Performance Evaluation of a Coagulation Laboratory Using Sigma Metrics and Quality Goal Index

Sigma methodology is a statistical calculation and quality management tool that provides information about process performance. If clinical laboratories start using sigma metrics to monitor their performance, they can more easily identify gaps in their performance, thereby improve their performance and patient safety. This study aimed to calculate sigma metrics and quality target index values by using internal quality control data from coagulation tests, thus evaluating the analytical performance.

Sigma levels were calculated using the formula: [total allowable error (TEa) - bias]/coefficient of variation (CV). Sigma values ≥ 6, between 3 and 6, and < 3 were classified as "world-class", "good" or "unacceptable", respectively. A biological variation database (BVD) was used for TEa. The quality goal index (QGI) is the reason behind a low sigma value, that is, lower precision, lower accuracy, or both due to the combination. QGI was calcu-lated using the formula QGI = bias/1.5 x % CV. With a QGI value of < 0.8, the measurement indicates that the accuracy of the procedure needs to be improved; QGI values > 1.2 indicate accuracy needs to be improved and values 0.8 ≤ QGI ≤ 1.2 indicate both precision and accuracy need to be improved.

Sigma and QGI of three-monthly two-level internal quality control values were calculated by using the laboratory automation system. In the prothrombin time (PT) and activated partial thromboplastin time (APTT) tests of the coagulation parameters studied, sigma values were found to be < 3 in both levels. When the QGI value was calculated, it was PT 0.45 and APTT 0.90 for level 1 and PT 0.16 and APTT 0.6 for level 2, respectively.

It was decided that sigma values of coagulation parameters at "low quality" levels and improvement studies should be carried out for coagulation parameters in our laboratory. By evaluating sigma levels, it is possible to identify tests with a high probability of failure, and these tests should undergo strict quality control inspections. In clinical biochemistry laboratories, appropriate quality control planning should be performed for each test by using the Six Sigma methodology and by calculating the quality target index.