Reevaluating the timing of specific immunoglobulin E measurement after anaphylaxis

The anergic period is defined as a period of around 3-6 weeks following a systemic allergic reaction when skin test results are negative. Therefore, most guidelines recommend that physicians should conduct skin test 6 weeks after an immediate hypersensitivity reaction. However, in vitro tests, including serum-specific immunoglobulin E (IgE) measurement, are generally deemed unaffected by the anergic period. Here, we present a case of a patient with initial cefaclor-specific IgE negativity, but when tested immediately after confirmed anaphylaxis to cefaclor, the results converted to positive after the resolution of the anergic period, 8 weeks post reaction. In patients with a strong clinical suspicion of drug-induced anaphylaxis, repeating in vitro tests after the anergic period may be warranted, even if the initial results are negative. Further investigation is needed to assess whether ImmunoCAP values, similar to skin test reactivity, exhibit significant short-term variability.