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.

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.