Screening for M-proteinemia: serum protein electrophoresis and free light chains compared.

Background: The objective of this study was to evaluate the efficiency of free light chain (FLC) analysis in comparison to serum protein electrophoresis (SPE) for detecting M-proteinemia. Methods: A total of 553 consecutive patients for whom evaluation of M-proteinemia was requested were included in this study. For all patients, serum FLC analysis and SPE followed by pentavalent immunofixation analysis was performed. Identification of monoclonal bands was performed using specific antisera. FLC analysis was performed using the Modular P analyzer in accordance with the manufacturer's recommendations. Local reference ranges for FLCs on this platform were established based on samples from patients with a normal electrophoretic pattern [no monoclonal bands, no hypo-or hypergammaglobulinemia, no acute phase pattern and normal kidney function, i.e., estimated glomerular filtration rate (eGFR) > 60 mL/min]. Results: Local reference ranges (95%) were established (n=243): kappa: 8.01-28.26 mg/L; lambda: 8.07-23.58 mg/L and k/lambda ratio: 0.74-1.66. Negative and positive predictive values were 98.6% and 49.5%, respectively, for screening for M-proteinemia by SPE alone, 94.3% and 21.7% for FLC concentration and 95.1% and 21.4% for FLC with the k/lambda-ratio included. Combining protein electrophoresis and FLCs resulted in a negative predictive value of 99.0% and a positive predictive value of 23.4%. Conclusions: Serum FLC analysis alone is not suitable for screening for M-proteinemia. Clin Chem Lab Med 2009;47:1507-11.