Serum beta-2-microglobulin in the differential diagnosis of monoclonal gammopathies

Serum beta-2-microglobulin (B2m) concentrations were determined in 43 southern African black patients with multiple myeloma (MM), in 130 black patients with monoclonal gammopathy of undetermined significance (MGUS) and in 70 control subjects. The results showed median values for serum B2m in patients with MM, MGUS and the control group to be 8.10 mg/l, 3.05 mg/l, and 2.35 mg/l, respectively; these values differed significantly from one another (P less than 0.01), even when patients with normal renal function (serum creatinine value less than 110 mumol/l) were considered separately. The median serum B2m concentration for IgG MM (22 cases) was 4.3 mg/l, for IgA MM (8 cases) 7.3 mg/l, and 24.2 mg/l for Bence Jones MM (12 cases). These differences were also significant (P = 0.001), but not in the restricted group of MM patients with normal renal function. In the 43 MM patients serum B2m concentrations had a significant positive correlation with serum creatinine (r = 0.706; P less than 0.005) and a significant negative correlation with haemoglobin values (r = -0.459; P = 0.006). In 28 MM patients with normal renal function, serum B2m values had a significant negative correlation with serum albumin (r = -0.602, P = 0.003). Sixty-five per cent of the 43 MM patients and 18.5% of the MGUS patients had raised serum B2m values (greater than 4.7 mg/l). An optimum cut-off value for serum B2m of 6.9 mg/l for differentiating MM from MGUS was determined using a classification rule. Despite lacking specificity, serum B2m measurement was useful in differentiating MM from MGUS, and was the best second choice variable in relation to serum albumin and haemoglobin in patients with normal renal function.