Monoclonal gammopathy of undetermined significance (MGUS), the precursor to multiple myeloma, is more common in blacks than whites. The serum free light chain (sFLC) assay is an important prognostic test in MGUS, but no study has evaluated sFLC levels and ratios in black MGUS patients. One-hundred and twenty-five black MGUS patients at two urban centers were compared to the white population of the Mayo Clinic. The median age for blacks was 73 years [41-94] and 75% were male. The M-protein isotype in blacks was 81% IgG, 13% IgA, 2% IgM, and 4% biclonal compared to 70%, 12%, 16%, and 2%, respectively, in whites, (P < 0.0005). The median M-protein concentration for blacks was 0.44 gm/dL (trace-2.33) compared to 1.2 gm/dl in whites. An abnormal sFLC ratio was present in 45% of black compared to 33% of white (P = 0.01) patients. Using the Mayo Clinic risk model, black patients had a significantly lower proportion of higher risk MGUS compared to whites: low 43%, low-intermediate 45%, high-intermediate 10%, and high 2% (P = 0.014). Black patients with MGUS have significantly different laboratory findings compared to whites. The biologic basis for these disparities and their effect on prognostic assessment is unknown. Prognostic models based on these biomarkers should be used cautiously in nonwhite populations.
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