From the Guidelines
For patients with suspected plasma cell disorders, I recommend obtaining a 24-hour urine collection, serum protein electrophoresis (SPEP), and free light chain (FLC) assay at initial evaluation. These tests are essential for diagnosing and monitoring monoclonal gammopathies such as multiple myeloma, MGUS, and amyloidosis 1. The 24-hour urine collection measures the amount of protein excreted over a full day, particularly Bence Jones proteins (free light chains) that may not be detected in serum. The serum protein electrophoresis identifies monoclonal proteins (M-proteins) in the blood, while the serum free light chain assay measures kappa and lambda light chains and their ratio, which is crucial for detecting light chain-only disorders.
Some key points to consider when interpreting these tests include:
- The serum FLC assay cannot replace the 24-hour urine protein electrophoresis (UPEP) for monitoring patients with measurable urinary M-protein 1.
- The FLC ratio is required for documenting stringent complete response (CR) according to the International Myeloma Working Group (IMWG) Uniform Response Criteria 1.
- Renal function can affect the serum FLC assay, and this should be taken into account when interpreting results 1.
- After the M-protein is quantified, it is essential to use the same test for serial studies to ensure accurate relative quantification 1.
For monitoring established disease, these tests should be repeated at intervals determined by disease status - typically every 1-3 months for active disease and every 3-6 months for stable disease. The combination provides complementary information about tumor burden, disease activity, and response to therapy, as some patients may show changes in one parameter before others during disease progression or treatment response.
From the Research
24-Hour Urine and Monoclonal Protein and FLC
- The 24-hour urine test is used to measure the amount of monoclonal protein in the urine, which can be an indicator of multiple myeloma 2.
- Monoclonal protein can be measured in both serum and urine, and the serum free light chain (FLC) assay is a useful tool for monitoring multiple myeloma 3, 4, 5.
- The FLC assay measures the levels of free kappa and lambda immunoglobulin light chains in the serum, which can be abnormal in patients with multiple myeloma 3, 6.
- Studies have shown that the FLC assay is more sensitive than urine electrophoresis for monitoring multiple myeloma, especially in patients with light chain multiple myeloma 4, 5.
- The International Myeloma Working Group guidelines recommend using the FLC assay in combination with serum protein electrophoresis and immunofixation for screening and monitoring multiple myeloma 6.
- However, there are no data to support using the FLC assay in place of 24-hour urine protein electrophoresis for monitoring or for serial measurements in patients with measurable disease by serum or urine protein electrophoresis 6.
Comparison of Serum FLC and Urine Electrophoresis
- A study compared the use of serum FLC measurements and urine electrophoresis for detecting the light chain component of monoclonal immunoglobulins in light chain and intact immunoglobulin multiple myeloma 4.
- The study found that the serum FLC test provided greater sensitivity than urine electrophoresis for monitoring multiple myeloma, especially in patients with light chain multiple myeloma 4.
- Another study found that the measurement of kappa/lambda ratio was more sensitive than serum protein electrophoresis and immunofixation for monitoring multiple myeloma 5.
Clinical Utility of FLC Assay
- The FLC assay has been shown to be a useful tool for monitoring multiple myeloma, especially in patients with oligosecretory plasma cell disorders 6.
- The assay can provide a rapid indication of response to treatment, and changes in FLC concentrations can be used to monitor disease progression 3, 4, 5.
- The FLC assay can also be used to monitor patients with light chain amyloidosis, oligosecretory myeloma, and non-secretory myeloma 6.