8 top allergens account for 90 percent of food allergies. Specific IgE levels (sIgE) that predict the likelihood of passing an oral food challenge are shown in the figure. (click to enlarge the image).
Here is the breakdown of the findings for the top 8 food allergens (remembered by the mnemonic TEMPS WFS):
Cow's milk
The sensitivities were 88% for SPT, and 87% for specific-IgE.
The specificities were 68% for SPT and 48% for specific-IgE.
Specific-IgE detects on average the same number of cases per 100 people with cow's milk allergy as SPT, but gives on average 20 additional false-positive diagnoses for every 100 people without the allergy.
Hen's egg
The sensitivities were 92% for SPT, and 93% for sIgE.
The specificities were 58% for SPT, and 49% for sIgE.
No significant differences in sensitivity and/or specificity were observed when SPT was compared to specific-IgE.
Wheat
The sensitivities were 73% for SPT, and 83% for sIgE.
The specificities were 73% for SPT, and 43% for sIgE.
SPT has a higher specificity than specific-IgE. Specific-IgE detects on average 11 more cases of every 100 people with wheat allergy than SPT, but gives on average 31 additional false-positive diagnoses for every 100 people without the allergy.
Soy
The sensitivities were 73% for SPT, and 83% for sIgE.
The specificities were 73% for SPT, and 43% for sIgE.
Specific-IgE had a higher sensitivity than SPT but lower specificity. Specific-IgE detects on average 28 more cases of every 100 people with soy allergy than SPT, but gives on average 30 additional false-positive diagnoses for every 100 people without the allergy.
Peanut
The sensitivities of SPT and specific-IgE were very similar: 95% for SPT, and 96% for sIgE.
There was no difference between the specificities: 61% for SPT, and 59% for specific-IgE.
Tree nuts
Only hazelnut was assessed. SPT sensitivities were 88% and 90% and specificities of 28% and 6% for hazelnut allergy using natural and commercial extracts, respectively. For specific-IgEs, sensitivities were 75–99% and specificities were 17–77%, depending on the cutoff.
Fish
Sensitivities 91% and 100% for SPT, specificity of 57% for SPT.
Sensitivities were 67–94% for sIgE, and specificities were 65–88% for sIgE.
Shellfish
Sensitivities were 100% for SPT. Specificities were 32% and 50% for SPT.
Sensitivity was 100% for sIgE, specificity was 45% for sIgE.
Component-specific-IgE
One study included 43 children and evaluated the accuracy of component-specific-IgEs (Ara h2) in peanut allergy. Sensitivity was 100% and specificity of 77%.
Summary
The systematic review suggests that both SPT and specific-IgE have good sensitivity, but poor specificity. In theory, this can be correct by the atopy patch test (APT) which has poor sensitivity, but good specificity. However, the evidence base for APT use is limited (results are in the reference source below).
Specific-IgE tend to have a higher rate of false positives than SPT.
Specific-IgE and SPT indicate the presence of IgE sensitization to a specific food. Sensitization is, however, not always associated with a clinical reaction to that food. Non-IgE-mediated immunological reactions to food result from the activation of other immunologic pathways (e.g. T-cell mediated) and manifestations include atopic eczema/dermatitis, food protein-induced enterocolitis, or proctocolitis. APT may be positive in some of these non-IgE-mediated conditions.
The double-blind, placebo-controlled food challenge (DBPCFC) is the ‘gold standard’ diagnostic test. DBPCFC is, however, time-consuming, resource-intensive and may induce anaphylaxis.
Food challenge is consuming increasing amounts of suspected food at fixed intervals under observation. It is done by feeding gradually increasing doses of the suspected food at 10-30 minutes until a reaction occurs or a normal amount of the food is eaten without causing symptoms. All negative blind challenges end with an open challenge. Oral food challenges are essential to the diagnosis of food allergy.
Oral Food Challenges (click to enlarge the diagram).
My personal opinion
My experience is based on more than 100,000 individual skin tests, multiple sIgE and oral food challenges. In my academic clinical practice, SPTs have more clinical relevance than sIgE. It is not uncommon for patients with a negative sIgE (lower than 0.35) and a positive SPT (larger than 3 mm) to have a clinical reaction during food challenges. It is very unusual for a patient with with a negative SPT to have a reaction during a food challenge. Therefore, SPTs appear to have more clinical relevance in the diagnosis of food allergy. SPTs results are also much faster (10 minutes) and generally less expensive than sIgE. SPTs also allow testing for non-standardized or complex food allergen sources, for example, fresh fruits and vegetables (apple, pear, plum, strawberry, mushroom), pasta sauce, etc.
Comparison of diagnostic methods for peanut, egg, and milk allergy - skin prick test (SPT) vs. specific IgE (sIgE) (click to see the spreadsheet). Sensitivity of blood allergy testing is 25-30% lower than that of skin testing, based on comparative studies (CCJM 2011).
References:
The diagnosis of food allergy: a systematic review and meta-analysis. K. Soares-Weiser et al. EAACI journal Allergy. Article first published online: 14 DEC 2013, DOI: 10.1111/all.12333
http://onlinelibrary.wiley.com/doi/10.1111/all.12333/full
Ethyl-chloride spray prevents itching secondary to allergy skin test, without masking the results http://buff.ly/1EFsQ0v
Comments from Twitter:
Priya Bansal, MD @Allergygal1: article confirming what we know-blood specific Ige has a high false + rate compared to prick food skin testing
Matthew Bowdish MD @MatthewBowdish" Ves, wonderful post on SPT vs immunocap!
Brian Schroer @Brian34Schroer: Nice quick review
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