The Quick and Owren prothrombin time assays: characteristics and performance in clinical routine

The prothrombin time assay was developed by Armand Quick and is used to monitor patients on vitamin K antagonist (VKA) treatment. Assay modifications were later made by Paul Arnor Owren. The differences between the Quick and Owren methods in assay reagents, protocols, calibration procedures, and factor sensitivities affect several aspects of assay performance, such as inter-laboratory variation, assay interference (of, e.g. DOACS), lupus anticoagulant, and preanalytical factors. In this review, several of these aspects are elucidated and compared between the methods. Most importantly, method-specific components-such as the final dilution of the patient sample, phospholipid origin, whether thromboplastin was used for activation or added later, and different coagulation factors affecting clotting time-are all of importance when it comes to the clinical usefulness of these methods. The Owren method has several advantages over the Quick method. First, with the Owren method, it is possible to use a simple calibration procedure, resulting in smaller differences between reagents and less instrument-dependent results in inter-laboratory comparisons. It is less sensitive to interference from DOACs and lupus anticoagulants, and less dependent on common preanalytic sample handling errors such as underfilling small-volume tubes, tube additives, and time and temperature for sample storage. Differences observed between the two methods are, in our opinion, clinically relevant, thus making awareness of method-specific limitations especially important.