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Background New Hevylite® assay quantifies the immunoglobulin classes, including IgM bound to light chains, allowing distinguishing immunoglobulins involved and uninvolved in plasma cell disorders. Objective To compare data obtained by IgM Hevylite® (IgM-HLC) assay with conventional methods used in routine laboratory practice for monitoring IgM plasma cell disorders. Methods Serum samples (n = 122) from 50 patients with IgM monoclonal protein (MP) identified by Immunofixation (IFE) before the beginning of the study were collected during monitoring from December 2012 to September 2014 (2 Waldestrom's macroglobulinemia, 4 NH-lymphoma, 44 MGUS) and were assessed using IgM Hevylite® (HLC) assay, Capillary Electrophoresis (CE), Immunofixation (IFE), serum Free Light Chain (FLC) assay and total IgM measurements. Results IgM MP was detected by IFE in 85/122 samples (71 IgMk, 10 IgMl, 4 IgMk/IgMl), while in 37/122 was undetectable although CE measured small MP, probably as a consequence of disease stimulating inflammatory immuno-response. Among the 85 positive samples, the HLC ratio but not the FLC ratio was altered in 36 samples while in 4 sera only FLC was altered. Out of 37 IFE negative samples 24 had normal HLC and FLC ratios. Conclusions Since the partial overlap of abnormalities identified by HLC and FLC assays, IgM Hevylite assay can provide valuable information on the evolution of IgM monoclonal disease and may support the recognition of a transitory monoclonality leading to an improvement in routine laboratory practice.