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

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.

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

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_-1), was repeat skin tested and had s-IgE measured (T₀), and then skin tested and had s-IgE measured 4 weeks later (T₁).

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 T₀. 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_-1 were reproducibly positive at T₀. For s-IgE, 54.2% (14 of 24) positive measurements were still positive at T₀ and 7 converted to positive at T₁.

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.