As demonstrated above, sFLC analysis is generally more sensitive than urine electrophoresis for indicating the presence of monoclonal FLCs. However, this advantage is dependent upon efficient renal reabsorption of FLCs (Section 3.5). The renal metabolism of filtered FLCs combined with the analytical sensitivity and specificity of sFLC analysis over conventional urine tests makes sFLC analysis superior for the diagnosis and monitoring of monoclonal gammopathies associated with FLC production.
Occasionally, patients are reported to have “positive” urine when sFLC results are normal. This may be due to a number a reasons: 1) serum and urine samples collected at different time points; 2) false positive urine results (caused by the staining of intact immunoglobulin in urine); or 3) true positive urine results (when proximal tubule reabsorption is impaired). Such discrepant results and other scenarios are further discussed in Section 7.7.