Low plasma cell 3(H) thymidine incorporation in monoclonal gammopathy of undetermined significance (MGUS), smouldering myeloma and remission phase myeloma: a reliable indicator of patients not requiring therapy

Abstract

The kinetics of bone marrow plasma cells were evaluated by means of in vitro 3(H)thymidine incorporation in 143 patients with monoclonal gammopathies. Fifty-three patients had symptomatic multiple myeloma (MM) at diagnosis, nine were in stable remission, six in unstable remission, and 16 in the relapse phase. Thirty-seven patients were classified has having monoclonal gammopathy of undetermined significance (MGUS) and 22 as smouldering myeloma (SM). A thymidine labelling index (LI%) of greater than 3 at initial diagnosis predicted a very short survival. High LI% values (median 2.8 +/- 1.1) were also seen at relapse. However, the major new finding was that the LI% could be used to discriminate precisely between the SM-MGUS group and the MM patients including stage I disease (P less than 0.0001). Only one patient developed MM during follow up, that being 8 months after the initial diagnosis of SM. During the unmaintained stable remission (plateau) phase a low proliferative activity was also observed (LI% = 0.6 +/- 0.2). Thus the LI% was extremely useful in identification of both poor risk groups with a LI% greater than 3 and stable patients requiring no immediate therapy with a LI% less than 1. The ability to discriminate between MGUS and SM and stage I MM should prove particularly useful clinically.