Antithrombin III Activity, Measured with a Chromogenic Substrate, in Patients with Hepatic Cirrhosis, with Prosthetic Heart Valves and During Parenteral Administration of Medroxyprogesterone Acetate as a Contraceptive Agent
Antithrombin III (AT III) activity was assayed on heat defibrinated plasma using a synthetic chromogenic substrate, benzoyl-Phe-Val-Arg-p. nitroanilide. AT III activity in 30 normal subjects averaged 95% ± 18 (± 1 SD). As AT III is synthesized in the liver, we measured its activity in 72 samples from 44 patients with hepatic cirrhosis. The mean activity (49% ± 23) was significantly lower than in the control group. AT III activity was also measured in 32 patients with prosthetic heart valves receiving sodium warfarin therapy. The mean activity in this group (90% ± 23) fell in the normal range. It also did not differ significantly from the mean activity (95% ± 29) in a similar (matched for sex and age) group of 32 subjects, also treated with sodium warfarin but for other reasons than bearing a prosthetic heart valve. As low AT III levels have been reported in women using a combined oral contraceptive, we also measured AT III in 19 women receiving trimonthly injections of 150 mg of medroxyprogesterone acetate as contraception. AT III levels in this group of women (102% ± 23) were found to be normal. So the oestrogen in the combined contraceptive may be responsible for the reported fall in serum AT III activity.