Expected long-term defect rate of analytical performance in the medical laboratory: Assured Sigma versus observed Sigma.

Reliability of laboratory results is determined by the ratio of incorrect results expected in long-term. Sigma is a measure of defect ratio, therefore long-term Sigma is a measure of the reliability of laboratory results. Commonly, long-term Sigma is estimated based on the short-term Sigma. The Six Sigma methodology assumes that in long-term performances will shift up to 1.5 Sigma, and therefore the long-term Sigma is considered 1.5 Sigma less than short-term Sigma. Analytical performance in the medical laboratory is prone to shifts larger than 1.5 Sigma. Thus, the 1.5 Sigma shift assumed in the Six Sigma is not a correct estimate in the medical laboratory. On the other hand, in the medical laboratory statistical quality control procedure (SQC) is applied to detect and correct shifts. Since SQC can be planned to trap shifts of different sizes, the threshold set for SQC determines the defect rate expected for long-term.