Differential diagnosis of IgM MGUS and WM according to B-lymphoid infiltration by morphology and flow cytometry

The distinction between IgM monoclonal gammopathy of undetermined significance (MGUS), asymptomatic Waldenstrom's macroglobulinemia (WM; aWM), and symptomatic WM (sWM) relies on two features: the presence of infiltration by lymphoplasmacytic lymphoma in the bone marrow (BM) biopsy and the existence of signs or symptoms attributable to the disease. Nevertheless, some patients lack a BM biopsy or it is not conclusive for diagnosis. In this study we have investigated 94 patients with IgM monoclonal gammopathies, in which a BM trephine biopsy and morphological and flow cytometry (FCM) evaluation of BM aspirate were available at diagnosis. We found a clear correlation between BM infiltration of B-lymphocytes assessed by morphology and by FCM with a Pearson correlation of 0.62 (P<.001). Moreover, in the absence of a BM trephine biopsy, the cut-off points that would help in the differential diagnosis between MGUS, aWM, and sWM would be 20% for morphology and 5% for FCM, both assessed in the BM aspirate.