Serum free light chains: an alternative to the urine Bence Jones proteins screening test for monoclonal gammopathies

BACKGROUND: Retrospective analyses have established the role of quantitative serum free light chains (FLCs) in the diagnosis of monoclonal light chain disorders. The aims of this study were to assess (a) whether the addition of serum FLCs to serum protein electrophoresis (SPEP) identified additional patients with monoclonal gammopathies; (b) whether serum FLC measurements could replace urinalysis for Bence Jones protein (BJP); and (c) the cost/quality implications of routinely measuring serum FLCs. METHODS: Serum FLCs were added to consecutive requests for SPEP from August to November 2004 and measured by automated immunoassay. RESULTS: Seventy-one of 923 patients had abnormal serum FLC ratios. Seven patients with monoclonal gammopathies and 1 patient with malignant lymphoma (but no monoclonal band) were detected among 43 patients with negative SPEP but positive serum FLC ratios. Thirty-five patients with negative SPEP had false-positive serum FLC ratios. The false-positive rate for a ratio >1.65 was higher than previously described and associated with polyclonal increases in immunoglobulins and renal impairment. Serum FLC ratios were normal in 2 of 13 patients with low-level persistent urine BJP. However, no significant pathology would have been missed by replacing BJP with serum FLCs. Revenue and manpower savings offset 60% of the costs of serum FLCs. CONCLUSIONS: Additional diagnostic information is gained by adding serum FLCs to SPEP as first-line tests for investigating possible B-cell disorders. The quality of the diagnostic service is enhanced by more confident exclusion of light chain disorders and improved interpretive assessment of SPEP and immunofixation electrophoresis.