Nested case–control study investigating the diagnostic role of tissue eosinophilia in adverse cutaneous drug reactions

Background: Although tissue eosinophilia has traditionally been considered diagnostically supportive of adverse cutaneous drug reactions (ACDRs), studies have suggested it is neither a sensitive nor a specific finding in drug eruptions (DEs). Objectives: Determining whether skin tissue eosinophilia is a reliable indicator of ACDR. Methods: A nested case–control retrospective study conducted in a cohort of 170 patients at a single institution. Tissue eosinophilia (number of eosinophils per high-power field (HPF)) was investigated in skin biopsies obtained from the following groups of patients who demonstrated: (i) in vitro assay and telephone interview-validated cutaneous drug reactions (true DE); (ii) initial clinical diagnosis of ACDR but drug aetiology was excluded by in vitro assay and telephone interview (false DE); and (iii) non-drug-associated cutaneous eruptions, skin tumours and nevi, randomly selected for evaluation (control). Results: Significantly higher number of eosinophils per HPF was observed in the false DE compared to the true DE group (P = 0.02). The false DE group demonstrated a higher number of eosinophils (P < 0.001) while the true DE group eosinophils’ number was not significantly higher as compared to control (P = 0.2032). Conclusions: Tissue eosinophilia is not a reliable indicator of ACDRs.