Warfarin is a very effective antithrombotic drug, but its dosage must be monitored carefully because its metabolic clearance varies among patients and the risk of clinically important bleeding rises sharply when the upper limit of the effective range is exceeded.
Warfarin therapy is usually monitored by the prothrombin time (PT).1 The PT is sensitive to three of the four vitamin K-dependent procoagulant clotting factors and is obtained by adding a mixture of calcium and thromboplastin to citrated plasma. The term thromboplastin describes a phospholipid-protein extract of tissues, usually lung, brain, or placenta, that contains both the tissue factor and the phospholipid necessary to promote the activation of factor X by factor VII.1,2 The PT is a simple test, but its interpretation can be problematic because thromboplastins vary markedly in their responsiveness to the anticoagulant effects of warfarin depending on their source and method of preparation.3-9 As a
Hirsh J. Substandard Monitoring of Warfarin in North AmericaTime for Change. Arch Intern Med. 1992;152(2):257–258. doi:10.1001/archinte.1992.00400140011002