Quantitation of the monoclonal plasma cell component in bone marrow from patients with serum paraproteinemia and nondiagnostic marrow morphology.

Twenty-one patients with serum monoclonal gammopathy but lacking acceptable morphological evidence of myelomatosis were studied with reference to the degree, if any, of monoclonal plasma cell expansion in aspirated marrow samples, enriched for plasma cells and analysed with respect to light chain distribution. Four of these patients had a biopsy-proven plasmacytoma of bone. Bone marrow aspirated from sites distant to the tumor showed clear evidence of infiltration by monoclonal plasma cells in two of the cases studied; the other two patients had normal results. Of the 17 other cases, 14 showed evidence of a monoclonal plasma cell component qualitatively concordant with the serum paraprotein as one would expect. These cases could be subdivided into those with myeloma (six cases) and those with monoclonal gammopathy of undetermined significance (eight cases) on the basis of conventional biochemical and radiological criteria. Three of the 17 patients, however, did not show evidence of monoclonal plasma cell infiltration, despite the presence of lytic lesions. It is important to recognize this minority group that simulates myeloma but that may well reflect alternative pathology that has not been identified.