A Case of Pustular Drug Eruption with a False-positive Result of Patch Testing for the Metabolic Product of S-carboxymethyl-L-cysteine

Abstract

　A 68-year-old man presented with red papules with small pustules on his trunk and extremities after taking tazobactam/piperacillin (TAZ/PIPC) , S-carboxymethyl-L-cysteine (SCMC) and Ambroxol hydrochloride for the treatment of aspiration pneumonia. Patch testing showed a positive reaction with 5% and 10% concentrations of thiodiglycolic acid (TDA) , which is the major metabolite nocturnally produced from SCMC. However, an oral challenge test for SCMC was negative. On the other hand, a challenge test with TAZ/PIPC reproduced erythema and fever 7 hours later, although the patch test was negative for TAZ/PIPC. Therefore, he was diagnosed with a pustular drug eruption caused by TAZ/PIPC. It was recently reported that patch testing for TDA often exhibits a false-positive result with an irritation response due to its acidity, as we experienced. If SCMC is suspected as a causative drug, an oral challenge test is recommended rather than a patch test with TDA to make a correct diagnosis.