Evaluation of the Optilite (TM) system for the determination of immunoglobulin free light chains in serum (sFLC)

The measurement of sFLC kappa and lambda and kappa/lambda ratio calculation is recommended for evaluation and management of plasma cell disorders. However, some analytical issues persist in their measurements, among which a too large long-term imprecision seems to be the main challenge. We evaluated the new Optilite system (The Binding Site) for sFLC determination by comparing its performance with specifications for bias, imprecision (as CV) and total error derived from biological variation of sFLC. We collected data during one year of routine use by employing three reagent lots. The system alignment was checked using the two-level (L and H) Optilite control material by comparing the obtained long-term experimental mean (n=233, both levels) with the manufacturer's assigned values. The protocol for CV evaluation employed the liquid-frozen BioRad Liquichek Control and a frozen serum pool tested for 125 and 79 runs, respectively. Inaccuracy was evaluated by results of four UK-NEQAS exercises [system-specific (Optilite) consensus value as reference]. Average cumulative bias [1.1% (L) and -2.0% (H) for sFLC kappa; 5.4% (L) and 0.1% (H) for sFLC lambda, respectively] fulfilled the desirable goals. CVs for sFLC kappa (7.1% for Liquichek and 6.6% for the pool, respectively), sFLC lambda (7.8% for both Liquichek and the pool) and kappa/lambda ratio (8.9% for Liquichek and 10.2% for the pool, respectively) failed however to reach minimum quality goals. In EQAS evaluation, all sFLC kappa and lambda and 3 out of 4 kappa/lambda ratio results were within the allowable total error. In our experience, the Optilite system shows a good method alignment suitable for sFLC interpretation using fixed cut-offs. However, the assay reproducibility is probably not suitable for optimal long-term monitoring of individual patients.