negative rast test with a positive skin test?

11/17/08

unisteph

Topics:Allergy Test

My daughter had a positive skin test for peanuts, cashews, and peacans. The peanut site was a definate reaction. I personally saw no reaction to the other nuts but the doctor said she saw a slight reaction.My duaghter took the rast test days after the skin test, however the rast test came back negative. She repeated the rast test today (4 mos later) and it still is negative. The doctor didn't repeat the skin testing but said she will in 6 months. Which test is more reliable the rast or the skin? Is it possible that it could just be a topical reaction oppose to an internal reaction/allergy?

Report Abuse

Answers (1)

Wednesday, December 10, 2008

susanphd

I have a similar situation with my son- and did with my daughter, who outgrew her main food allergy.

My understanding is if the child us under 3, the RAST is not as reliable as the skin test. There is an increased rate of false negatives on the RAST for children under 3.

A negative RAST and a postive skin test could also be a sign that your daughter is outgrowing her food allergy. I was told that the skin test often takes 6 months to a year to follow a RAST with negative results.

If you are very anxious and want to know- try a patch test , a fresh food test (where they use the fresh food as a skin test) and then a food challenge (in office!).

I decided to wait until my son is able to communicate well before trying a food challenge with his small positive skin test or wait until there is no reaction at all.

Also- the size of the welt/reaction is not a reliable indicator about the severity of the reaction if the food is ingested- especially if the child has not been directly exposed to the allergen. Sometimes it takes several times of ingestion before the body results and anaphylaxis or other reactions occur.

Talk to your allergist about your concerns and questions- if she is good, she will be able to answer them! I may be completely wrong on these answers as this is not as hard of a science as other medical areas!