Influence of demographic factors on antithrombin III activity in a healthy population.

Antithrombin III (AT III) activity has been measured in 9669 healthy blood donors (5525 male and 4144 female). The distribution of AT III is approximately 'normal' with mean 105.6 IU/dl and standard deviation 11.2; however, definite age and sex related variations are evident. Pre-menopausal females have lower mean AT III compared to their male contemporaries who have remarkably stable mean AT III until 45 years, after which there is a gradual decline. In contrast, post-menopausal females have higher mean AT III than both males of the same age and younger pre-menopausal females. Concurrent hormone replacement therapy inhibits this rise. The use of hormonal preparations is associated with a 4 IU/dl reduction of mean AT III in younger females but not in those over 30 years. Smoking may result in a mild increase in AT III of doubtful clinical significance. On-going genetic and family studies are expected to predict a prevalence rate of congenital AT III deficiency in excess of the previously reported figure of 0.02%. The authors consider these observed variations as minor and recommend the use of a single reference range for AT III activity, but that particular care be taken when interpreting results in pill-taking females and the elderly.