Skin test is better than blood test (sIgE) for evaluation of milk allergy, but oral challenge is the best

Food allergy is an immunologically mediated adverse reaction to food protein. Cow's milk protein allergy (CMPA) is the most frequent type of food allergy and may be difficult to diagnose.

This Brazilian study included 190 children aged 1-5 years who underwent:

- skin prick test (SPT) for cow's milk, eggs, wheat and peanuts
- specific IgE test for cow's milk, eggs, wheat and peanuts
- atopy patch test (APT) for cow's milk, eggs, wheat and peanuts

122 children underwent open oral challenge ("the gold standard") for the food to which the child was sensitive or had suspected sensitivity.

Presence of food allergy was confirmed for 41% of children. Among these cases, 88% were due to allergy to cow's milk protein.

Sensitivity and specificity of the tests:

- SPT had 31.8% sensitivity, 90.3% specificity, 66.7% PPV and 68.4% NPV
- IgE test had, respectively, 20.5%, 88.9%, 52.9% and 64.6%
- APT had 25.0% sensitivity, 81.9% specificity, 45.8% PPV and 64.1% NPV.

According to this study, oral challenge is the best method for diagnosing CMPA, because of the low sensitivity and PPV of skin and specific IgE tests.

It is important to note that in 10–25% of food allergy reactions, sIgE can be undetectable by blood test (http://buff.ly/1e7HAmY).

I agree that an oral food challenge is the best method to diagnose a food allergy. Skin tests and blood tests play an important role in making the decision if the oral challenge is safe. However, the sensitivity of the skin test and blood test is higher than the reported values in the study. Please have a look at the table below for reference:



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).


Oral Food Challenges (click to enlarge the diagram).

References:

Costa AJF, Sarinho ESC, Motta MEFA, Gomes PN, de Oliveira de Melo SM, da Silva GAP. Allergy to cow's milk proteins: what contribution does hypersensitivity in skin tests have to this diagnosis? Pediatr Allergy Immunol 2011; 22: e133-e138.

Practical guide to skin prick tests in allergy to aeroallergens (free full text). Allergy, 2011.

Oral food challenges

Milk allergy resolution calculator from CoFAR http://bit.ly/U6TkNS - Mobile version: http://bit.ly/U6TmFn

Image source: Wikipedia, GNU Free Documentation License.

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