Clinical usefulness of free light chain concentration as a tumor marker in multiple myeloma.
Monoclonal immunoglobulin, as a marker for monoclonal gammopathy, is evaluated by protein electrophoresis (PEP) and immunofixation electrophoresis (IFE). However, PEP and IFE are not satisfactory in sensitivity, objectivity, and facility. Recently, a highly sensitive, automated immunoassay for measurement of free light chain (FLC) concentrations in serum and urine has been developed for the identification and monitoring of patients with monoclonal gammopathy. To explore the clinical usefulness of measurement of FLC concentrations, we measured the kappa and lambda FLC concentrations and calculated the kappa/lambda FLC ratios for three groups [multiple myeloma (MM), other diseases, and control] and compared the results of the FLC assay with the results of PEP or IFE. The concentrations of serum kappa and lambda FLCs and the kappa/lambda FLC ratios for the MM group and non-MM groups were distinct. In the MM group, some sera and urine samples had no evidence of M protein on PEP and IFE, but FLC assay showed abnormal concentrations of FLCs and abnormal kappa/lambda FLC ratios in most cases. As compared with the PEP, the kappa/lambda FLC ratio revealed higher sensitivity in all diagnostic ranges with different cutoff values. Particularly, when the cutoff value 2.0 for kappa/lambda FLC ratio was used, specificity and positive predictive value were largely improved than when the cutoff values 1.2 and 1.5 were used. These findings indicated that FLC assay enables to detect myeloma patients with very low M protein due to early stage or after therapy and to distinguish patients with monoclonal increase of FLC from patients with polyclonal increase of FLC due to other conditions, particularly using kappa/lambda FLC ratio 0.3-2.0 as a diagnostic range. Despite some technical limitations of the assay, the incorporation of kappa/lambda FLC ratios with FLC concentrations is useful in the detection of M protein, particularly with negative serum or urine IFE results, and differentiation of monoclonal gammopathies from patients with polyclonal increase in FLC due to other conditions.