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.

Blood Testing

One of the toughest moments for the parents of a child with food allergy related anaphylaxis is the first allergy test. Our daughter's skin-prick test felt like an exercise in confirming what we as parents already knew. We had seen severe reactions to peanut and egg so we knew the result before we went in.

The test did not provide us with new information though it was helpful to confirm the sensitivity to peanut and egg allergens. Unfortunately the test is not able to assess severity. It was also interesting to learn that it is possible to trigger an allergic reaction and as a result our daughter was tested using peanut or egg extract without breaking the skin due to her history of prior reactions.

Another option is to go the blood test route. There is an interesting article in the NY Times today about the value of blood tests. Here is a key excerpt from the article:

"Allergists have typically turned to blood testing as a last resort when skin testing cannot be used. ... Yet studies have found that newer blood tests are as sensitive as skin tests and less subjective. The blood test is also part of a larger debate about who should be treating allergy sufferers. Blood testing would allow pediatricians and other primary care doctors to diagnose allergies and treat many patients. But allergists contend that these generalists are not qualified to assess the laboratory results."

This will likely be a hot topic for debate in the short term.