Oral contraceptives, ABO blood types and thromboembolism

An interesting article in CMAJ by Sode and colleagues1 reported that in groups of patients with increased susceptibility to thromboembolic disease because of factor V Leiden R 5062 or prothrombin mutation G 20210A, the highest incidence of disease occurred in patients with A, B or AB blood types. In previous studies,2^–4 we reported that among 348 women treated with various oral contraceptive agents and observed for 5877 months, thromboembolic complications occurred only in patients with preexisting blood coagulation deficiencies, but all patients had A or AB blood types.

The coagulation deficiencies we observed produced no clinical problems until oral contraceptives were initiated. Abnormalities included increased levels of factors V, VII, VIII, II (prothrombin) or its mutations or decreased levels of plasminogen or combinations of these pathologies. The question is whether patients should be tested for factors of the blood coagulation and fibrinolysin systems as well as blood groups before starting oral contraceptive therapy. Although the incidence of thromboembolism in these patients is relatively low, and the tests are expensive, the potential of serious complications in otherwise healthy women may warrant testing. Positive findings may justify consideration of other methods of pregnancy prevention, or of the use of oral contraceptives with the lowest estrogen concentrations, which have the lowest risk of thromboembolic complications.