Retrospective Application of the Current International Myeloma Working Group Disease Definitions in 20 Cats With Immunoglobulin Secreting Neoplasms.

Studies describing feline myeloma related diseases (MRD) are limited and reported prognostic indicators are inconsistent. Cats more commonly exhibit visceral plasma cell tumours (PCTs) than marrow-predominant disease, suggesting that diagnostic approaches adapted from the International Myeloma Working Group (IMWG) criteria, incorporating extramedullary involvement and myeloma defining events (MDEs), may better capture clinically relevant disease. We sought to evaluate cats with serum protein electrophoresis (SPE) and immunofixation (IF) confirmed M-proteins using IMWG-adapted criteria, determine how often cases satisfy IMWG-adapted versus classic multiple myeloma (MM) definitions, compare outcomes, characterize clinical and laboratory findings and assess prognostic indicators. A retrospective cohort of 20 cats with SPE/IF-confirmed M-proteins and clinical data was included in the study applying IMWG-adapted and classic MM criteria using available clinical records, laboratory data and cytologic or histologic findings. Survival was evaluated using the log-rank test. 14/20 cats met IMWG-adapted MM criteria, whereas 7 cats met classic criteria. Median survival time did not differ between classic MM (226.5 days) and IMWG-reclassified MM (141 days; p = 0.99). Visceral organ involvement was common; splenic PCTs occurred in 71.4% (10/14) and hepatic PCTs in 87.5% (7/8) of sampled organs. Lytic bone lesions were infrequent (3/14). IgG M-proteins predominated (71.4%). No clinicopathologic variable was significantly associated with survival. IMWG-adapted criteria reclassified MM without altering survival, supporting their diagnostic utility in feline MRD. The predominance of visceral PCTs and infrequent bone involvement highlights species-specific disease patterns and the diagnostic value of extramedullary sampling. Larger prospective studies should refine feline-specific biomarkers and validate MDEs.