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

Vancomycin and digoxin are associated with potential toxicity and serum concentrations need to be monitored in certain patients. Previous reports suggested IgM paraproteins could interfere with vancomycin assays, and no paraprotein interference has been reported with digoxin assays. Here we present a suspected case of free-kappa light chains-mediated falsely low digoxin and vancomycin concentrations with Abbott particle-enhanced turbidimetric inhibition immunoassay (PETINIA) method. A 53-year-old patient received multiple doses of vancomycin and digoxin intravenously, but trough vancomycin and random digoxin concentrations repeatedly measured as <1.1 μg/mL and <0.2 ng/mL respectively with Abbott PETINIA method. Results from alternative methods showed concentrations reaching toxic levels and administration of the drugs was immediately terminated. A significantly elevated level of free-kappa light chains, possibly in polymeric form as suggested by protein electrophoresis result, was suspected to be the cause of falsely low results. During the laboratory investigation, absorbance curves revealed increased agglutination in the patient's samples in the latter part of the reaction, suggesting interfering substances led to production of turbidity after reagents were added. Protein-free filtration partially recovered the drugs with Abbott PETINIA. When drug concentrations do not correlate with clinical judgment, clinicians and pharmacists should consult clinical laboratories for investigation of potential interfering substances.