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Analytical quality of prothrombin time and activated partial thromboplastin time using six sigma

BACKGROUND: Prothrombin time (PT) and activated partial thromboplastin time (aPTT) are global coagulation tests for diagnosis of coagulation disorders and therapy management. PT expressed as INR is used to monitor warfarin therapy whose desirable value should be 2.5 (range 2- 3). It is expected that target value should be reached in 5 days. Although widely used standardization of these tests is main deficiency. The aim of this study was to assess analytical quality of these tests for general use and specifically for monitoring warfarin therapy, and express it as six sigma value. METHODS: Reagents used for PT and aPTT were: Innovin and Actin FS respectively (Siemens Healthcare Diagnostics, Germany). Six sigma value was calculated using equation: Sigma= (TEa-bias)/CVa. A total allowable error (TEa) criterion was selected from CLIA requirements for quality control (15% for both tests). Bias was assessed from the last external quality assessment (Croatian Center for External Quality Control Assessment: CROQALM): PT=0% and aPTT=3.62%. CVa values were calculated from internal quality control measurements based on 6 month period. Obtained CVa for normal level were: PTsec:4.07%, PT%:7.06%, INR:4.69% and for pathological level: 4.90%, 5.26%, 4.71% respectively. For aPTT expressed in seconds CVa were for normal level 4.15% and for pathological level 3.39%. Additionally six sigma value was calculated using clinical requirement for warfarin therapy 50% change from basal PT value was selected as TEa. RESULTS: Six sigma for PT normal range was: PTsec: 3.7, PT%:2.1, INR: 3.2, and pathological: 3.1, 2.9 and 3.2 respectively. For aPTT six sigma was 2.7 for normal level and 3.4 for pathological level. When using clinical criterion for PT expressed as INR, six sigma was 10.7. CONCLUSIONS: When used for achieving desirable INR for warfarin therapy, INR has excellent quality performance. However, our results show that six sigma differ for both PV and aPTT between reference and pathological range, indicating that different internal quality control strategy is required for these tests at different levels of control material.

six sigma, activated partial thromboplastin time, prothrombin time

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