Oral anticoagulant therapy is usually monitored by the one-stage prothrombin time (PT) (Quick-time). The PT result can be expressed as time in seconds, as a ratio (patient time in seconds/control time in seconds), as an index (which is control time/ patient time multiplied by 100), or as a percent of normal control plasma using a saline dilution curve. The guidelines for the optimal therapeutic range for the control of anticoagulant therapy have been debated for 40 years, and the recommendations have differed widely between experts. As a consequence, patients in North America are generally treated with higher doses of oral anticoagulants (and, therefore, exposed to a higher risk of bleeding) than their counterparts in the United Kingdom.
The main reason why patients in North America are treated with higher doses of oral anticoagulants is largely historic and related to the marked differences in sensitivity of the various PT reagents (thromboplastins)
Hirsh J, Levine M. Therapeutic Range for the Control of Oral Anticoagulant Therapy. Arch Neurol. 1986;43(11):1162–1164. doi:10.1001/archneur.1986.00520110052014
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