Field evaluation of Spaplus system for the determination of free light chains (FLC)in serum

Background: Measurements of serum immunoglobulin k and λ FLC and FLC ratio calculation are recommended for the evaluation of plasma cell disorders. Several practical issues for the analytical measurement of FLC have, however, been identified. Searching for a solution able to fulfil the performance goals for the effective use of FLC in clinical setting, we evaluated the suitability of SPAplus analyzer using Freelite reagents (both from The Binding Site) for FLC determination. Particularly, we compared the system performance with allowable goals for bias, imprecision (CV) and total error (TE) derived from biologic variation of FLC. Methods: We evaluated the performance of SPAplus FLC using data collected during a six-month time period of routine use,employing two different reagent lots. The two-level (N and H) liquid SPAplus control material was used for bias estimate by comparing the obtained long-term experimental means (n=34, both levels) with the corresponding assigned values. The protocol for CV evaluation employed the liquid-frozen Bio-Rad Liquichek Unassayed Chemistry Control, measured in each performed run for a total of 29 runs. Inaccuracy was checked by results from three UK NEQAS exercises (system-specific (SPAplus) consensus value as reference). Goals (desirable/minimum quality levels) for bias, CV and TE were ±4.1%/6.1%, <4.0%/6.0% and ±10.7%/16.1% for κFLC and ±7.1%/10.6%, <3.5%/5.3% and ±12.9%/19.3% for λFLC,respectively. In addition, CV and TE for FLC ratio should be <2.3%/3.4% and ±7.7%/11.6%. Results: Average cumulative bias was -6.0% (control N) and - 6.2% (control H) for κFLC, and 4.3% (N) and 6.1% (H) for λFLC, respectively. Overall CV resulted in 10.8% for κFLC (mean 11.9 mg/L), 7.3% for λFLC (mean 13.6mg/L) and 8.8% for FLC ratio (mean 0.9). On EQAS evaluation all λFLC and two out of 3 results for κFLC were within the minimum allowable TE, while the FLC ratio achieved the minimum goal only in one exercise. Conclusions: Considering our previous experience with other analytical systems, the SPAplus solution undoubtedly represents a significant step forward. A further improvement in measurement imprecision (priority) and method alignment is probably needed to fulfil the stringent analytical goals derived from biologic variation.