Challenge Test Results in Patients With Suspected Penicillin Allergy, but No Specific IgE.

Purpose: Patients with a history of allergic reaction to penicillin, but with no detectable specific IgE, are common and pose a dilemma. Challenge tests are considered to be the diagnostic gold standard. The aim of this study was to identify subgroups of patients with very low risk for reactions who could be safely tested using a more rapid and simple procedure.
Methods: A total of 580 consecutively referred adult patients with a history of non-serious cutaneous allergic reactions to penicillin, but with no IgE, were challenged with therapeutic doses of penicillin V (phenoxymethylpenicillin), penicillin G (benzylpenicillin), or both.
Results: Only 14 of 580 patients had a positive challenge test. In 11 of the 14, a reaction to challenge occurred within 2 hours, and none were anaphylactic. The year of the original reaction was known for 555 patients; a positive challenge was seen in only 0.4% of those with an original reaction >15 years before challenge, but in 4.6% of those with a more recent original reaction (P=0.001). Onset of a reaction within the first day of the original exposure was a predictive factor for a positive challenge (P=0.001) in patients challenged within 15 years of the original reaction.
Conclusions: Among suspected penicillin-allergic patients with non-severe skin reactions and no detectable specific IgE, the subgroup of patients who originally reacted more than 15 years previously had very low risk for reacting to a challenge. The risk was higher in patients with a more recent original reaction, especially if the symptoms had occurred within the first day of exposure.