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Detection of and Light Chain Monoclonal Proteins in Human Serum: Automated Immunoassay versus Immunofixation Electrophoresis

Associated Regional and University Pathologists Institute for Clinical and Experimental Pathology,¹ Departments of Pathology,² Pediatrics and Medicine, University of Utah School of Medicine, Salt Lake City, Utah 84108³

Received 28 September 2005/ Returned for modification 14 November 2005/ Accepted 7 December 2005

Recently, turbidimetric immunoassays for detecting and quantifying and 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 and 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 -FLC and -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 / FLC ratios using the turbidimetric immunoassays. In conclusion, the and FLC assays are significantly less sensitive (72.9 to 91.4%) than IFE, but specific in detecting serum monoclonal proteins. Moreover, the / ratio has little value in routine screening since the majority of sera with abnormal IFE patterns had normal / FLC ratios.