Diagnosis and Endotyping
The gold standard for the diagnosis of FA remains the double-blind
placebo-controlled food challenge (DBPCFC); however, they are
time-intensive with high-risk of severe reaction, necessitating a need
for alternative diagnostic techniques (Figure 2). Allergen-specific IgE
(sIgE) assays are readily available in clinical settings but have a high
rate of false positives as they cannot distinguish true FA from
sensitivity without clinical allergy. The ImmunoCAP assay is a
fluorescent method which is currently the standard for sIgE
quantification. It is sensitive but requires a large amount of blood,
which can be problematic when testing young children. A method that has
shown to be comparable to ImmoCAP is IMMULITE, a chemiluminescent
method.11 Recently, LuLISA, an bioluminescent method
which requires 1 μL or less of plasma sample has been
published.12 Additionally, a peanut bead-based epitope
assay was developed using the LEAP cohort and validated in CoFAR-2 and
POISED studies. It uses two sIgE antibodies in sequential fashion to
diagnose PA and has demonstrated good sensitivity (92.3%), specificity
(94.1%), and accuracy (93.4% concordance with DBPCFC). Although
requiring less than 100 µl of plasma/serum and being easily adapted for
high-throughput use in clinical labs, gaps in the molecular
characterization of non- peanut allergens has limited its
use.13
Beyond IgE characterization, the potential of basophil activation tests
(BAT) has been increasingly recognized in recent
years.14,15 However, the effectiveness of BAT differs
significantly between allergens 15,16. Despite
variation in sensitivity, BAT has demonstrated high levels of
specificity enabling it to complement skin-prick and sIgE tests, which
lack specificity but provide considerable sensitivity. Using BAT as a
second-round diagnostic test after skin-prick and sIgE pre-screening was
shown to reduce the number of required diagnostic OFC by 5-15% for
peanut, sesame, and cashew17. The MONAS study found
that a single BAT was efficacious in predicting clinical allergy status
across peanut (AUROC 0.98), cashew (0.97), hazelnut (0.92), pistachio
(0.95), and walnut (0.97), outperforming sIgE testing for peanut and
hazelnut in a sub-analysis of sensitized patients undergoing
OFC15. In addition to a potential role in the
diagnosis of FA, BAT may also predict response to
OIT.18 The POISED study found that patients who failed
DBPCFCs after a period of desensitization followed by peanut avoidance
had higher %CD63high basophils upon peanut
stimulation and had significantly higher Ara h 1, Ara h 2, Ara h 3, and
sIgE/total IgE than those who passed DBPCFCs19.
Classification of patients into “non/low”, “intermediate”, or
“high” basophil responders at baseline was additionally able to
predict success of DBPCFCs following treatment. Sustained
unresponsiveness (SU) to peanut was observed in patients with low
basophil activation at baseline and those with a greater than 80%
reduction in peanut-induced basophil activation after
OIT20, supporting the utility of BAT in predicting and
monitoring response to OIT. These new technologies including others
using novel gating strategies with optimization of storage and
automation of measurements and analysis may enable routine high
throughput analysis in the future.21
Mast cell activation tests (MAT) is another in vitro diagnostic test,
similar to BAT. The BAT uses whole blood whereas the MAT uses plasma or
serum to sensitize mast cells. Expression of activation markers are
measured on stimulation with allergen. The MAT has similar specificity
in the diagnosis of PA but lower sensitivity.22Ongoing research and novel biomarkers in addition to IgE and
basophil/mast activation biomarkers for diagnosis of FA are being
developed.23-26
In addition to the inherent risks associated with DBPCFCs, there exists
considerable variation across trial design, providers, and academic
sites making DBPCFCs challenging to standardize. Several groups,
including DeFASe27, Dribin et al.28,
and CoFAR29,30, and others31 are
attempting to more uniformly approach the characterization of reactions
during food challenges through standardized grading scales for
FA-related adverse events (AEs). As there is wide variability of AE
severity, there is also the push to understand and develop tools for
prediction of patient-specific response to
DBPCFC32-35. These tools can assist in diagnostic and
treatment strategies in those at risk for the most severe reactions.