Falsely Elevated Vancomycin Concentrations in a Patient Not Receiving Vancomycin

Therapeutic drug monitoring (TDM) of vancomycin is commonly performed using immunoassays. This case describes falsely elevated vancomycin serum concentrations, possibly secondary to endogenous protein interference. Vancomycin was prescribed for a patient with a suspected septic knee. A blood sample for TDM was inadvertently collected before the first dose. The reported concentration was 36.1 mg/L using the Roche Modular P analyzer and remained high over the next 48 hours and 8 months later in the absence of vancomycin therapy. Vancomycin was undetectable in the patient sample by liquid chromatography-tandem mass spectrometry. The sample was subsequently investigated for endogenous protein interference. The responsible interference was removed by polyethylene glycol precipitation and heat inactivation. Four alternative immunoassays with varying test principles measured vancomycin concentrations ranging from undetectable to 108 mg/L. A glucose-6-phosphate dehydrogenase detection method was common to the two immunoassays exhibiting the greatest interference. To our knowledge, this is the first report of falsely elevated vancomycin concentrations on the Roche Modular P analyzer. Immunoassays are generally robust in facilitating TDM but are susceptible to cross-reactivity. Assay interference should be considered and laboratory professionals contacted when vancomycin levels do not correlate with clinical expectations.