Association of oral but not transdermal estrogen therapy with enhanced platelet reactivity in a subset of postmenopausal women.

OBJECTIVE: We sought to determine the effects of oral versus transdermal estrogen therapy on platelet function in postmenopausal women.
METHODS: Blood obtained from 84 postmenopausal women was tested for closure times using the Platelet Function Analyzer-100 before and after administration of oral or transdermal estrogen for 8 weeks.
RESULTS: Women with normal closure times at baseline (n = 71) demonstrated no significant change after receiving estrogen therapy with oral (n = 29) or transdermal (n = 42) estrogen. Women with borderline closure times of 61 to 66 seconds (n = 13) showed a significant acceleration of closure times (P = 0.0008) after oral estrogen therapy (-6.8 +/- 0.7 seconds, n = 5) but no significant change from baseline after transdermal estrogen therapy (1.1 +/- 0.5 seconds, n = 8).
CONCLUSIONS: An acceleration of closure times as measured by the Platelet Function Analyzer-100 in women with borderline baseline closure times is associated with the use of oral, but not transdermal, estrogen therapy. These results suggest that oral estrogen therapy increases platelet reactivity in a subset of women.