A patient presented with peritoneal dialysis-associated peritonitis, initially appearing susceptible to vancomycin by conventional AST (VITEK 2 and BMD). However, treatment failed, prompting further investigation. Population analysis profiling area under the curve subsequently confirmed vancomycin heteroresistance in the infecting Staphylococcus epidermidis strain. Whole-genome sequencing did not reveal known genetic mechanisms for this heteroresistance, indicating a diagnostic challenge. This case highlights that standard susceptibility testing can misclassify vancomycin-heteroresistant organisms, leading to treatment failure in critical infections like peritonitis. Clinicians should consider heteroresistance when empirical vancomycin therapy fails despite apparent susceptibility.
Key note: Conventional antimicrobial susceptibility testing may miss vancomycin heteroresistance, leading to treatment failure.