Falsely decreased vancomycin caused by rheumatoid factor: A case report

Background: Vancomycin is associated with potential nephrotoxicity and trough concentrations need to be monitored in certain patients. Falsely decreased vancomycin measurement may result in overtreatment and need to be identified promptly by clinicians and pharmacists to avoid toxicities.Methods and Results: We report a case of rheumatoid factor-mediated falsely low vancomycin measurement with Abbott particle-enhanced turbidimetric inhibition immunoassay (PETINIA) method. Reanalyzing the sample using an alternative method, removing the interferences using heterophile blocking reagent as well as rheu-matoid factor clean-up solution all helped to solve the false results. Results from alternative method and interference studies showed vancomycin concentrations reached toxic concentrations in the patient and administration of the drug was immediately terminated. The patient experienced a transient increase in serum creatinine. Conclusions: Even though most modern immunoassays use blocking agents to neutralize interfering antibodies such as rheumatoid factor, it is important for health care professionals to understand that occasional interference still occurs due to the heterogeneous nature of rheumatoid factor.

Automated summary

This article reports a case of rheumatoid factor-mediated falsely low vancomycin measurement. Reanalyzing the sample with an alternative method, using heterophile blocking reagent and rheumatoid factor clean-up solution helped to solve the false results. The patient's vancomycin concentrations reached toxic levels and drug administration was immediately terminated. The patient experienced a transient increase in serum creatinine. Even though most immunoassays use blocking agents to neutralize interfering antibodies like rheumatoid factor, occasional interference can still occur.