Low or borderline food test results

October 23, 2002

I have recently had both a skin prick test and a RAST done. The skin prick came back negative for all foods. The RAST came back with low equivocal for peanuts-558, pork-575, and baker's yeast-613. Should I completely avoid these foods, should I go by the skin or RAST test? -- G.L.M., Illinois

Skin testing and RAST testing, when done correctly, usually correlate pretty well, but there are exceptions. Many doctors consider skin testing to be a little more accurate.

We normally don't do both, but we had a situation recently where a patient had some RAST testing to molds, pollens and environmental items we'd already skin tested her for.

At first we questioned our skin testing since the RAST picked up some things the skin testing had not found. But as we examined further, we found the low equivocal RAST results were items we'd tested for only at a scratch or percutaneous level. We hadn't felt they were significant enough to test at a stronger intradermal (ID) level.

For explanations of scratch (percutaneous) testing, see:
Allergy testing methods -- Percutaneous testing

For intradermal (ID) testing, see:
Allergy testing methods -- Intradermal testing

For RAST and other forms of testing, see:
Allergy testing methods -- Rast, patch, challenge, and elimination testing

However, when the RAST test showed positive, we did go ahead and test these with ID testing to see what our results would be. The items that had shown very low positives to RAST also revealed very low positives with ID testing to the same items.

The fact that this patient had a low equivocal to these items did not change this patient's course of therapy.

An equivocal RAST test is usually considered just barely borderline or essentially negative by the doctors I've worked with. However, it is possible to have a false negative RAST test.

Lots of things can affect both skin testing and RAST testing. Some of these factors include accuracy of technique, proper storage of extracts or blood samples, and medications used in the days prior to skin testing.

There are no hard, fast guarantees. There have been reports of people who tested negative or very low with RAST, yet they had a serious allergic reaction when they ate the food. We had a patient who had been told she was allergic to tuna as a child. She showed a low positive to tuna with RAST testing. She ate tuna and broke out in a rash.

Your physician is not only knowledgable about the testing methods he uses. He is also able to evaluate your health history and present symptoms and correlate that with skin testing.

He can best advise you on what foods are safe for you in relation to your food allergy testing.

References:

  1. A nurse with anaphylaxis to stone fruits and latex sensitivity potential diagnostic difficulties to consider, Weiss, Dr. Steven, Annals of Allergy, Asthma, & Immunology, December, 1996.
  2. Radioallergosorbent Test (RAST) Methods for Allergen-Specific Immunoglobulin E (IgE) 510(k)s; Final Guidance for Industry and FDA, U.S. Food and Drug Administration.