Effect of warfarin on the activated partial thromboplastin time

Outpatients followed in an anticoagulation clinic were studied retrospectively to determine the effect of warfarin on the activated partial thromboplastin time (APTT). Twenty-nine patients were studied in part 1 of the trial to determine whether their APTT values were elevated when their prothrombin time (PT) was within 1.5 to 2.5 times the control PT. Part 2 was carried out using the data of 32 patients to determine whether a correlation existed between the degree of elevation of the patients' PT values due to warfarin and the concurrent degree of elevation of their APTT results. Baseline PT and APTT values and a minimum of three concurrent PT and APTT values determined during anticoagulation therapy with warfarin were used. Data were collected by chart review and review of Hematology Department records. Results indicated that a statistically significant difference was evident between the baseline APTT (30.79 sec) and the mean APTT (55.10 sec) when the PT was within the therapeutic range of 1.5 to 2.5 times the control while on warfarin therapy. Good linear correlation was evident (r = 0.821) between the degree of elevation of the PT and the degree of elevation of the APTT for the group. In most cases, a good linear correlation was also evident for individual patients. Routine ordering of concurrent APTT and PT tests for patients receiving warfarin therapy is not needed since the PT alone can be monitored under most circumstances.