Abstract
Recently, turbidimetric immunoassays for detecting and quantifying kappa and lambda free light chains (FLC) have become available and are promoted as being more sensitive than immunofixation electrophoresis (IFE) in detecting FLC monoclonal proteins. In this study, we assessed the ability of these turbidimetric assays to detect serum monoclonal proteins involving both free and heavy-chain-bound kappa and lambda light chains compared to standard immunofixation electrophoresis. Sera demonstrating a restricted band of protein migration (other than a definite M spike) by serum protein electrophoresis (SPE), which may represent early monoclonal proteins, were also examined. When compared to IFE, percent agreement, sensitivity, and specificity for the kappa-FLC and lambda-FLC were 94.6, 72.9, and 99.5% and 98.5, 91.4, and 99.7%, respectively, in detecting monoclonal proteins involving free and heavy-chain-bound light chains. The majority of sera (73.7%) demonstrating a restricted band of protein migration on SPE demonstrated abnormal IFE patterns suggestive of multiple myeloma or monoclonal gammopathy of unknown significance, but gave normal kappa/lambda FLC ratios using the turbidimetric immunoassays. In conclusion, the kappa and lambda FLC assays are significantly less sensitive (72.9 to 91.4%) than IFE, but specific in detecting serum monoclonal proteins. Moreover, the kappa/lambda ratio has little value in routine screening since the majority of sera with abnormal IFE patterns had normal kappa/lambda FLC ratios.