[Usefulness of serum free light-chain assay for evaluation of therapeutic efficacy in non-secretory multiple myeloma].

: Non-secretory multiple myeloma (NSM) is characterized by the absence of detectable monoclonal proteins in serum and urine using an immunofixation assay; therefore, invasive bone marrow examinations are required for monitoring the disease activity. Recent studies reported that abnormal κ/λ ratios or elevated involved free light-chains were detected by the free light-chain (FLC) assay in 70∼80% of patients with NSM. Here we report a case of a 59-year-old male with NSM, in which the FLC assay was useful in monitoring the response to treatment. He was diagnosed with NSM and showed abnormal serum FLC. Serum kappa free light-chain levels were reduced to normal ranges immediately following the initial treatment and a complete response was confirmed by a bone marrow examination. During the clinical course, we observed a good correlation between serum FLC levels and the findings of the bone marrow examination. The FLC assay is noninvasive and has the potential to be useful for monitoring the disease activity of non-secretory multiple myeloma.