Falsely Elevated Vancomycin Concentrations in a Patient Not Receiving Vancomycin

Abstract

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.

Probable Drug Interaction Between Etanercept and Cyclosporine Resulting in Clinically Unexpected Low Trough Concentrations: First Case Report

“…During investigation of this patient case, assay interference from endogenous proteins or peptides was also considered as a possible cause of clinically unexpected CsA concentrations, as we noticed that false results due to immunoassay interference were common during therapeutic drug monitoring (Tsoi et al, 2019). For example, it was reported in literature that patients receiving etanercept may develop antibodies that could interfere with immunoassays, which posed one challenge for reading laboratory results of patients taking biological therapeutics (Russell et al, 2000;Bolstad et al, 2013).…”

Long‐Acting Lipoglycopeptides Can Interfere With Vancomycin Therapeutic Drug Monitoring

“…24 Previous studies have highlighted the potential for false vancomycin concentrations in vancomycin TDM, 25,26 including one study in which a patient had an elevated vancomycin concentration before initiation of therapy. 27 Other studies have highlighted the impact of renal function on accumulation of the metabolite, CDP-1, that can also falsely increase vancomycin concentrations. 6,28 These falsely elevated levels can have deleterious clinical consequences.…”

Use of Antibiotics in Preterm Newborns

“…This aspect may be problematic within TDM practice, where cross-reactivity with drug metabolites and other analytes can lead to imprecision results [ 47 ]. Tsoi et al (2019) describes a case of falsely elevated vancomycin serum concentrations as a consequence of endogenous protein interference [ 219 ].…”