Polyclonal free light chain of Ig may interfere with interpretation of monoclonal free light chain κ/λ ratio

There is controversy about whether a sensitive assay for the serum Ig free light chain (FLC) κ/λ ratio can replace urine immunofixation electrophoresis (UIFE). This report describes two untreated patients in whom monoclonal FLCs were identified in urine despite normal serum FLC κ/λ ratios. Unlike the classical serum electrophoretic patterns in multiple myeloma, both serum samples showed adequate amounts of polyclonal Ig. The most likely explanation is a masking effect by polyclonal FLC on the serum κ/λ ratio when sufficient concentrations of polyclonal FLC exist. These cases illustrate this likely effect and attest to the continued importance of UIFE for initial screening of patients for Bence-Jones protein.