
as increased mast cell numbers and neovascularization have been
described.

The most important finding of our study is that cold-induced whealing in
patients with typical ColdU is linked to markedly increased blood
perfusion and PPG amplitudes. This demonstrates that whealing comes with
distinct and detectable changes in skin microvasculature and PPG metrics
that permit objective skin readings of provocation test responses. This
finding may be relevant beyond ColdU, for example, for the assessment of
skin responses to provocation testing in symptomatic dermographism and
solar urticaria.

How can PPG-based measurements improve CTT readings in ColdU? Our
findings suggest that PPG can precisely determine the edge of a
cold-induced wheal, which is critical for assessing the CTTs of
individual ColdU patients, i.e. the highest temperature that suffices to
produce a wheal. TempTest\textsuperscript{(r)}-based CSTs in patients with
typical ColdU produce linear wheals that start and end where the skin is
exposed to 4degC and the CTT, respectively. PPG objectively identifies the
end of a TempTest\textsuperscript{(r)}-induced wheal unlike how the CTT is
currently determined, i.e. by inspection, which is subjective and
subject to intraoperator and interoperator variability of measurements.
Accurate PPG-based CTT measurements may, thus, benefit the monitoring of
treatment responses in routine practice and clinical trials.

Our study has several limitations. These include the small sample size
and the lack of diversity of patients in terms of age and skin type.
Further studies are needed to confirm and extend our results.

Taken together, our findings suggest that photoplethysmographic
assessments of ColdU CST responses appear to be accurate and can provide
objective verification of positive and negative test results. Thus, PPG
may assist in diagnosing ColdU, distinguishing typical and atypical
ColdU, and making threshold testing more precise.

REFERENCES

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and angioedema. Classification, diagnosis and
therapy{]}.\emph{Hautarzt}. 2010;61(9):743-749.
doi:10.1007/S00105-010-1932-9 2. Bizjak M, Ko\selectlanguage{polish}š\selectlanguage{english}nik M, Dinevski D, et al.
Risk factors for systemic reactions in typical cold urticaria: Results
from the COLD-CE study.\emph{Allergy}. 2022;77(7):2185-2199.
doi:10.1111/ALL.15194 3. Bizjak M, Ko\selectlanguage{polish}š\selectlanguage{english}nik M, Dinevski D, et al.
Adrenaline autoinjector is underprescribed in typical cold urticaria
patients. \emph{Allergy}. 2022;77(7):2224-2229. doi:10.1111/ALL.15274 4.
Katsarou-Katsari A, Makris M, Lagogianni E, Gregoriou S, Theoharides T,
Kalogeromitros D. Clinical features and natural history of acquired cold
urticaria in a tertiary referral hospital: a 10-year prospective study.
\emph{Journal of the European Academy of Dermatology and Venereology}.
2008;22(12):1405-1411. doi:10.1111/J.1468-3083.2008.02840.X 5. Maltseva
N, Borzova E, Fomina D, et al. Cold urticaria -- What we know and what
we do not know. \emph{Allergy}. 2021;76(4):1077-1094.
doi:10.1111/ALL.14674 6. Maurer M, Hawro T, Krause K, et al. Diagnosis
and treatment of chronic inducible urticaria. \emph{Allergy}.
2019;74(12):2550-2553. doi:10.1111/ALL.13878 7. Hochstadter EF,
Ben-Shoshan M. Case Report: Cold-induced urticaria: challenges in
diagnosis and management. \emph{BMJ Case Rep}. 2013;2013.
doi:10.1136/BCR-2013-010441 8. den Uil CA, Klijn E, Lagrand WK, et al.
The Microcirculation in Health and Critical Disease. \emph{Prog
Cardiovasc Dis}. 2008;51(2):161-170. doi:10.1016/J.PCAD.2008.07.002 9.
Plyer A, Colin-Koeniguer E, Weissgerber F. A New Coregistration
Algorithm for Recent Applications on Urban SAR Images. \emph{IEEE
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doi:10.1109/LGRS.2015.2455071 10. Guven G, Hilty MP, Ince C.
Microcirculation: Physiology, Pathophysiology, and Clinical Application.
\emph{Blood Purif}. 2020;49(1-2):143-150. doi:10.1159/000503775 11.
Kyriacou A. Panicos, Allen John. Photoplethysmography : technology,
signal analysis and applications. 12. Eriksson S, Nilsson J, Sturesson
C. Non-invasive imaging of microcirculation: a technology review.
\emph{Med Devices (Auckl)}. 2014;7:445. doi:10.2147/MDER.S51426 13.
Natalello G, de Luca G, Gigante L, et al. Nailfold capillaroscopy
findings in patients with coronavirus disease 2019: Broadening the
spectrum of COVID-19 microvascular involvement. \emph{Microvasc Res}.
2021;133:104071. doi:10.1016/J.MVR.2020.104071 14. Gallucci F, Russo R,
Buono R, Acampora R, Madrid E, Uomo G. Indications and results of
videocapillaroscopy in clinical practice.\emph{Adv Med Sci}.
2008;53(2):149-157. doi:10.2478/v10039-008-0038-4 15. Machikhin AS,
Shulzhenko AE, Potemkin A v., et al. Exoscope-based videocapillaroscopy
system for in vivo skin microcirculation imaging of various body areas.
\emph{Biomedical Optics Express, Vol 12, Issue 8, pp 4627-4636}.
2021;12(8):4627-4636. doi:10.1364/BOE.420786 16. Andersson T, Wardell K,
Anderson C. Human in vivo cutaneous microdialysis: estimation of
histamine release in cold urticaria.\emph{Acta Derm Venereol}.
1995;75(5):343-347. doi:10.2340/0001555575343347 17. Meyer J, Gorbach
AM, Liu WM, et al. Mast Cell Dependent Vascular Changes Associated with
an Acute Response to Cold Immersion in Primary Contact Urticaria.
\emph{PLoS One}. 2013;8(2):e56773. doi:10.1371/JOURNAL.PONE.0056773 18.
Weller K, Groffik A, Church MK, et al. Development and validation of the
Urticaria Control Test: a patient-reported outcome instrument for
assessing urticaria control. \emph{J Allergy Clin Immunol}. 2014;133(5).
doi:10.1016/J.JACI.2013.12.1076 19. Finlay AY, Khan GK. Dermatology Life
Quality Index (DLQI)---a simple practical measure for routine clinical
use. \emph{Clin Exp Dermatol}. 1994;19(3):210-216.
doi:10.1111/J.1365-2230.1994.TB01167.X 20. Liu P, Zhu Z, Zeng C, Nie G.
Specific absorption spectra of hemoglobin at different PO2 levels:
potential noninvasive method to detect PO2 in tissues. \emph{J Biomed
Opt}. 2012;17(12):125002. doi:10.1117/1.JBO.17.12.125002 21. Park J,
Seok HS, Kim SS, Shin H. Photoplethysmogram Analysis and Applications:
An Integrative Review. \emph{Front Physiol}. 2021;12.
doi:10.3389/FPHYS.2021.808451 22. Kumar M, Suliburk JW, Veeraraghavan A,
Sabharwal A. PulseCam: a camera-based, motion-robust and highly
sensitive blood perfusion imaging modality. \emph{Scientific Reports
2020 10:1}. 2020;10(1):1-17. doi:10.1038/s41598-020-61576-0 23.
Siebenhaar F, Degener F, Zuberbier T, Martus P, Maurer M. High-dose
desloratadine decreases wheal volume and improves cold provocation
thresholds compared with standard-dose treatment in patients with
acquired cold urticaria: a randomized, placebo-controlled, crossover
study. \emph{J Allergy Clin Immunol}. 2009;123(3):672-679.
doi:10.1016/J.JACI.2008.12.008

\textbf{Figure legends}

\textbf{Figure 1.} Images of the skin of patients with typical ColdU and
a positive response to cold stimulation testing with an ice cube (A) and
TempTest\selectlanguage{ngerman}\textsuperscript{®} (B).

\textbf{Figure 2.} Experimental setup: scheme (A), appearance (B) and
running state (C).

\textbf{Figure 3.} Data processing pipeline.

\emph{PPG,} photoplethysmogram

\textbf{Figure 4A.} Change in skin perfusion at sites of cold exposure
is significantly higher in patients with typical ColdU as compared to
patients with atypical ColdU and HCs

\emph{PPG,} photoplethysmogram, \emph{CTT} , critical temperature
threshold, \emph{Standard CST} , standard cold stimulation
test,\emph{ColdU} , cold urticaria, TempTest\textsuperscript{®},\emph{R}
\textsubscript{PPG,} ratio of post-provocation and baseline PPG
amplitudes

Vertical bars indicate CTT values for patients with a positive ICT and
TempTest\textsuperscript{®} (pink) and with negative standard CSTs
(cyan). For participants with a negative TempTest\textsuperscript{®},
the bar height is shown as 1°C. Circles stand for the measured
value\emph{R} \textsubscript{PPG}. \emph{R} \textsubscript{PPG} varies
between typical ColdU patients and HCs. By introducing a proper\emph{R}
\textsubscript{PPG} threshold of 3 (black line) we may clearly
distinguish participants with (red circles) and without (blue circles)
positive reactions to standard CSTs.

\textbf{Figure 4B} illustrates the similarities (green) and differences
(black) between our PPG-based technique and CST with and an ICT or
TempTest\textsuperscript{®}, in the same order.

\textbf{Figure 5.} ROC-curve of \emph{R} \textsubscript{PPG} and CST

\emph{FN} , false negative, \emph{FP} , false positive, \emph{TN} , true
negative, \emph{TP,} true positive.

\textbf{Table 1}\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
\begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
\textbf{Total} \textbf{n=63}\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
\textbf{Typical ColdU n=39}\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
\textbf{Atypical ColdU n=24}\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
\textbf{p-value}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
Gender: Female Male\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
50 (79\%) 13 (21\%)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
31 (80\%) 8 (20\%)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
19 (79\%) 5 (21\%)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
0.976\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Age (years) (min-max)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
42 (18-69)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
42 (18-69)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
42 (19-66)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.804\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
CSU\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
8 (13\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
5 (13\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
3 (13\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.970\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Comorbid CIndU:\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
18 (29\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
9 (23\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
9 (38\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.218\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
- symptomatic dermographism,\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
12 (19\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
6 (15\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
6 (25\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.345\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
- cholinergic urticaria\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
7 (11\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
5 (13\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2 (8\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.582\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
- heat urticaria\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
1 (2\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
1 (4\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.199\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
- delayed pressure urticaria\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
4 (6\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2 (5\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2 (8\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.612\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
- vibratory urticaria\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
1 (2\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
1 (4\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.199\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
- solar urticaria\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2 (3\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2 (8\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.067\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
- contact urticaria\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
1 (2\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
1 (3\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.429\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
- aquagenic urticaria\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
-\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
- 2 types or more\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
13 (21\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
8 (21\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
5 (21\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.976\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Angioedema\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
33 (52\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
22 (56\%0\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
11 (46\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.414\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Hypotensive episodes\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
4 (6\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
1 (3\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
3 (13\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.116\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Allergic rhinitis\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
20 (32\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
10 (26\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
10 (42\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.185\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Asthma\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
14 (22\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
8 (21\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
6 (25\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.677\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Atopic dermatitis\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2 (3\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2 (5\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.423\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Autoimmune disorders\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
15 (24\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
10 (26\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
5 (21\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.663\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Oncology\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2 (3\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2 (5\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.260\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Family history of ColdU\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
6 (10\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
3 (8\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
3 (13\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.528\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
H1-Antihistamines licensed doses\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
50 (79\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
31 (79\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
19 (79\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.976\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
H1-Antihistamines escalated doses\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
9 (14\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
6 (15\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
3 (13\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.751\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Omalizumab\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2 (3\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2 (5\%)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0.260\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Duration of ColdU (months)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
60 (11-360)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
24 (11-71)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
212 (60-360)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{\textless{}0.001}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
UCT\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
11 (6-16)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
10 (6-13)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
13 (9-16)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{0.028}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
DLQI\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
4 (0-10)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
6 (2-10)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
0 (0-4)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{0.006}\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textbf{Table 1.} Demographic and clinical features of patients with
typical and atypical ColdU.

\emph{ColdU} , cold urticaria, \emph{CSU} , chronic spontaneous
urticaria,\emph{CIndU} , chronic inducible urticaria, \emph{UCT} ,
Urticaria Control Test, \emph{DLQI} , Dermatology Life Quality Index\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
\textbf{Metric} & \textbf{Typical ColdU} & \textbf{Atypical ColdU and
HCs} & \textbf{All study subjects}\tabularnewline
Sensitivity & 0.974 & - & 0.875\tabularnewline
Specificity & - & 0.875 & 0.974\tabularnewline
PPV & 1.000 & - & 0.927\tabularnewline
NPV & - & 1.000 & 0.955\tabularnewline
Accuracy & 0.974 & 0.875 & 0.936\tabularnewline
\bottomrule
\end{longtable}

\textbf{Table 2.} Statistical metrics of \emph{R}
\textsubscript{PPG}predictive capability

\emph{ColdU} , cold urticaria, \emph{HCs} , healthy controls, \emph{NPV}
, negative predictive \emph{PPV} , positive predictive value, PPG
amplitudes, \emph{PPG} , photoplethysmography value,\emph{R}
\textsubscript{PPG}, ratio of post-provocation and baseline

\textbf{Supplemental material}

\textbf{Figure 1.} Scatterplot of \emph{R} \textsubscript{PPG} and CTT.

\emph{ICT} , ice cube test, \emph{TT} ,
TempTest\selectlanguage{ngerman}\textsuperscript{®},\emph{CTT} , critical temperature threshold

The correlation coefficient (CC) of \emph{R} \textsubscript{PPG} of
ICT/TT-positive patients and the CTT is 0.12, at once the CC of\emph{R}
\textsubscript{PPG} of all study objects and the CTT is 0.70, the CC of
post CST PPG amplitudes for typical ColdU and CTTs is 0.15.

\textbf{Figure 2.} Typical PPGs before (A, C) and after ICT (B, D) for a
healthy person (A, B) and a ColdU patient (C, D).

Figure illustrates PPGs in one of the ColdU patients with positive SPT
and one of the HCs. The frequency of the pulses indicates the heartbeat.
The pulse shape may be associated with the state of tissue and its
pathologies. Before ICT, PPG amplitudes for ColdU and healthy volunteers
have similar values but after cold exposure they grow in a different
way. The relative increase of PPG amplitude is much higher for ColdU
patients than for healthy ones. To quantify the effect of exposure to
cold on microcirculation, we introduced the value\emph{R}
\textsubscript{PPG}~as a ratio of PPG amplitudes after and before the
ICT.

ColdU, cold urticaria, ICT, ice cube test, \emph{PPG} ,
photoplethysmography

Orcid

Sofia Serdotetskova https://orcid.org/0000-0001-8472-115

Alexander Machikhin https://orcid.org/0000-0002-2864-3214

Daria Fomina https://orcid.org/0000-0002-5083-6637

Mikhail Volkov https://orcid.org/0000-0002-9658-9467

Anastasia Guryleva https://orcid.org/0000-0003-2239-3725

Mojca Bizjak https://orcid.org/0000-0003-2595-468X

Valeriya Bukova https://orcid.org/0000-0001-9564-4308

Marina Lebedkina https://orcid.org/0000-0002-9545-4720

Olga Mukhina https://orcid.org/0000-0002-3794-4991

Gerelma Andrenova https://orcid.org/0000-0001-7053-3900

Irina Mukatova https://orcid.org/0000-0002-5804-8643

Inna Danilycheva https://orcid.org/0000-0002-8279-2173

Marcus Maurer https://orcid.org/0000-0002-4121-481X\selectlanguage{english}
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