The Wheal Deal

Today researchers from Australia announced their discovery that up to a third of children who tested positive to peanut allergy on a skin prick test did not react to an oral challenge. This is somewhat encouraging to parents with children that have been diagnosed as peanut allergic via skin prick test but who have not had a known allergic reaction. There is a chance that their child is not actually allergic ... wouldn't that be great!

I would not recommend that skeptical parents do their own oral challenge as the reaction could be fatal! In this study, the oral challenges were done carefully and in the presence of trained medical professionals. When the challenge data was compared to the skin prick data the researchers noticed that the children who had a positive skin prick test and a negative challenge all had wheal diameters below 13MM.

This is not a new discovery. As seen on Dr. John Weisnagel's site, studies have indicated this correlation before, for example a June 2002 study noted that "the performance characteristics of the skin tests were superior using raw extracts because the negative predictive value was 100% (if the wheal diameter was less than 3mm, they could be quite certain the child was not allergic). On the other hand, if the wheal diameter was larger than 3mm, the predictive value was only 74%. If the skin test wheal was 16mm or larger in diameter, the predictive value was 100% that the child was allergic.

There is indeed a strong correlation between a positive skin prick diagnosis and the results of the oral challenge, albeit less than 100%. In the majority of cases, a positive skin prick test does correctly indicate that the child has an allergy. Parents should consult an allergist before making a decision to confirm or refute the skin prick diagnosis with an oral challenge. It's best to err on the side of assuming that your child does have the allergy unless proven otherwise.