A comparison of seven prothrombin time reagents--development of an evaluation strategy.

The control of p.o. anticoagulant therapy by the use of prothrombin times or ratios is one of the major functions of a clinical coagulation laboratory. European prothrombin reagents tend to be more sensitive to the effects of p.o. anticoagulants than traditional North American rabbit brain thromboplastin reagents. The clinical importance of this in vitro observation has been emphasized by the results of two recent clinical trials which both suggested that the less intense anticoagulation therapy that results from the use of a European-type prothrombin reagent may be safer. To minimize the clinical effects of differences in prothrombin time reagent formulation, a global correction factor has been developed--the international normalized ratio (INR). Despite this advance in international standardization, individual laboratories need a rapid and efficient method to help select the most appropriate prothrombin reagent for the clinical management of their local patient population. This study has examined the response of a variety of commercially available prothrombin time reagents to normal plasmas and those from anticoagulated patients. Using both classical sampling theory and multiple range testing, it was demonstrated that as few as 20 estimations on normal plasmas and 50 on anticoagulated specimens will permit a clinical laboratory to select a suitable prothrombin time reagent for anticoagulant control of their specific local patient population.