Talking Allergies

I haven't heard of that correlation....is that coming from you allergist?

Peanut allergy can be tested in 3 ways.

1-Allergy skin test - this is a simple and fast way to test for peanut allergy. It involves placing a drop of a weak solution of peanut (plus you need positive and negative control solutions to interpret the tests properly). The drops are pricked and the results are measured 15 minutes later. A positive result looks like a little mosquito bite. If there is a good history of a peanut allergic reaction the skin test is a simple way to confirm the allergy. If there is no history of a reaction to peanut a positive test needs careful interpretation. The size of the skin test reaction does not indicate what reaction you may have.

2-Peanut RAST test - the RAST test is done on blood, and measures the amount of IgE against peanut. As with the allergy skin test, if there is a good history of a peanut allergic reaction the RAST test is a simple way to confirm the allergy. The RAST result is usually reported in two ways - either as 1+ to 4+ or as an exact number.

The exact number gives a more precise indication of how likely it is a reaction will occur on exposure to peanut. For example a result of 15kUA/l means it is 95% likely there will be a reaction. It does not tell you what the reaction will be. Three people with levels of 15kUA/l could have very different reactions such as eczema in one, hives in another, and anaphylaxis in the third.

The RAST can also be used to assess the chance that peanut allergy has resolved (see below).

3-Peanut challenge - very occasionally there is a good history suggesting peanut allergy but negative results on skin and RAST tests. In this situation a peanut challenge may be considered. This should only be undertaken with careful hospital supervision in case a reaction occurs. The challenge will usually start with a tiny amount of peanut (eg. 1/8th of a gram) to try and avoid causing a significant reaction.
___________________________________________________________

Peanut challenge may also be suggested for older children who have possibly outgrown their peanut allergy (see below).

The problem is that all the studies show that most of the children who do grow out of peanut allergy will continue to have positive skin tests and RAST tests to peanut. This means the tests alone cannot determine which children have grown out of the allergy.

However if the RAST result is very high (eg. above 15kUA/l) it is extremely unlikely the allergy has gone away.

Given the difficulty of managing peanut allergy, repeat evaluation is warranted. It is probably best to delay this until age 4 years in order to give the best chance that a child has outgrown the allergy. At that time it's worth repeating the allergy tests if there have been no recent reactions on accidental exposure.

If the skin test/RAST test is negative then a cautious peanut challenge should be considered.

If the skin test is positive and the RAST test is low positive then a cautious peanut challenge should be considered.

If the RAST test is highly positive then it's very likely the allergy is persistent, and most likely that it will be life long. Ongoing avoidance and an up to date action plan are needed.

The exact number gives a more precise indication of how likely it is a reaction will occur on exposure to peanut.

I think I'm really confused now....so, the RAST isn't a predictor of the severity of an allergic reaction, but does it accurately reflect the presence of an allergy? What do you mean -- false positives and false negatives with the RAST?