As an alternative to urine electrophoretic tests, FLC immunoassays can be used on urine samples (Section 4.5.3). Although Van Hoeven et al. [518] reported that the sensitivity of urine FLC assays was greater than UPE for FLC measurement (75% vs. 44%; n=73), all data indicate that uIFE remains the most sensitive assay for the identification of monoclonal proteins in urine [166][519]. However, urinary FLC immunoassays do not solve any of the problems relating to renal threshold, urine collection and urine measurement indicated above. Furthermore, the diagnostic sensitivity of sFLC ratios is superior to urine FLC measurements [520]; this is a function of the proximal tubule absorption of FLCs and the much wider normal ranges for urinary FLC concentrations and the κ/λ FLC ratio (Section 6.4) [166].

The International Myeloma Working Group (IMWG) guidelines recommend sFLC assays in combination with SPE and serum IFE (sIFE) to screen for pathological monoclonal plasma cell proliferative disorders other than AL amyloidosis, which also requires a 24-hour uIFE [20]. The guidelines also state that “Measurement of urine FLC levels is not recommended” (Chapter 25).