THE LABELING INDEX OF PRIMITIVE PLASMA-CELLS DETERMINES THE CLINICAL BEHAVIOR OF PATIENTS WITH MYELOMATOSIS

For patients with multiple myeloma the most import-ant laboratory corr elate of prognosis and disease activity is the bromodeoxyuridine (BrdU rd) plasma cell labelling index (LI), However, the traditional immunof luorescent microscope LI technique, like other manual enumeration assa ys, can suffer from poor precision and accuracy, In this study the LI of different subpopulations of plasma cells (CD38(++)) as determined b y flow cytometry was correlated with disease state. The mean LI of the total CD38(++) population was significantly higher (2.7 +/- 0.4%) tha n the LI determined by the traditional slide technique (0.6 +/- 0.1%) for 65 samples tested. Primitive plasma cells (CD38(++), CD45(++)) had a higher labelling index than mature plasma cells (CD38(++), CD45(-)) (7.0 +/- 1.3% v 1.8% +/- 0.3%) and in one patient the LI of the primi tive plasma cells was 46%, in addition, the LI of the mature plasma ce lls was lower than the total plasma cell population. As expected, ther e was a significant difference between the LI of patients in plateau p hase and progressive disease but this difference was greatest when the LI of the primitive plasma cells was studied (9.2 +/- 2.9% v 2.2 +/- 0.7%: z = 19.9, P < 0.001). This study has raised some concerns about the sensitivity and accuracy of the traditional labelling index and ha s shown that the increased LI associated with progressive disease is a lmost entirely attributable to an increase in the LI of the primitive plasma cell subpopulation and that the LI of primitive plasma cells pr ovides a more clinically significant correlation with disease status t han the traditional assay.