A number of studies and EQA schemes have compared the absolute values returned by the Freelite and N Latex FLC assays (Section 39.3) [199][922][201][175][203][209][210][211][212][921][926][957]. All have reached similar conclusions: the two assays do not compare well and are not interchangeable. For example, a well-designed study by Lock et al. [201] compared quantitative results of Freelite and N Latex FLC assay for 327 serum samples submitted for analysis to four routine diagnostic laboratories in the UK; a total of 79% were from patients with known monoclonal gammopathies. Comparing the results produced by the two assays, standard linear regression gave r2 values of 0.86 and 0.71 for κ and λ sFLCs, respectively. This level of agreement is well below the requirements of the Clinical Laboratory Standards Institute (which requires an r2 ≥0.95 to establish that two assays are equivalent) [213]. Bland-Altman plots identified 17/327 (5.2%) samples that had the most discrepant results (Figure 8.8). This included 14 patients whose sera contained clonal FLC that were “poorly detected” by the N Latex FLC but not the Freelite assays [201]. Of most concern was the fact that 2 of the 14 patients had LCMM (prior to treatment) but their N Latex FLC assay results indicated normal sFLC ratios while their Freelite assay results were clearly abnormal (Table 8.8). These 2 patients and other examples of missed diagnoses are discussed further in Section 8.5.6 below. Whilst some comparison studies have been limited to small numbers of patients with monoclonal gammopathy [208] or a lack of clinical information [200] all have reported a similar poor concordance of Freelite and N Latex FLC assay results. This lack of agreement can be clinically important and is discussed further in Section 8.5 below.