Beware of False Positives

A few minutes into your first visit to your allergist, you can start to feel as though you walked into the acupuncturist’s office by mistake. Your allergist may perform dozens of skin-prick tests to identify the substances that trigger your reactions.

Although these tests are useful in spotting the most common suspects, positive test results don’t necessarily mean that you’re allergic to a particular food that’s been tested. In the world of food allergies, false positives are all too common. A false positive is a test result that incorrectly indicates that you are allergic to something that you are not really allergic to.

Caution: Some estimates show that up to 60 percent of all positive skin test results are false positives. If you are mistakenly instructed to avoid 60 percent of all foods, you’re likely to end up with a severely restricted diet and perhaps even malnutrition.

What skin tests test

When your doctor performs a skin test, she’s determining whether your body contains a high enough concentration of an antibody (IgE or Immunoglobulin E) to trigger a reaction. Food allergies are often said to be IgE-mediated if the presence of a food-specific IgE is triggering the reaction. Skin tests are not useful for diagnosing non-IgE-mediated food allergies.

Interpreting negative results

Negative skin test results are the easiest to interpret. When you test negative for a particular food, the likelihood that you have an IgE-mediated reaction to that food is next to nothing. The main exceptions occur in babies in the first six to nine months of life, during which time occasional false negatives occur.

Interpreting positive results

Interpreting positive food skin test results is more problematic. Positive tests indicate that IgE is present but do not, without confirmation from other sources, prove that a reaction will occur when you eat the food. In other words, the test can show a positive result or a false positive (a skin reaction even though you don’t react to the food when you eat it).

False positives occur in the following scenarios:

  • You have a small amount of IgE antibody to a food but are not be truly allergic to that food. You can eat the food and experience absolutely no reaction to it.
  • Some proteins in foods are cross-reactive with similar proteins in other foods or even environmental allergens like pollens. This cross reactivity can lead, for example, to a falsely positive skin test for soy in a person with peanut allergy, or a positive test to wheat in a person with grass pollen allergy.

Some studies show that the larger the skin test (the bigger the bump on your skin at the site of the test), the more likely a true allergy is at work, although this has not proven true in other studies.

The problem with false positives

Skin test results are only one component of the diagnostic picture, so allergists should evaluate them carefully and in the context of the big, clinical picture. In order to determine the likelihood that a positive result is indicative of a true food allergy, your doctor should confirm the result with one or more of the following:

  • A medical history that shows a probable reaction to the same food. For example, if you’ve had two severe reactions after eating eggs, you obviously need to avoid eating eggs.
  • A positive RAST (radioallergosorbent test) for the same food. RAST results indicate the levels of food-specific IgE in the blood.
  • A reaction to a food challenge. Your doctor may have you consume a small amount of the food under supervision and with emergency medicine and equipment close by to confirm the food allergy. (Never attempt a food challenge on your own.)

Remember: Not all allergists are well-schooled in diagnosing and treating food allergies. If your allergist performs multiple skin tests and places you on a highly restrictive diet based solely on the results of those tests, question the diagnosis. You may need to seek a second opinion. And if you do end up on a highly restrictive diet, consult a nutritionist to make sure you are getting the proper nutrition.