Unexpected Falsely High Vancomycin Concentrations in a Patient with Endocarditis During Routine Therapeutic Drug Monitoring.

Introduction: Vancomycin is a glycopeptide antibiotic indicated for the treatment of infections caused by methicillin-resistant Case description: An 87-year-old female was admitted with diagnosis of endocarditis. She was started on antibiotic therapy with intravenous vancomycin. The first vancomycin level registered in our hospital was 36.4 μg/ml. Because this value exceeded the critical threshold (15-20 μg/ml) the prescription was interrupted. New vancomycin concentration tests were indicated for the next days with 65.8 μg/ml, 73 μg/ml, 55.1 μg/ml, 77.9 μg/ml and 100 μg/ml results. Two samples were reanalysed in Hospital Universitario La Paz and were undetectable. Our Clinical Analysis Department found an analytical interference; however, the responsible molecule could not be identified, so the clinical team changed antibiotic treatment to daptomycin, and the infectious condition of the patient improved. Conclusions: It is recommended that clinicians have a high index of suspicion to investigate cases where drug concentrations do not correlate with clinical expectations. Healthcare professionals need to understand that occasional interference still occurs due to the binding of proteins. LEARNING POINTS: Unknown compounds or mechanisms may interfere with vancomycin assays.Assay interference should be suspected in patients with persistently high vancomycin levels who continue to deteriorate clinically.