Discussion
We report here the compared performance of two sIgE platforms and their
clinical cut-offs for 10 common food allergens. Clinical cut-offs for
sIgE have been previously proposed multiple times, in particular for
food allergens, as indicators of the probability of presenting with
allergic symptoms, rather than thresholds accurately predicting the
occurrence of symptoms (14, 23, 24). Thus, the quantitative nature of
sIgE measurements is essential for allergy diagnosis, and physicians
must be aware of the characteristics of the employed methods. Indeed,
several routine methods of sIgE quantitation co-exist, because they
belong to successive generations and to different times of availability
for clinical use. First generation tests were radioimmunoassays which
used an anti-IgE reagent labelled with a radio-isotope, usually125I: the RAST (RadioAllergoSorbent Test,
Pharmacia Diagnostics AB, Uppsala, Sweden ) was commercialized in 1974
(25). The main second-generation test is based on the ImmunoCAP™
technology (originally from Pharmacia AB, now ThermoFisher Scientific),
where allergens are covalently attached to a nitrocellulose sponge (4).
Third generation sIgE tests are represented by the IMMULITE 2000 system
(Siemens Healthcare SAS, Saint-Denis, France), which uses biotinylated
soluble allergens bound to a large diameter (25mm) avidin-coated unique
bead, and a chemiluminescent signal is used for detection (26). Recently
(2020), fourth-generation technologies for sIgE determination were made
available (NOVEOS, Hycor, Garden Grove, CA, USA and IDS-iSYS, Bolton,
UK), differing from third generation tests through the use of
biotin-labelled allergens bound to avidin micro-beads and
chemiluminescent detection (16, 27).
We found that NOVEOS sIgE results were significantly lower than those
obtained with ImmunoCAP™, by a mean value of 15%. This discrepancy is
not due to a defect in the linearity of NOVEOS technology (28) which is
similar to that of ImmunoCAP™ (27). The differences we observed between
the two methods could be due to an underestimation by NOVEOS, to an
overestimation by ImmunoCAP™, or both. Significant discrepancies had
been reported previously between ImmunoCAP™ and Immulite™, a
third-generation technology developed by SIEMENS (29). It is notable
that our data show very few differences for low values (0.1 to 1
kUA/L). These levels of sIgE are important for early
detection of sensitization against food allergens in children (30, 31).
By contrast, if the two methods produce significantly different results
for high sIgE concentrations, these discrepancies are of lesser clinical
significance, especially if they are greater than the clinically
relevant cut-offs.
For clinical performance, NOVEOS is better able to exclude allergy in
sensitized individuals having low sIgE values (0.35-3.5
kUA/L) and we also found some false positive results in
non-allergic patients tested with ImmunoCAP™ for peanut and hazelnut
extracts. Despite these discrepancies, we report that NOVEOS and
ImmunoCAP™ have mostly similar performance for discriminating between
food allergic and food tolerant individuals. It is impossible to
determine clinically relevant universal thresholds of sIgE
concentrations due to important variations from one population to
another one. For example, the cut-off for rAra h 2 sIgE concentration in
peanut allergy varies from 0.10 to 42.2 kUA/L between
studies (15). However, the establishment of “local” clinical cut-offs
is of utmost importance for the management of a given population of
patients including the design of OFC protocols. In support of this
assertion, a 2002 study conducted in our center found a clinical
threshold for ImmunoCAP™ peanut extract sIgE (cut-off of
15kUA/L with 95% specificity and 44% sensitivity)
which was similar to the values we report here (cut-off of
14kUA/L, 86% specificity and 51% sensitivity) (24).
Thus, cut-off values can be established for a given population on the
condition of using similar protocols and seem to be stable for extended
periods (20 years in this example). Our study further supports that sIgE
measurements by both ImmunoCAP™ and NOVEOS, are highly informative on
the risk of allergy in the patients we studied based on OR values
>10 and RR>2.
We investigated two potential causes of discrepancies in clinical
performance between NOVEOS and ImmunoCAP™ with hazelnut and peanut
extracts, namely Cor a 1 spiking of ImmunoCAP™ hazelnut extract and the
presence of CCD. While Cor a 1 spiking of ImmunoCAP™ hazelnut extract
did not contribute to clinical performance discrepancies, CCD did. Thus,
our study supports the view that glycosylated epitopes are more
accessible to sIgE with ImmunoCAP™ than with NOVEOS. This could be due
to the avidin-coated beads and the biotinylation of NOVEOS allergens.
Another possibility is that anti-CCD sIgE react both with CCD
determinants on allergen molecules and also with the nitrocellulose
sponge matrix on ImmunoCAP™ (32, 33). Unlike the animal-derived
galactose-α−1,3-galactose epitope, plant CCD (e.g. MUXF3), are
currently considered devoid of clinical relevance in allergy (34). This
could explain our findings of better sIgE - confirmed plant food allergy
correlation with NOVEOS than with ImmunoCAP™. For recombinant MA which
are non-glycosylated in both systems we found similar clinical
performance as expected.
There are several limitations to our study. Firstly, this is a
retrospective, monocentric study. Secondly, the population was mainly
comprised of children (85%). In addition, and depending on the
allergen, the percentage of patients under a strict avoidance diet
varied from 12% (peanut) to 53% (seafood) and an OFC was not
systematically performed for food allergy diagnosis, except for peanut.
On the other hand, our study encompasses a large number of comparisons
including both extracts and molecular food allergens. Moreover, the
heterogeneity of the patients and of their therapeutic protocols mirrors
our regular clinical practice.
In conclusion, we demonstrate here that, for 10 common food allergen
extracts and molecules, assayed in a large pediatric cohort, sIgE
determination performed with NOVEOS or with ImmunoCAP™ are highly
correlated with and predictive of the actual diagnosis of food allergy
or tolerance. Despite a ten-fold lower test sample volume requirement (4
µL) compared to ImmunoCAP™ (40 µL), NOVEOS has an overall better
capacity to identify patients at risk of allergy versus asymptomatic
sensitization (p=0.03 when AUC are compared). Further confirmatory
studies are warranted including more allergens (i.e. other food
allergens, respiratory, venom, drugs) and both adult and pediatric
patients from other geographical areas.