| Summary: | Background Patients with ischemic stroke have a transiently increased risk of subsequent venous thromboembolism (VTE). Prothrombotic genotypes may augment VTE risk under conditions of high thrombosis risk related to stroke. Aims To investigate the effect of prothrombotic genotypes in patients with ischemic stroke on the risk of VTE in a population‐based case–cohort study. Methods Cases with incident VTE (n = 664) and a randomly selected age‐weighted subcohort (n = 1817) were sampled from three surveys of the Tromsø Study (1994–2008). Participants were genotyped for ABO (rs8176719), F5 (rs6025), F2 (rs1799963), FGG (rs2066865) and F11 (rs2036914) single‐nucleotide polymorphisms (SNPs). Cox regression models were used to calculate hazard ratios (HRs) for incident VTE according to individual SNPs and categories of risk alleles (5‐SNP score; 0–1, 2, 3–4 and ≥ 5) in participants with and without ischemic stroke. Results There were 192 patients with incident stroke, of whom 43 developed VTE during a median of 15.2 years of follow‐up. The risk alleles of individual SNPs augmented the elevated VTE risk brought about by ischemic stroke. In stroke patients, a one‐category increase in the genetic risk score was associated with a 50% higher relative risk of overall VTE (HR 1.5, 95% confidence interval [CI] 1.3–1.8) and an 80% higher relative risk of provoked VTE (HR 1.8, 95% CI 1.5–2.1). Stroke patients with ≥ 5 risk alleles had a 12‐fold (HR 11.7, 95% CI 4.1–33.3) higher relative risk of VTE than stroke‐free participants with 0–1 risk alleles. Conclusions Prothrombotic genotypes increased the risk of VTE in stroke patients, and the risk increased with an increasing number of risk alleles.
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