determinants of anticoagulant control in patients receiving warfarin 1 a hospitalbased drug information system has been used to assess the time for which patients treated with warfarin were outside the range of thrombotest values 510 and 515 and to examine possible contributory factors in situations where anticoagulation fell outside these ranges 2 anticoagulant control varied with the age of the patient and with concomitant drug therapy but not with patient sex or indication of anticoagulation 3 most patients were underanticoagulated at some stage but patients over 70 years spent significantly longer in the 510 range than those in the age range 3059 years and thrombotest values of less than 5 were found predominantly in the older group 4 patients given drugs known to interact with warfarin spent least time in the defined thrombotest ranges those on drugs known to potentiate warfarin effect had significantly lower thrombotest values than the other patients studied