Six sigma and calculated laboratory tests

New quality assessment (QA) systems such as Six Sigma have become more popular because they offer a different approach to problems in the clinical laboratory. Laboratory QA programs, however, deal only with measured tests, and thus QA procedures are not applied for most calculated tests, such as measurement of LDL, total globulins, unconjugated bilirubin, creatinine clearance, urea reduction ratio, corrected total calcium, international normalized ratio, plasma osmolality, and anion gap, for which the test results are not measured directly but are calculated based on relationships between measured values (1).