When not to trust therapeutic drug monitoring: Table 1:

Abstract

Therapeutic drug monitoring (TDM) is the measurement of serum or plasma drug concentration to allow the individualization of dosing. We describe the case of a patient who was prescribed inappropriately large doses of vancomycin due to inaccurate TDM. Specifically, our laboratory reported progressively lower vancomycin concentrations despite dose increases. Eventually, when duplicate samples were sent to a different laboratory vancomycin concentrations were found to be in the toxic range. We hypothesize this was due to the patient generating immunoglobulin antibodies against her infection that interfered with the original TDM immunoassay. Immunogenic TDM interference has been known to rarely occur in patients with immune related comorbidities; however, if we are correct, this is a unique case as this patient did not have such a background. This case illustrates the importance of using clinical judgement when interpreting TDM as, in this case, substantial harm to the patient was likely only narrowly avoided.

A suspected case of falsely low digoxin and vancomycin concentrations caused by free kappa light chains with PETINIA method

“…It has also been suggested that paraproteins can bind to the analyte itself occasionally, or a component of the assay system, and subsequently cause false results [ 4 , 6 ]. In contrast to previous reports that suggested IgM paraproteins as the root cause of interference for vancomycin assays [ 5 , 7 , 8 ], the patient in our report did not have elevated IgM, or an IgM paraprotein based on the immunofixation result ( Supplemental Fig. 1 ).…”

Falsely Elevated Vancomycin Concentrations in a Patient Not Receiving Vancomycin

“…Falsely low vancomycin concentrations, secondary to immunoglobulin (Ig)‐mediated interference, have been reported with PETINIA platforms including those manufactured by Beckman‐Coulter (DxC 800, Synchron) and Siemens (VISTA 1500) . In five publications involving seven cases of falsely low serum vancomycin concentrations, the reported results were considered suspect because all of the patients were receiving vancomycin before the time of serum sampling …”

“…5 In five publications involving seven cases of falsely low serum vancomycin concentrations, the reported results were considered suspect because all of the patients were receiving vancomycin before the time of serum sampling. [8][9][10][11][12] An English-language literature search (PubMed, Ovid; database inception to April 2019) identified a single case report of falsely elevated vancomycin concentrations. The authors of this case attributed the spurious result to interference secondary to high IgM levels with the Abbott Architect immunoassay.…”