Evaluation of cytokine analytical performance and optimization of quality control strategies using the six sigma...

The six sigma model is widely applied in laboratory quality management. This study used total allowable error (TEa) based on National Center for Clinical Laboratories (NCCL) external quality assessment (EQA) criteria and "desirable" biological variation (BV) specifications as quality goals to evaluate the analytical performance of 12 cytokines in five laboratories and develop individualized quality control (QC) strategies.

Imprecision and trueness data for 12 cytokine assays were collected from five laboratories and used to calculate sigma metrics. Assay performance was visualized using a normalized sigma method decision chart. The Westgard sigma rules run-size flowchart and quality goal index (QGI) were further applied to design customized QC protocols and identify priority areas for improvement.

Sigma metrics varied by concentration level, with higher-concentration materials generally yielding better sigma performance. For the same analyte, sigma values also differed according to the selected quality goal. Compared with "desirable" BV specifications, NCCL criteria produced lower sigma values for interleukin-6 (IL-6), whereas interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α) showed the opposite trend. These differences were clearly demonstrated by the normalized sigma method decision chart. Based on Westgard sigma rules and QGI, individualized QC strategies and targeted improvement measures were established for assays with sigma values below 6.

The six sigma model provides a practical framework for cytokine assays quality management, supporting targeted QC design and focused analytical performance improvement.