Light chain myeloma and detection of free light chains in serum and urine of dogs and cats

BACKGROUND: Detection of free light chains (fLC) in animals relies on protein electrophoresis or the Bence‐Jones protein test on urine. OBJECTIVE: To describe the detection of both serum fLC (sfLC) and urine fLC (ufLC) in 8 dogs and 2 cats using a commercially available human immunofixation (IF) kit. ANIMALS: Archived serum or urine samples from 27 dogs and 2 cats submitted to the Colorado State University Veterinary Diagnostic Laboratory for routine diagnostics. METHODS: Retrospective study evaluating the presence of fLC in dogs and cats using agarose gel electrophoresis and routine and fLC IF performed on serum and urine. The performance of the fLC IF reagents was evaluated using samples characterized by routine IF, tandem mass spectrometry, and a combination of fLC IF and western blotting. Free light chains were documented by paired electrophoresis and fLC IF. RESULTS: The fLC only myeloma case developed end‐stage renal failure 5 months post initial diagnosis. All electrophoresis‐defined urinary Bence‐Jones proteins were labeled by the anti‐free λ light chain (anti‐fλ) reagent; none were labeled by the anti‐free κ light chain (anti‐fκ); 2 of these were identified as fκ by mass spectrometry. An electrophoretically identical protein restriction that was labeled by the anti‐fλ reagent was present in the paired serum from 5/8 of cases, documenting sfLC. CONCLUSIONS AND CLINICAL IMPORTANCE: Commercially available human IF reagents identified sfLC and ufLC in both dogs and cats. Free light chains may be nephrotoxic in dogs.