Heparin cofactor II levels are increased by the use of combined oral contraceptives.

Heparin cofactor II (HCII) was assayed by a microtitre amidolytic substrate technique. A linear response was obtained up to 1.5 U/ml and HCII levels were not affected by freezing and thawing the plasma. The assay was validated by comparing HCII and antithrombin III (AT III) levels in AT-III-deficient plasmas and samples from critically ill patients. Higher HCII levels were found in healthy normal women than in healthy normal men (means 1.16 and 0.97 U/ml, respectively, P less than 0.01). A significant increase in HCII levels from 0.86 to 1.10 U/ml (mean values) was seen in healthy normal women starting on combined oral contraceptive (COC) preparations (P less than 0.001). Increased HCII levels were maintained over a 6-month period, but fell towards normal 14 days after stopping COC, although they were still significantly higher than before starting COCs. The discrepancy in HCII level between normal men and women may be due to COC use. In clinical studies, different reference ranges should be used for men and women, and the need for careful questioning about the use of COCs is emphasized.