Researchers refine testing method for cashew nut allergy
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Testing for Ana o 3, a 2S albumin of cashew nut, provided the most value in predicting clinical reactivity for cashew nut allergy compared with other methods, according to a study published in Annals of Allergy, Asthma & Immunology.
Petteri Röntynen, MD, of University of Helsinki and Helsinki University Hospital, and colleagues evaluated the utility various tests in evaluating pediatric cashew nut allergy, including specific IgE (sIgE) to Ana o 3, sIgE to cashew nut, skin prick testing, basophil activation testing and serum tryptase measurement, as well as combinations of these.
The study involved 106 children (median age, 5.04 years; age range, 1.41-6.66 years; 21.7% aged younger than 3 years; 48% boys) with suspected cashew nut allergy.
The researchers drew a blood sample from each participant before they underwent oral food challenge. Also, the researchers assessed baseline tryptase and complete blood count before each challenge and 60, 120 and 240 minutes after significant allergic symptoms occurred.
The researchers additionally performed SPT on each child’s forearm for cashew and nine other nuts as well as for birch before each OFC. BAT was performed for cashew nut before each OFC as well.
During the OFC, each patient received 50 mg, 100 mg and 500 mg of raw cashew nut protein at 30-minute intervals, with a cumulative dose of 680 mg of cashew protein, or 3.5 cashew nuts. OFCs were halted when significant objective or subjective symptoms emerged. The median cumulative reactive dose was 80 mg of protein.
Following OFC, 72 children (68%) developed symptoms to cashew protein, with 14 (19%) characterized as mild, 26 (36%) as moderate and 32 (44%) as severe. Also, 34 children (47%) had positive-challenge reactions defined as anaphylactic, whereas 19 (29%) had negative challenges despite a reliable history of earlier reaction.
The researchers found a moderate negative correlation between the cumulative reactive dose and Ana o 3-IgE (Spearman’s rho, –0.583) and cashew nut SPT (Spearman’s rho, –0.545).
However, researchers observed a weak negative correlation between the cumulative reactive dose and cashew nut IgE (Spearman’s rho, –0.493) and the percentage of CD63+ basophils (BAT%; Spearman’s rho, –0.469).
Further, the severity score correlated to cashew nut SPT (Spearman’s rho, 0.412), BAT% (Spearman’s rho, 0.448,), Ana o 3-IgE (Spearman’s rho, 0.338) and cashew nut IgE (Spearman’s rho, 0.379).
A BAT of 22.8% or greater best predicted reaction severity with 91.7% specificity and 60.7% sensitivity but, the researchers noted, the cutoffs were age-specific. BAT’s usefulness, they added, was best demonstrated among children aged 5 years and older, which could help providers decide the optimal time for cashew nut challenge to avoid severe allergic reactions.
Researchers also noted that SPTs associated well with severity, especially among older children, and are associated with lower costs and greater availability than BAT.
But overall, the researchers wrote, Ana o 3-IgE seemed to be the best single test for diagnosing cashew allergy, with a 0.97 kU/L cutoff showing 94.1% specificity and 61.1% sensitivity. Various test combinations did not improve diagnostic values.
Perspective
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The results of this study are not too surprising. We have known that testing to the component of the allergen (or component-resolved diagnostics) can result in a more accurate test for certain food allergens. For example, testing to a certain peanut component can improve the diagnostic accuracy of testing for peanut allergy compared with testing to whole peanut extract.
This study lends further support to the idea that using component testing for cashew may offer a more accurate test compared with testing with the whole cashew extract. While I personally do not use component testing for cashew currently in my practice, the results of this study also line up with other data on this subject.
If component testing is available for cashew, you could argue to use the component testing in lieu of using the cashew IgE serum test and feel comfortable in doing so. However, I think it is very important to point out that while the cashew component testing was “the best overall test,” as the researchers wrote, it really was not that much better than the SPT.
I would like to see a cost-effectiveness model and a patient-centered model to see which test is best overall. If you look at the accuracy of skin testing in this population, and given that it is point of care, takes 15 minutes and does not require a needle, one may argue that the SPT is a better overall test.
While the authors focus heavily on the best overall test, you can see by the numbers that the tests really overlapped. Although the component test was the best one, it really wasn’t best by much. They are almost interchangeable. If cost and access are all the same, then maybe you could argue that component testing is the best overall test.
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