Serum free light chain (FLC) has the following advantages Therefore, in 2006, the international standard for determining the efficacy of myeloma has adopted free light chain as one of the criteria for strict and complete remission. Serum free chain has a unique and irreplaceable value in the management of IgD, light chain, non-secretory myeloma and amyloidosis; it can be used as an indicator of renal function in these patients; serum free chain values and abnormal κ/λ ratios in these patients can be used to determine Abnormal serum free light chain values and κ/λ ratios are important criteria for determining benign and malignant plasma cell disease. The serum free light chain test and its clinical application Abstract: The monoclonal free light chain (FLC) test is an important diagnostic aid for many plasma cell diseases (e.g., multiple myeloma, primary systemic amyloidosis, monoclonal gammopathy of undetermined significance, macroglobulinemia, etc.). It is an important diagnostic and monitoring tumor marker especially in patients with multiple myeloma. Existing methods for the identification and quantification of monoclonal immunoglobulins, such as protein electrophoresis and immunofixation electrophoresis, are not sensitive to the identification and quantification of free light chains. The serum free light chain assay [1] is a recently applied method for automated quantification of free light chains in blood with high sensitivity and good specificity. Combined application of conventional M protein identification methods can improve the early diagnosis of many malignant plasma cell diseases; in monitoring, serum free light chain assay can respond to treatment and disease recurrence earlier than other indicators. It also provides very meaningful prognostic information in patients with MGUS. Monoclonal immunoglobulin free light chain (FLC), originally identified 150 years ago in the urine of patients with myeloma and defined as a periplasmic protein, is an important tumor marker, a homogeneous κ or λ free molecule produced by the uncontrolled proliferation of monoclonal malignant plasma cells. It appears in the serum and urine of patients with many malignant plasma cell diseases, including multiple myeloma, primary systemic amyloidosis (AL), primary macroglobulinemia, and light chain deposition disease. The usual qualitative and quantitative measurement of urinary FLC to determine disease status is not ideal because the concentration of urinary FLC is largely influenced by the renal tubular reabsorption capacity and does not accurately reflect the patient’s disease status. Recently kits for serum free light chain quantification have been commercialized and used in many countries, and were written into the guidelines for the diagnosis and treatment of multiple myeloma and AL in 2006. In this paper, the metabolism of FLC in normal human, serum free light chain detection and clinical application are reviewed as follows: 1. Physiology of serum free light chain metabolism Immunoglobulins are synthesized by plasma cells and are tetramers consisting of two identical heavy chains and two identical light chains, with five heavy chains γ, α, μ, δ, and ε. They determine the classification of immunoglobulins, which correspond to IgG, IgA IgM, IgD, and κ and λ light chains, and each immunoglobulin either contains κ light chain or λ light chain. Human plasma cells produce approximately twice as many κ light chains as λ light chains. The polypeptide chains of each light chain contain approximately 220 amino acids, which fold to form a constant and variable region. The yield of free light chains is about 40% more than that of heavy chains, which is for the synthesis of the proper conformation of the intact immunoglobulin molecule [6]. Immunoglobulin light chains that are not bound into the tetrameric form are secreted in the free form. These free light chains can exist as monomers (22C27 kDa) or can be covalently or non-covalently bound into dimers (44C55 kDa) . In normal humans, plasma cells synthesize immunoglobulins with a large number of FLC molecules produced and distributed intravascularly and in the interstitial space. The remaining FLC is cleared by glomerular filtration, after which FLC is taken up and broken down by proximal tubular cells. Studies have shown that a large amount of FLC is reabsorbed by the kidneys daily (10-30 g/day). Normal individuals can excrete 1-10 mg per day of free light chain into the urine, along with secretory IgA and other immunoglobulins [6]. When malignant plasma cell disease occurs, monoclonal plasma cells proliferate and produce large amounts of homogeneous monoclonal free κ or λ light chain molecules that are filtered by the glomerulus. When the filtered FLC exceeds the catabolic and reabsorptive capacity of the proximal tubule, it is excreted from the urine or reaches the ascending branches of the medullary collaterals to precipitate with Tamm-Horsfall protein in a tubular fashion, often resulting in myeloma nephropathy.