The Quality of Laboratory Testing Today
An Assessment of σ Metrics for Analytic Quality Using Performance Data From Proficiency Testing Surveys and the CLIA Criteria for Acceptable Performance
James O. Westgard, Sten A. Westgard



To assess the analytic quality of laboratory testing in the United States, we obtained proficiency testing survey results from several national programs that comply with Clinical Laboratory Improvement Amendments (CLIA) regulations. We studied regulated tests (cholesterol, glucose, calcium, fibrinogen, and prothrombin time) and nonregulated tests (international normalized ratio [INR], glycohemoglobin, and prostate-specific antigen [PSA]). Quality was assessed on the σ scale with a benchmark for minimum process performance of 3 σ and a goal for world-class quality of 6 σ. Based on the CLIA criteria for acceptable performance in proficiency testing (allowable total errors [TEa]), the national quality of cholesterol testing (TEa = 10%) estimated σ values as 2.9 to 3.0; glucose (TEa = 10%), 2.9 to 3.3; calcium (TEa= 1.0 mg/dL),2.8 to 3.0; prothrombin time (TEa= 15%), 1.8; INR(TEa=20%), 2.4 to 3.5; fibrinogen (TEa= 20%), 1.8 to3.2; glycohemoglobin (TEa= 10%), 1.9 to 2.6; and PSA (TEa =10%), 1.2 to 1.8. The analytic quality of laboratory tests requires improvement in measurement performance and more intensive quality control monitoring than the CLIA minimum of 2 levels per day

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