The worldwide development of bacterial resistance to β-lactam antibiotics and the increasing incidence of methicillin-resistant Staphylococcus aureus (MRSA) infections have necessitated the use of older antibiotics such as vancomycin. Vancomycin is a glycopeptide antibiotic isolated from Streptomyces orientalis. It has activity against gram-positive bacteria, including Streptococcus, methicillin-sensitive and resistant S. aureus, coagulase-negative Staphylococcus, and Enterococcus species (1). Vancomycin is toxic and is reserved for treating severe bacterial infections. Two reported toxicities of vancomycin are ototoxicity and nephrotoxicity. Therefore, therapeutic drug monitoring is important in the clinical management of patients receiving vancomycin.
Measurements of serum vancomycin concentrations are performed by using immunoassays, and in general these assays are robust and accurate. Simons et al. report a patient with a history of lymphoplasmacytic lymphoma whose serum showed a falsely low trough vancomycin concentration of <0.1 mg/L, a value incompatible with vancomycin therapy. Retesting of the specimen by use of a different method in another laboratory revealed a concentration of 9.8 mg/L. Another patient with a history of non-Hodgkins lymphoma also demonstrated a similar problem with the competitive turbidimetric assay. Both patients had monoclonal immunoglobulins, and the authors speculated that these paraproteins in addition to immunoglobulins such as IgG and IgM might have caused this interference. Because there is no easy way to detect such interferences, the report by Simons et al. is a timely reminder to laboratorians that, although rare, such interferences in immunoassays used for therapeutic drug monitoring are problematic. Because both paraproteins and immunoglobulins are high–molecular weight molecules, reanalysis of this specimen by use of protein-free ultrafiltrate (free vancomycin) may have eliminated this interference.
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Authors’ Disclosures of Potential Conflicts of Interest: No authors declared any potential conflicts of interest.
Role of Sponsor: The funding organizations played no role in the design of study, choice of enrolled patients, review and interpretation of data, preparation or approval of manuscript.
- © 2009 The American Association for Clinical Chemistry