Plasma cell acid phosphatase activity as prognostic factor in multiple myeloma: relationship to the thymidine-labeling index.

Plasma cell acid phosphatase (AP) activity and thymidine labeling index (LI%) were evaluated concomitantly in 52 patients with monoclonal gammopathies. AP score, percentage of AP positive plasma cells, and LI% were significantly higher in 26 patients with multiple myeloma (MM) at the time of diagnosis than in 11 monoclonal gammopathy of undetermined significance (MGUS) and eight smoldering myeloma (SM) patients. LI% had the highest statistical correlation with disease status. A 1% cutoff could clearly separate the patients with progressive MM compared to those with stable disease (SM-MGUS) (P less than .001). There was a significant overall correlation between the AP score and LI% (P less than .005). Since LI% is a recognized powerful prognostic factor, this correlation suggests that the AP score can also be a reliable test predicting patient survival duration. In addition, we identified a subgroup of IgG MM patients with very high tumor mass who had a low LI% but a high AP score. This was associated with very poor patient survival and indicated the discrete prognostic importance of AP score in this subgroup with low LI%. Thus, both the LI% and AP score can be recommended as helpful clinical tests in patients with monoclonal gammopathies.