The importance of patch tests in the differential diagnosis of adverse drug reactions.

Exudative erythema multiforme is an acute self-limited skin disease often associated with infections (usually viral), and also with systemic diseases and drugs. We report the case of a 39-year-old woman diagnosed with systemic lupus erythematosus, who presented at the emergency clinic with exudative erythema multiforme which started 10 days after taking amoxicillin and clavulanic acid for tonsillitis together (almost simultaneously) with the pneumococcal vaccine. Rowell's syndrome was also considered to be a possibility. Skin patch tests were carried with the standard battery of patches (GPEDC) and the active ingredients of the suspected drugs (Chemotechnique ®), with readings at D2 and D3. The tests were positive for amoxicillin 10% pet (++), ampicillin 10% pet (+ +) and penicillin G potassium 10% pet (+). We accepted the diagnosis of erythema multiforme due to amoxicillin, confirmed by patch testing.