Ratio of urinary free immunoglobulin light chain kappa to lambda in the diagnosis of Bence Jones proteinuria.

The aim of this study was to evaluate the diagnostic efficacy of the ratio of urinary free light chain (FLC) kappa to lambda (kappa/lambda ratio) for the detection of Bence Jones protein (BJP). Urine specimens were collected from 243 patients suspected of having BJP. Immunofixation identified 59 BJP-positive specimens among them. The kappa/lambda ratios of all specimens were determined by FLC immunoassays and then the cut-offs for the kappa/lambda ratio were defined as 5.5 for BJP K and 0.1 for BJP lambda by ROC curve analyses. Using the cut-offs, we detected abnormal kappa/lambda ratios in 51 (86%) of the 59 BJP-positives and 11 (6%) of the 184 BJP-negatives identified by the results of immunofixation. High-resolution urinary protein electrophoresis (UPE), a sensitive method for BJP screening, showed almost equal sensitivity to the kappa/lambda ratio, detecting monoclonal band(s) in 52 (88%) of the 59 BJP-positives. However, in UPE analysis these positive specimens should be followed by redundant immunofixation analysis to determine the isotypes. We further evaluated the combination method of FLC assays with UPE that correctly diagnosed 82% of the specimens as positive or negative for BJP, with only two false-negative results. These results suggest that quantitative FLC immunoassays provide an alternative or complementary method for the detection of BJP.