Positive Skin Test or Specific IgE to Penicillin Does Not Reliably Predict Penicillin Allergy.

INTRODUCTION

According to guidelines, patients are diagnosed with penicillin allergy if skin test (ST) result or specific IgE (s-IgE) to penicillin is positive. However, the true sensitivity and specificity of these tests are presently not known.

OBJECTIVE

To investigate the clinical relevance of a positive ST result and positive s-IgE and to study the reproducibility of ST and s-IgE.

METHODS

A sample of convenience of 25 patients with positive penicillin ST results, antipenicillin s-IgE results, or both was challenged with their culprit penicillin. Further 19 patients were not challenged, but deemed allergic on the basis of a recent anaphylactic reaction or delayed reactions to skin testing. Another sample of convenience of 18 patients, 17 overlapping with the 25 challenged, with initial skin testing and s-IgE (median, 25; range, 3-121), months earlier (T), was repeat skin tested and had s-IgE measured (T), and then skin tested and had s-IgE measured 4 weeks later (T).

RESULTS

Only 9 (36%) of 25 were challenge positive. There was an increased probability of being penicillin allergic if both ST result and s-IgE were positive at T. Positive ST result or positive s-IgE alone did not predict penicillin allergy. Among the 18 patients repeatedly tested, 46.2% (12 of 25) of positive ST results at T were reproducibly positive at T. For s-IgE, 54.2% (14 of 24) positive measurements were still positive at T and 7 converted to positive at T.

CONCLUSIONS

The best predictor for a clinically significant (IgE-mediated) penicillin allergy is a combination of a positive case history with simultaneous positive ST result and s-IgE or a positive challenge result.