Application of serum pools in insulin harmonization: Commutability and stability.

Background: Recalibration using serum pools assigned by higher-order reference methods had been demonstrated to be effective in improving the agreement among insulin immunoassays. To promote the application of serum pools in insulin harmonization, this study analyzed serum pools' commutability between insulin immunoassays, and their short- and long-term stability at different temperatures. The agreement between commonly used immunoassays was also evaluated.

Methods: Insulin in 69 individual serum samples, 10 serum pools, and three EQA samples (lyophilized powder of serum pools) were detected by six widely used immunoassays. The commutability of serum pools and EQA samples was evaluated according to the IFCC-recommended approach. Serum pools' stability at different temperatures was investigated by placing them at various temperatures for varying lengths of time. Individual serum samples' results were analyzed using the Bland-Altman and Passing and Bablok regression analyses.

Results: Serum pools were commutable among most assays, the EQA samples-lyophilized serum pools-were non-commutable among most assays. Serum pools can be stably stored at -20°C and -80°C for at least one year, but can only be stably stored at room temperature for twenty-four hours. Significant relative differences were observed among assays. Recalibration using serum pools can only improve the assays' agreement at middle and high insulin levels, but not at low levels.

Conclusions: Serum pools were commutable and stable for insulin measurement and can be used in insulin harmonization. The existing EQA materials were non-commutable between most assays, and other EQA materials, such as serum pools, should be studied.