When one is faced with the problem of selecting patients for anticoagulant therapy or for evaluating results of such therapy, there are 3 main points which must be considered.
We are concerned with a prophylactic and not a curative measure. Therefore, it is in fact too late to start prophylaxis when thrombosis has already occurred. The only effect which can be hoped for under such circumstances is to prevent additional, secondary thrombi, locally or at other sites.
The antithrombotic efficacy depends on: (a) the level of hypocoagulability actually obtained and (b) the stability of this level. These are critical points which are too often neglected when therapeutic results are discussed. Results of anticoagulant prophylaxis have often been poor or negative because these points have been disregarded, and anticoagulation as a therapeutic measure has been criticized when in fact the doctor in charge has been most to blame.
OWREN PA. The Results of Anticoagulant Therapy in Norway. Arch Intern Med. 1963;111(2):240-247. doi:10.1001/archinte.1963.03620260100018