Sigma Metrics as a Measure to Evaluate Internal Quality Control Performance in a Clinical Chemistry Laboratory

To determine the sigma metrics of biochemical parameters in a clinical chemistry laboratory and to evaluate their individual performance on the sigma scale using the quality goal index (QGI) ratio.

Retrospective cross-sectional study. Place and Duration of the Study: Department of Pathology, Shalamar Medical and Dental College, Lahore, Pakistan, from October 2023 to September 2024.

After ethical approval from the Institutional Review Board, data for 20 biochemical parameters enrolled in the proficiency testing programme were collected. Data were obtained for the internal quality control coefficient of variation percent (%CV) and the external quality assurance scheme (EQAS)-%bias for included parameters. Sigma values were calculated by using the formula (TAE-Bias) / CV. After the calculation of sigma values, the QGI ratio was utilised to analyse the cause of low sigma values for particular analytes.

Out of 20 biochemical parameters, both levels of uric acid, bilirubin, alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), triglycerides (TGs), high-density lipoprotein (HDL), potassium, creatine phosphokinase (CPK), and level 2 glucose, urea, albumin, calcium, magnesium, and phosphate showed world-class performance with sigma values ≥6. Level 1 calcium, magnesium, sodium, as well as level 2 total protein and total cholesterol showed excellent performance with sigma values 5. Unacceptable performance was shown by level 1 of urea, creatinine, albumin, total protein, and total cholesterol with sigma values <3. The QGI ratio calculated for the evaluation of the problems with sigma score ≤3 showed that low sigma value of level 1 glucose, urea, creatinine, and total proteins were due to inaccuracy; that of level 1 total cholesterol was due to both imprecision and inaccuracy, while that oxsf level 1 phosphorous was due to imprecision.

World-class performance on the basis of sigma values were observed for uric acid, total bilirubin, ALT, AST, ALP, TGs, HDL, CPK, and potassium, while certain parameters of level 1, such as urea, creatinine, total protein, albumin, and total cholesterol, showed unacceptable performance. Sigma metric analysis provides a standard for improving assay performance and optimising quality control (QC) operations in the clinical chemistry laboratory.

Quality control, Internal quality control, Assay performance, Sigma metric, Six sigma, Sigma score, Internal quality control performance.