Background: Densitometric quantitation using serum protein electrophoresis (SPE) is used to monitor monoclonal proteins (M-proteins) in human patients but has not been validated in the dog. Serum globulin concentrations, species-specific radial immunodiffusion (RID), and ELISAs are currently used in veterinary medicine. Objective:We aimed to compare four methods that quantify M-proteins using densitometry and biuret protein (dM-protein) measurements. We also validated the best performing method and compared it with the RID and ELISA methods for measuring canine serum M-protein. Method: Serum from six normal dogs and 83 serum samples from 46 dogs with confirmed monoclonal gammopathies were used. A spike and recovery experiment with purified monoclonal IgG and IgM, inter-run and intra-run variability, linearity under dilution, and lower limit of detection were performed. Results of commercial canine RID and ELISA kits for total class-specific immunoglobulin were compared with dM-proteins. Results: The corrected perpendicular drop gating method had <20% error for IgG/γglobulin and IgM/β-globulin M-protein quantifications. Linearity (r > .99), intra-run CV (1.1%-2.3%), and inter-run CVs (2.0%-3.5%) were acceptable. Correlation between the RID and densitometry results ranged from r = .25 to r = .88, depending on the class. The RID result was greater than that of the biuret total protein in 26/63 (41%) IgA cases. A panel of IgG, IgA, and IgM RIDs failed to correctly identify an IgM paraproteinemia in 6/6 (100%) cases. Densitometry was not comparable with any other tested method. Conclusion: Densitometric quantitation is a valid technique for measuring M-proteins in the β-and γ-globulin regions. Immunotyping via RID using the tested kit does not appear to detect IgM. Densitometry is recommended for measuring M-proteins in canine patients. K E Y W O R D S electrophoresis, immunoglobulin, monoclonal gammopathy, radial immunodiffusion

Background Current recommendations for monitoring disease progression and response to treatment in humans with multiple myeloma include evaluation of serum paraprotein (M‐protein) concentration. Densitometry, species‐specific radial immunodiffusion (RID) and ELISA methods can be used to quantify M‐proteins. Objective Retrospectively evaluate use of the International Myeloma Working Group (IMWG) response criteria for humans in dogs with multiple myeloma. Animals Sixteen dogs with a diagnosis of multiple myeloma, M‐protein documented by serum protein electrophoresis (SPE) and immunofixation (IF) in an initial sample and subsequent electrophoretic evaluation of serial samples. Methods Retrospectively, densitometric M‐proteins, RID and globulins were measured and characterized according to IMWG criteria. Available clinical history was reviewed. Overall survival time (OST) was calculated from initial electrophoretic evaluation to death or last contact. Results All cases received some form of nonstandardized chemotherapy. Complete response (CR), a lack of detectable M‐protein by SPE and IF, was documented in 1 case. Median survival was longer for dogs that attained ≥90% densitometric M‐protein reduction (630 days) than for those that did not attain at least 50% reduction in densitometric M‐protein (284 days; log rank P = .006). Five dogs were defined as having progressive disease (M‐protein increase of >25% and at least 0.5 g/dL from nadir), which correlated with concurrent or subsequent clinical deterioration. Response criteria categorized by serum globulins or RID was not correlated with OST or clinical findings. Conclusions and Clinical Importance Densitometric M‐protein characterized using IMWG response criteria correlated with OST and clinical findings. Densitometric M‐protein detection should be used to monitor dogs with multiple myeloma.

Three dogs under 12 months old were diagnosed with atypical multiple myeloma (MM), having an aggressive multifocal anaplastic round cell sarcoma in bone marrow, viscera, and/or peripheral blood, which were confirmed by cytology and immunohistochemistry to be of plasma cell origin. The intramedullary sarcomas caused myelophthisis, osteolysis, and hypercalcemia. Complete or free light chain monoclonal gammopathy in the serum and/or urine was demonstrated by protein electrophoresis and immunofixation. The polymerase chain reaction for antigen receptor rearrangement assay performed on 2 cases identified a clonally rearranged immunoglobulin gene. Neoplastic cells lacked expression of CD45, CD3, CD18, CD21, CD34, and MHCII by flow cytometry. Immunohistochemistry revealed MUM1 immunoreactivity of the neoplastic cells. Combining all data, the diagnosis was MM. An aggressive form of MM in young dogs should be a differential diagnosis for patients with an immunoglobulin-productive, B cell-clonal, CD45-negative, MUM1-positive discrete cell neoplasm arising from the bone marrow.