Factor XIII Val34Leu and the risk of venous thromboembolism in factor V Leiden carriers

A mutation in factor XIII (Val34Leu) was reported to protect against venous thromboembolism. We evaluated the effect of Val34Leu on thrombotic risk in 352 factor V Leiden carriers who were first‐degree relatives of 132 thrombotic propositi carrying factor V Leiden. The total observation period was 2594 years in 92 Val34Leu carriers and 7444 years in 260 non‐carriers. The annual incidence of a first episode of venous thromboembolism was 0·31% in Val34Leu carriers and 0·44% in non‐carriers [relative risk (RR) for venous thromboembolism: 0·7, 95% CI 0·3–1·5]. Age‐specific RR for venous thromboembolism were (for Val34Leu carriers and non‐carriers respectively): 1·0 (95% CI 0·3–3·2) in the age group of 15–30 years, 0·4 (95% CI 0·05–3·0) in the age group of 30–45 years, 0·6 (95% CI 0·1–2·9) in the group aged 45–60 years and 0·5 (95% CI 0·06–4·5) in relatives older than 60 years. In conclusion, the impact of FXIII Val34Leu on the venous thromboembolic risk is modest, suggesting that screening for this mutation in factor V Leiden carriers is not justified.