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As the measuring ranges of Freelite and N Latex FLC assays are different, samples used in precision studies must be suitably selected to ensure a fair comparison. This was illustrated in a precision study by Pretorius et al. [175] in which the samples with the lowest concentrations were well within the standard N Latex FLC measuring range (above the second calibrator point), but at or below the standard Freelite measuring range. Unsurprisingly, the total %CV of the lowest sample for the Freelite assays was >20%, but <7% for N Latex FLC (Figure 8.3.)

A better-designed precision study by Lock et al. [201] used samples that either fell within the reference range or were moderately elevated by both FLC assays. The authors concluded that the within-batch and between-batch precision of both Freelite and N Latex FLC assays was acceptable (Table 8.3). A report by Sharrod-Cole et al. [202] reached similar conclusions. Finally, Carr-Smith et al. [921] used UK NEQAS external quality control data to compare the inter-laboratory variation of Freelite assays on the SPAPLUS® with that of N Latex FLC assays on the Siemens BN™II. The authors conclude that the performance of Freelite was at least equivalent, and often superior, to N Latex FLC for the majority of distributions (Section 39.3).

Maintaining batch-to-batch consistency is an essential requirement for any FLC assay, which may be used to monitor patients over the course of many years. Freelite polyclonal antisera are produced using standardised manufacturing procedures that include careful blending to achieve balanced, reproducible recognition of FLC molecules, with good between-batch consistency. This is further discussed in Section 5.5.

Assay Precision fluid Freelite (%CV) N Latex FLC (%CV)
Within-batch precisionκ sFLCNormal 9.8 2.2
λ sFLCNormal 7.6 1.8
Between-batch precisionκ sFLCNormal 7.1 5.1
Elevated 5.8 6.4
λ sFLCNormal 7.0 3.6
Elevated 6.4 5.6

Table 8.3. Precision of Freelite and N Latex FLC assays [201]. Reproducibility data based on 10 replicates, except N Latex FLC between-batch which was based on 3 replicates.

A theoretical advantage of monoclonal over polyclonal antibodies is reduced variation between reagent lots. Pretorius et al. [175] assessed variance between two N Latex FLC reagent lots, calibrated with the same material. Whilst the κ N Latex FLC assay demonstrated negligible bias (-1.7%), the λ N Latex FLC assay demonstrated significant bias (-5.4%). For both κ and λ N Latex FLC assays the difference in values for a number of samples was >20% (Figure 8.4). This included one notable extreme value which showed a >80% difference in λ sFLC concentration between reagent lots (Figure 8.4B). This indicates that use of monoclonal antibodies does not eliminate between-batch variation.

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