Quantitation of circulating peripheral blood plasma cells and their relationship to disease activity in patients with multiple myeloma

Background. The analysis of peripheral blood mononuclear cells for plasma cells and B‐cell surface light chain ratios may provide additional insight on the pathogenesis of multiple myeloma and predict disease activity. The goals of this study were to correlate these parameters with clinically determined disease activity and establish a cutoff value of circulating plasma cells that could be used in future clinical trials. Methods. Peripheral blood samples from 84 patients with monoclonal gammopathies were analyzed by an immunofluorescence, slide‐based, labeling index (LI) technique for detection of plasma cells, the LI, and light chain ratios. These parameters were compared with disease activity. Results. Of the 84 patients studied, 27% had inactive and 73% had active disease. The mean number of plasma cells for patients with inactive disease was 0.61 × 10⁶/l, compared with 139.6 × 10⁶/l for patients with active disease (P < 0.001). The mean ratios of involved to uninvolved light chain percentages for patients with inactive and active disease were 1.6 and 9.2, respectively (P = 0.002). The absolute number of plasma cells better predicted disease activity than the light chain ratio. By use of a cutoff value of 3 × 10⁶/l, 67% of patients with active disease were determined to have 3 × 10⁶/l or more plasma cells, and 96% of those with inactive disease had less than 3 × 10⁶/l plasma cells. Conclusions. These results suggest that detection of circulating plasma cells is a marker of disease activity in patients with plasma cell disorders and that an appropriate cutoff value is 3 × 10⁶/l or more circulating plasma cells. Cancer 1993; 72:108–113.