The relative contributions of antithrombin iii during heparin treatment, and of clinically recognisable risk factors, to early recurrence of venous thromboembolism.

The influence of antithrombin III (ATIII) level and ATIII activity, measured during intravenous heparin treatment for venous thromboembolism (VTE), on 'heparin requirement' (the heparin dose required to prolong the activated partial thromboplastin time (APTT) into its designated therapeutic range), and on the likelihood of recurrent VTE during the first month of anticoagulant therapy, were examined in a prospective study of 232 patients with VTE treated according to a standard protocol. 15 patients with recurrent VTE (6.5%) had a lower mean APTT during heparin treatment than patients without recurrence; a finding due partly to their heparin requirement. However, there was no measurable relationship between ATIII level or ATIII activity and either heparin requirement or recurrence of VTE. By contrast, both the presence of disseminated malignancy and the development of heparin induced thrombocytopenia were powerful, clinically recognisable, risk factors for recurrence during or soon after heparin therapy.