[Multiple myeloma and analytical discrepancy in serum free light chain assay: a case report].

In multiple myeloma, the measurement of serum free light chains (sFLC) is an essential biomarker for establishing the diagnosis, assessing the risk of disease progression and monitoring treatment response. However, analytical techniques are not yet standardized and results obtained with different analyzer/reagent combinations are not interchangeable. Moreover, an excess of antigen (the hook effect) may lead to an underestimation of the true concentrations. We report the case of a female patient with IgG kappa multiple myeloma, admitted to the emergency department for general deterioration following a clinical relapse. The measurement of (sFLC) performed by immunoturbidimetric method (Diazyme®) on a Cobas c311® analyzer yielded low values that were inconsistent with the clinical findings and the results of serum protein electrophoresis. Reassessment using the Optilite® analyzer with Freelite® reagents provided results consistent with the patient's clinico-biological status. This observation highlights the analytical limitations of certain assay systems and emphasizes the importance of interpreting results in the light of clinical and biological data.