Archived Issues

2007, Volume 18, Number 2

A Negative RAST Becomes Positive.
What to do?

By David F. Keren, M.D.

Recently, I received a call from a doctor’s office that was concerned about one of our recent RAST (radioallergosorbent) tests. The serum was from a child who had had a reaction to legumes, but one of our RAST tests for this allergen had come back negative. Because of this, the doctor’s office sent another sample to another laboratory that reported a moderate level of activity (high Class II)—RAST testing is scored on a scale of 0 (no reactivity) to 6 (high reactivity).

By the time I was called, our original sample was no longer available. I also learned that the second sample had been drawn a month after our sample and requested that we be allowed to test an aliquot of that sample. Our RAST test also gave a moderate level of activity (low Class III) on the second sample.

The client did not understand how a negative could turn into a positive in one month’s time. Of course, it is possible that the incorrect sample was processed initially. If it was mislabeled anywhere from the patient’s arm to when we received it, we could have a false negative. The automated system we use is bar-coded and highly reliable, but no system is perfect. However, the sequence of the testing was suspicious to me.

I reviewed the matter with an expert on RAST laboratory testing, Dr. Robert Hamilton at Johns Hopkins Hospital. He told me that for years this has been an issue not just with RAST testing but even with skin testing. He was most familiar with this phenomenon with regard to hymenoptera testing, especially in young children where a child is stung by a bee, has a bad response and then is tested (by RAST or skin) within a day or so and tests negative or very weakly. Because of the history, in a month or two the child is retested and comes up with a strong positive.

Presumably, the mechanism is stimulation of the IgE immune response by the antigen to form more IgE against the allergen. When you see a patient with an allergic reaction to what seems to be an obvious allergen, yet receive a negative, or weakly reactive test shortly after the stimulation, please consider repeating the test in 1-2 months to see if the IgE response becomes stronger.

More importantly, however, our policy at Warde Medical Laboratory is that when any of our clients has a concern about a test done here, that the client can just call me, discuss the case, and I'll send the same tube to another reference laboratory for free. We have that much confidence in our testing.