Lack of correlation between plasma cell thymidine labelling index and serum beta-2-microglobulin in monoclonal gammopathies.

Simultaneous evaluation of bone marrow plasma cell thymidine labelling index (LI) and serum beta-2-microglobulin (SB2M) was performed in 146 patients with multiple myeloma (MM) or monoclonal gammopathy of undetermined significance (MGUS). Eighty patients had MM on diagnosis, 11 were in relapse and 12 were in remission phase; 43 patients had MGUS. All the evaluated patients had normal renal function with a creatinine level less than 1.4 mg%. Overall there was no direct correlation between LI% and SB2M. LI% best reflected the proliferative capacity of the tumor clone itself being less than or equal to 1% in MGUS and MM in remission, but greater than 2% at relapse of MM. SB2M correlated best with the stage of disease and tumor burden. These two factors therefore have different clinical utility: LI is a useful parameter to detect disease stability (e.g., MGUS) or highly proliferative disease (aggressive MM at diagnosis or early relapse). SB2M remains the best single predictor of patient tumor burden and associated survival duration.