On the value of the activated partial thrombo-plastin time (aptt) in monitoring heparin therapy.

Of 1068 blood samples referred for evaluation of the haemostatic mechanism 135 contained heparin as judged by the polybrene titration method. Of the 57 samples with a heparin concentration in the defined therapeutic range, 65 per cent had APTT values in the "therapeutic range". Of the samples from patients who had not received any anticoagulant, 213 (24 per cent) had prolonged APTT, and 48 of these samples had APTT values in the "therapeutic range" for heparin therapy. It is suggested that the APTT test is of limited value in monitoring heparin therapy.