Detection and Characterization of Paraproteinemia in Canine Chronic B‐cell Lymphocytic Leukemia Using Routine and Free Light Chain Immunofixation

BACKGROUND: Hyperglobulinemia is reported in 26% of canine chronic B‐cell lymphocytic leukemia (B‐CLL) cases. However, few cases have been characterized by protein electrophoresis and immunofixation (IF), and the incidence of a monoclonal protein (M‐protein) is unknown using these techniques. OBJECTIVE: To characterize and determine the proportion of canine B‐CLL cases with an M‐protein using plasma protein electrophoresis (PPE), routine and free light chain (fLC) IF, and to assess if productive B‐CLL cases express MUM1/IRF4 by cell tube block (CTB). METHODS: PPE, routine (targeting IgG, IgA, IgM, IgG4, and light chain) and fLC IF were performed using 48 dog B‐CLL plasma samples from patients diagnosed via peripheral blood flow cytometry. CTB was performed on a separate cohort of 15 patients. RESULTS: Hyperproteinemia (>7.5 g/dL) was present in 17/48 cases (35%). An M‐protein was detected in 32/48 cases (67%). Of these, 19/32 cases (59%) had only complete (monoclonal heavy and light chain) M‐proteins detected, 10/32 cases (31%) had both complete and fLC M‐proteins detected, and 3/32 cases (9%) had only an fLC M‐protein detected. IgM was the most common clonal immunoglobulin isotype detected (23 cases). CD21(+) cell counts were higher in cases with detectable M‐protein. Plasma fLC IF suggested β‐γ region interference, likely caused by clotting proteins. All B‐CLL cases consistently expressed PAX5 and did not express MUM1/IRF4. CONCLUSIONS: Most B‐CLL cases had an M‐protein and were not hyperproteinemic. Most cases with paraproteins had a complete IgM monoclonal gammopathy; a subset had documented fLCs. The prognostic significance of heavy and fLC presence should be evaluated.