At present there are no medical tests for food allergies that are more accurate than the detective work of a parent who is a keen observer and accurate recorder. In most cases, a carefully done elimination diet will uncover what the allergy is to. Your doctor or allergist can help by performing one or both of the following tests:
- Skin test. A skin test is helpful in uncovering hidden food allergies, but it has a high incidence of false positives, meaning the skin test is likely to show that you or your child is allergic to a food when really you are not. A negative skin test (i.e., your child does not react to a certain food allergen injected in his skin) is a reliable indication that you are unlikely to be allergic to that food.
- Blood test. This test, called a RAST (Radio-Allergo-Sorbent Test), measures the antibodies in your bloodstream to certain food allergens. Unlike the skin test, a RAST test has a high degree of false negatives, meaning it does not detect food allergies that your child really has. A positive RAST test is a reliable indicator that you are likely to be allergic to that food. If a certain food, say peanuts, show up positive on a RAST test, that means you are more likely than not to be allergic to peanuts. If a skin test and a RAST test agree, you can give the results even more weight. But keep in mind that neither a skin test nor blood test is as reliable as your own observations, and they are certainly more costly. Testing may be most helpful when the results of an elimination diet are confusing.