The efficacy of long-term oral anticoagulant therapy and its laboratory assessment.

The activated partial thromboplastin time is compared with the corresponding prothrombin ratio in 6378 samples of platelet-poor plasma from 446 patients treated for a total of more than 4500 patient/months with oral anticoagulatnts. A relative decrease in the activated partial thromboplastin time following deep vein thrombosis is described, which tends to become less obvious during the first year of treatment and is greater in older patients. Although this relative decrease is also found in patients treated after cerebrovascular accidents, it is not found in patients treated after myocardial infarction or in patients with mitral valve disease treated prophylactically with long-term oral anticoagulants. It is though possible that these changes following deep vein thrombosis might be useful in helping to determine the duration of oral anticoagulant treatment.