Q:

11/6/2013
How would you approach a 7 year old female patient, with history of immediate urticaria associated with first time cashew ingestion as a toddler, who presently has a negative SPT (Greer) to pistachio, borderline positive SPT to cashew (3 mm), and specific IgE (ImmunoCAP) to both cashew and pistachio in the 40 kU/L range? She was negative on sIgE and SPT to the other common tree nuts. Pistachio sensitization was expected given its botanical relationship with cashew (both anacardiaceae). I would probably repeat the SPT (perhaps with both fresh prick to prick and commercial extract), and if the results are similar, use an oral challenge as a tie-breaker, perhaps to pistachio first and then cashew.

There seems to be a good SPT NPV for clinical tree nut allergy overall, which would give me confidence to do the challenge, however the significantly elevated sIgE and specificity of her results consistent with her clinical history gives me pause. Would you (or other expert food allergists) tend to give the SPT or sIgE more weight when making the decision to challenge? My "gut" finds a reliably negative SPT a very compelling basis for doing an oral challenge regardless of sIgE level.

A:

Thank you for your inquiry.

I am going to forward your inquiry to Dr. Wayne Shreffler, who, as you know, is a nationally known expert in food allergy. As soon as we receive his response, we will forward it to you.

Thank you again for your inquiry.

Sincerely,
Phil Lieberman, M.D.

We have received a response from Dr. Wayne Shreffler. Thank you again for your inquiry.

Sincerely,
Phil Lieberman, M.D.

Response from Dr. Wayne Shreffler:
It's my pleasure to help out, but I do wish in this case (as in many) I had a better evidence base to stand on.

On the one hand, I do give a reproducibly negative skin test considerable weight when deciding on whether to perform a food challenge. On the other hand, several features in this case point to high pre-test probability for true cashew/pistachio allergy. The child is exclusively (at least among tree nuts) and strongly co-sensitized to cashew and pistachio on serum specific IgE testing. Cashew/pistachio testing is less subject to false positives from PR-10 and LTP sensitization. And, of course, the child has a history consistent with immediate food allergy. I would be more skeptical about the sensitization were there greater discrepancy between cashew and pistachio. My anecdotal impression is that strong correspondence is more often seen with true allergy. Ana o 3 testing could be considered (with negative results also supporting a rationale for food challenge) along with the plan to do additional skin testing as proposed. This suggestion, however, is based on an extrapolation not on actual test performance data that I am aware of.

Best,
Wayne Shreffler, M.D.

AAAAI - American Academy of Allergy Asthma & Immunology