Figure 8. Boxplots of the most important variables (VOMS) for
discriminating between (a) COVID-CTRL and (b) COVID - RECOV. MTA -
3-methoxy-5-(trifluoromethyl)aniline; TND -
1,1,6-Trimethyl-dihydronaphthalene.
Some VOMs, such as D-carvone, MTA, TDN, and α-terpinene, are associated
with diet [32-35], so their interpretation as potential biomarkers
of COVID-19 infection and progression is not straightforward.
3. DISCUSSION
This study focused on the analysis of the volatile composition of the
urine of patients infected with SARS-CoV-2 upon recovery. Overall, 101
different VOMs were identified in the urine of the recruited subjects,
and statistically significant differences were found between these
groups and control subjects. Accordingly, a volatile signature composed
of D-carvone, MTA, TDN, 2-heptanone, and TONEA and nonanoic acid,
α-terpinene, β-damascenone, α-isophorone, and t-furan linalool were
defined for COVID-19 patients and patients recovered from the disease,
respectively. Correspondent boxplots show sharp variations in the levels
of the referred VOMs between analysed groups. The interpretation of
these variations in urine composition is hindered by the fact that they
undergo modifications due to various factors, including metabolic
processes, pH fluctuations, bacterial activity, and the degradation of
urine components. Additionally, external factors such as diet, lifestyle
habits, health conditions, physical stress, and the environment also
affect the urine composition [16]. Terpenes are often associated
with exogenous sources such as beverages, foods, and flavouring
ingredients, although they can also be produced through the mevalonic
acid pathway [28,35]. Carotenoid-rich foods are a source of volatile
norisoprenoids, which are produced through enzymatic degradation and
have been reported in other studies involving volatile urinary
fingerprinting [36]. Phenolic compounds are often found in urine as
by-products of metabolic processes, but they can also be produced
through the consumption of food, beverages, industrial chemicals, and
environmental pollutants [37]. Ketones are a key subgroup of
chemicals detected in urine, and multiple metabolic pathways are
involved in their production, including carbohydrate metabolism,
decarboxylation of oxo-acids, and lipid peroxidation [38,39]. Some
studies have suggested that a considerable portion of ketones in urine
may stem from gut bacterial activity, external sources such as
foodstuffs, beverages, flavouring ingredients, or pollution [31,40].
To our knowledge, this is the first study to reveal changes in the
urinary volatilomic profile following SARS-CoV-2 infection and recovery
from COVID-19. Such changes define volatile signatures with the
potential to be used in noninvasive COVID-19 diagnosis and management.
In this context, the number of samples constitutes a limitation of this
study, which can be circumvented by future disease outbreaks. In this
scenario, the experimental conditions, safety protocols, and
collaboration between the research entities involved in this study can
be promptly activated, allowing the recruitment of more subjects and
relevant clinical information. Age, diet, previous clinical condition,
sex, and gender, for instance, were interferents that could not be
included in the present study.
4. CONCLUSION
A novel, fast, and sensitive analytical approach was developed and
successfully applied in diagnosis of COVID-19 infection. This
exploratory study using HS-SPME/GC-MS, unveiled significant differences
in the volatilomic patterns of COVID-19 patients and recovered patients
compared to control subjects, evidencing that SARS-CoV-2 infection
triggered metabolic changes that also affect the urinary volatile
composition of the infected patients. This constitutes a signature with
potential for COVID-19 diagnosing and monitoring of the disease
progression with potential to be successfully used in clinical
applications. Overall, the results show the feasibility of using urine
samples for the non-invasive COVID-19 diagnosis and further studies with
larger cohorts are desirable, envisaging the developing of this
complementary tool for COVID-19 diagnosis.