Freelite sFLC assays are well established for monitoring haematological response in AL amyloidosis, and have been incorporated into a number of national and international guidelines (Section 28.6). By contrast, FLC assays from other manufacturers, including N Latex FLC assays, have not been formally validated. A study by Mahmood et al. [924] was the first to compare haematological responses assigned (using consensus criteria) with Freelite or N Latex FLC assays in 90 newly diagnosed AL amyloidosis patients. Although there was broad agreement in responses at 2, 4, and 6 months, the N Latex assay response was often earlier: 18 patients reached a PR or VGPR earlier by N Latex FLC than by Freelite. The authors highlight that patients classed as early responders by the N Latex FLC assay may get under-treated, and hence carry a risk of shorter time to disease progression.

Palladini et al. [945] reported that during follow-up, the reduction in dFLC concentration measured using either method was of prognostic significance in AL amyloidosis, although the optimal cut-off values, determined by ROC analysis were different (>50% dFLC decrease by Freelite; >33% dFLC decrease by N Latex FLC). In conclusion, Freelite and N Latex assays cannot be used interchangeably when monitoring haematological response in AL amyloidosis, and new response criteria would need to be derived and validated for the N Latex FLC assays if they are to be used in routine clinical practice.