Has the Quick or the Owren prothrombin time method the advantage in harmonization for the International Normalized Ratio system?

Prothrombin time (PT) is the most commonly used coagulation test in health care. We sought here to compare two major PT methods (Quick and Owren) for harmonization of International Normalized Ratio (INR) results. We measured PT with an ACL 7000 analyser. We used three Owren and three Quick reagents for PT, and estimated the International Sensitivity Index (ISI) for each reagent using two local and two manufactured ISI calibrator sets. The coagulation time was measured using five different normal plasmas to assess variation for every reagent and both methods. We studied the analytical bias for every reagent and both methods at INR = 1.0 and INR = 2.5. The mean percentage coefficient of variation of the Owren reagent ISI was 2.40% and that of the Quick reagent ISI was 12.85%. The mean percentage coefficient of variation of normal plasma seconds for the Owren method was 2.54% and that for Quick was 4.02%. The absolute error at INR = 1.0 and INR = 2.5 was 0.00 and 0.04 INR for Owren, and 0.01 and 0.16 INR for Quick. The Owren PT method has the advantage over the Quick PT method in ISI calibration, normal plasma variation, within-run analytical variation and absolute error at INR = 2.5. The INR system is more demanding on analytical quality than earlier units (Ratio,%). The data would indicate that the Owren PT method has advantages over the Quick PT method in harmonization of the INR system.