Assessment of Serum Free Light Chain Assay for Screening for Plasma Cell Disorders.

Introduction and Methods: The purpose of this study was to evaluate the serum free light chain (FLC) assay in its ability to improve performance of protocols designed to screen for plasma cell disorders. We measured M-protein levels using serum protein electrophoresis (SPEP) in 312 consecutive patients being screened for plasma cell disorders at the Veterans Administration Medical Center - Puget Sound. The serum kappa and lambda free light chain levels were quantitated using the serum FLC assay in these same patients. The kappa/lambda ratio was calculated using the free kappa and free lambda results from the serum FLC assay.

Results: SPEP results indicated the presence of a possible monoclonal gammopathy in 77 of the 312 patients in this study. In this group of 77 patients, a plasma cell disorder was diagnosed in 27 of them. The serum FLC assay showed an abnormal kappa/lambda ratio in 20 of these 77 patients, all 20 of whom were diagnosed with multiple myeloma. In the group of 235 patients with normal SPEP results, 17 were found to have an abnormal kappa/lambda ratio. Of these 17 patients, 15 were diagnosed with multiple myeloma, one with lymphoma, and one with bladder cancer.

Conclusions: Because a number of disorders and diseases can increase production of immunoglobulins, there were a significant number of false positives in the SPEP results. At the same time, there were also several false negative SPEP results. The number of both false positives and false negatives was smaller for the serum FLC assay. Further, use of SPEP and the serum FLC assay together resulted in significantly improved sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV). (See Table 1.) These results indicate an important role for the serum FLC assay in screening for monoclonal gammopathies.