M components-a review of 1242 cases.

Among 1242 patients referred for immunologic investigation 1255 M components were detected in the serum. Of these patients 50.9% had multiple myeloma, 18.1% had nonmyelomatous malignant diseases such as macroglobulinemia, lymphoma, leukemia or cancer, 4.3% had connective tissue diseases, 2.5% had primary generalized amyloidosis (PGA) and the rest had various "benign" conditions. Whereas IgG was the commonest M component in multiple myeloma, connective tissue diseases and the other benign conditions, IgM was the commonest M component in lymphoma and leukemia; Bence Jones proteinemia was most frequently observed in PGA. The ratio of kappa to lambda light chains varied from 1.7:1 in IgG myeloma to 1:9 in IgD myeloma, and was 1:2.1 in PGA. Bence Jones protein was detected in 422 (66%) of 640 urine samples tested, the prevalence ranging from more than 70% in multiple myeloma and PGA to as low as 36% in various benign conditions. It is evident that the class and type of M components and the presence of Bence Jones proteinuria have no definite significance with regard to the diagnosis. Therefore, thorough investigation and follow-up at regular intervals are required when M components are detected.