The Urine Free Light Chain assay measures the levels of free kappa (κ) and lambda (λ) immunoglobulin light chains excreted in urine. Normally, small amounts of light chains pass into urine, but elevated levels can indicate abnormal production by plasma cells, as seen in multiple myeloma, light chain amyloidosis, or other monoclonal gammopathies.

The kappa/lambda ratio in urine helps distinguish monoclonal (single clone) light chain production from polyclonal (reactive) increases due to other conditions. Urine testing is especially useful in detecting light chains that are filtered by the kidneys and may not be apparent in blood tests.