Patients who are treated by self-medication with intranasal mupiricin (Bactroban™) for controlling meticillin-resistant Staphylococcus aureus may, or may not, adhere to their regimen. Herein, we describe a potential methodology for assessing adherence by measuring the gastric degradation product, monic acid A (MA), as a biomarker in urine. MA was isolated (~80% recovery) through a Waters Oasis HLB cartridge and detected (e.g. 25 pg on the column) by HPLC/MS/MS (API4000). Within a calculated 10(6)-fold margin, this analytical sensitivity should facilitate urinary MA quantitation if, for example, 1% of intranasal mupirocin is swallowed and degraded characteristically to MA by gastric acidity.