Agreement of Owren and Quick Prothrombin Times: Effects of Citrate and Calcium Concentrations and International Sensitivity Index Correction

Abstract

The prothrombin time (PT) is commonly measured by either the “Quick method”, which is based on the technique described by Quick and co-workers in 1935 (1)(2), or by the Owren method (3) (combined thromboplastin reagent). The latter is the predominant approach used in the Nordic countries, Benelux, and Japan. These two methods are rarely mentioned or noted in the scientific literature (4)(5), and more attention is paid to the thromboplastin in the reagent and to its origin (6)(7). The Owren method is sensitive to coagulation factors II, VII, and X. Fibrinogen (factor I) and factor V are in the reagent. The Quick method is affected by deficiencies of fibrinogen and factors II, V, VII, and X. Both methods are suitable for the control of anticoagulant treatment. The most important technical difference between the two is the sample volume in the reaction mixture: 5% in the Owren method, and 33% in the Quick method. This has the effect of making the coagulation reaction in the Quick method more sensitive to many preanalytical variables, e.g., citrate concentration in the sample (8)(9). The small sample volume in the reaction mixture with the Owren method makes it possible to use even EDTA plasma (10). The International Normalized Ratio (INR) is recommended to harmonize PT results for oral anticoagulant therapy (11). The recommendation conforms with those made by WHO using reference thromboplastins (12). Regardless of the reagent, instrument, or method used, the results should be the same when the INR system is used. Because these two major PT methods have not previously been compared, the aim of this study was to compare the PT results obtained with two Owren reagents as INR units in samples containing 0.129 mol/L (3.8%) citrate solution and to compare the …