Introduction: Multiple myeloma (MM) is characterized by plasmacelullar proliferation and accumulation, organ impairment and production of monoclonal immunoglobulin. Plasma cells also produce free light chains, not bound in immunoglobulin molecule. Aims: The aim of the study is the evaluation of serum free light chain (FLC) levels and their ratio (κ/λ ratio) between individuals with monoclonal gammopathy of undetermined significance (MGUS) and MM patients, and also between active and stable stage of multiple myeloma. Methods: 130 MM patients were enrolled in the study, 31 patients were examined in time of diagnosis, 99 during the course of the disease, 72 patients were in active stage of the disease, and 58 patients had stable disease. 52 MGUS individuals were examined also. Serum FLC levels were assessed by FreeLiteTM Immunotech system, U-test according to Mann-Whitney was used for statistical evaluation. Kappa or lambda secretion was evaluated separately. Results: Comparing serum FLC levels in MM patients and individuals with MGUS, in kappa group were assessed significantly higher levels of dominant kappa chain (p = 0.001), lower values of alternative chain lambda (p = 0.033) and higher κ/λ ratio (p = 0.0004) in patients with MM. Also in lambda group there were detected differences in levels of dominant light chain lambda (p = 0.029), κ/λ ratio (p = 0,003) and in suppression of alternative light chain kappa (p = 0.032). Matching active and stable phase of MM were detected significant higher levels of dominant chain kappa (p = 0.0001), κ/λ ratio (p = 0.001) and suppressed values of alternative chain lambda (p = 0.001) in patients with active disease in kappa group. In lambda active MM group were assessed differences in dominant chain lambda (p = 0.011) and also in κ/λ ratio values (p = 0.004). Conclusions: The above study confirmed differences in serum FLC levels and κ/λ ratio between MGUS individuals and MM patients, but also among active and stable stage of multiple myeloma. κ/λ ratio seems to be more sensitive indicator than serum FLC levels themselves.

        Key words: multiple myeloma, monoclonal gammopathy of undetermined significance, immunoglobulin free light chain.