Thank you for your inquiry.
I am referring your question to Dr. Michael Land who is an authority on food allergy in children. When we receive Dr. Land's response, I will forward it to you.
Thank you again for your inquiry.
Phil Lieberman, M.D.
We received a response from Dr. Michael Land. Thank you again for your inquiry and we hope this response is helpful to you.
Phil Lieberman, M.D.
Response from Dr. Michael Land:
This is a great teaching/learning case and a classic example of why we (as allergists) still need to use the food challenge as the gold standard in the diagnosis of immediate IgE-mediated food allergy. I would like to refer the audience to the recent food allergy guidelines published in JACI in 2010, where the section on the Diagnosis of food allergies comments on skin prick testing with the following statement by the expert panel: Negative SPTs occasionally occur in patients with IgE mediated FA. Therefore, in cases where the history is highly suggestive, further evaluation (for example, physician-supervised oral food challenge) is necessary before telling a patient that he or she is not allergic to a suspected food and may ingest it. If we recall from reading Middleton in our training, the negative predictive value of skin prick tests for predicting IgE-mediated food allergy is greater than 95%. This means that there is still a small percentage (~5%) of patients with negative skin tests who may still have IgE-mediated food allergy.
Under the section for specific IgE tests, they also state that: Undetectable sIgE levels occasionally occur in patients with IgE-mediated FA. Therefore, in cases where the history is highly suggestive, further evaluation (for example, physician-supervised oral food challenge) is necessary before telling a patient that he or she is not allergic to a suspected food and may ingest it. An important point to stress here is that even patients who have an Undetectable sIgE may still have a low level of specific IgE that is below the level of detection by the assay (i.e. if it is 0.34 kU/L, then the result would be <0.35 kU/L, but this would be a significant enough level for this patient). Because of the limitation of the assay, I generally try to avoid using the term negative IgE test, but rather undetectable.
With this patient's reproducible immediate hives on multiple occasions and the positive food challenge, the practitioner has done the right thing in recommending avoidance, for now.
Regarding the question of when to re-test the patient, the Expert Panel states that Insufficient evidence exists for the EP to recommend a specific optimal interval for FA follow-up testing for each food and go one to say Annual testing is often the practice for determining whether allergy to milk, egg, soy, and wheat have been outgrown, and the testing interval is extended to 2 to 3 years for allergy to peanut, tree nuts, fish, and crustacean shellfish. However, the EP noted that these testing schedules are not supported by objective evidence.
In this patient's case, given the negative skin test and undetectable sIgE with the questionable hives as well, in my personal opinion, a reasonable time to re-test the sIgE and skin test would be 6-12 months from the last reaction. If he still has a negative SPT and undetectable sIgE, then a repeat food challenge could then be done at that point. It is plausible to theorize that this patient may be either losing tolerance and developing a peanut allergy and a subsequent skin test will be positive or the peanut IgE will be detectable--or in contrast, it may be that he is in the process of developing tolerance and this reactivity will disappear. This current snapshot of the patient is certainly a challenging point (a grey zone). As with all of our patients, we should keep in mind that food allergy is a dynamic disorder that can often surprise us in terms of reactivity, the time to develop tolerance, and the development of food allergy in patients who had previously tolerated a food (i.e. adults developing food allergy). Despite all of our technology and testing, we still have a lot to learn that hopefully research can explain in the future.