Factor V Leiden and the Risk for Venous Thromboembolism in the Adult Danish Population

Context Estimates of risk for venous thromboembolism associated with factor V Leiden vary. Contribution This population-based cohort study found that heterozygotes and homozygotes for factor V Leiden had about 3 and 18 times higher risks for venous thromboembolism than noncarriers. Absolute 10-year risks for thromboembolism were 0.7% and 3% among heterozygotes and homozygotes younger than 40 years of age who did not smoke and were not overweight. The 10-year risks in heterozygotes and homozygotes older than age 60 years who smoked and were overweight were 10% and 51%. Implications Risks for thromboembolism associated with factor V Leiden are important but probably lower than previously reported. The Editors Venous thromboembolism is associated with more than 300000 hospitalizations and 50000 deaths every year in the United States alone (1). Factor V Leiden is the most frequent hereditary risk factor for venous thromboembolism; it is present in 1% to 7% of white persons and almost never affects black and Asian persons (1, 2). The reported excess risk for venous thromboembolism associated with factor V Leiden (or its phenotype, resistance to activated protein C) varies considerably among studies: Compared with noncarriers, odds ratios between 3 and 16 have been reported for heterozygotes (3-10), while a single study reported an odds ratio of 79 for homozygotes (4). Most of these studies are casecontrol studies (3, 4, 6-10), and no population-based prospective study has been published. Because odds ratios from casecontrol studies may overestimate risk in healthy factor V Leiden heterozygotes and homozygotes, a study to estimate such risks in the adult population at large is warranted. Our main purpose was to estimate hazard ratios for venous thromboembolism in factor V Leiden heterozygotes and homozygotes in the adult Danish population. We also investigated whether factor V Leiden increased hazard ratios for primary and secondary thromboembolic events equally and whether hazard ratios for deep venous thrombosis and pulmonary embolism differed. Finally, we estimated absolute risks for venous thromboembolism according to factor V Leiden genotype that depended on the presence or absence of other thromboembolic risk factors. For these purposes, we performed genotyping on 9253 individuals from the adult Danish population who were participants in the Copenhagen City Heart Study (11). Methods Study Design The Copenhagen City Heart Study is a prospective cardiovascular study of individuals randomly selected according to the Central-Population-Register code to reflect the adult Danish population at large. Those invited were stratified into 5-year age groups ranging from 20 to 95 years; 35- to 70-year-old persons were emphasized. In 19761978, 19329 individuals were invited, of whom 74% (14223) participated. In 19811983, the original cohort supplemented with 500 20- to 25-year-olds was invited to participate; 70% (12698) participated. Finally, in 19911994 the cohort was further supplemented with 3000 20- to 49-year-old persons, and of those invited 61% (10135) participated. More than 99% of participants were white persons of Danish descent. Of the 10135 participants who attended the 19911994 examination, 9259 gave blood for DNA analyses. Of these, 9253 underwent genotyping for factor V Leiden as previously described (11, 12). We did not exclude participants with a thromboembolic event before entry into the Copenhagen City Heart Study. Of the 9253 participants who were analyzed for factor V Leiden, all attended 1 examination, 80% attended 2 examinations, and 77% attended 3 examinations. Examinations included a self-administered questionnaire, a phys ical examination, and blood samples. Clinical and demographic data as well as data on nonresponders have been published previously (13-15). At some time during follow-up, 555 women used oral contraceptives and 1125 used postmenopausal hormone replacement therapy. No women experienced a thromboembolic event while using oral contraceptives, but 11 women had an event while using postmenopausal hormone replacement therapy. We followed all individuals from baseline until the occurrence of a venous thromboembolic event or until censoring. Baseline was defined as the date at which the participant was first selected for inclusion into the Copenhagen City Heart Study. For 7166 participants, baseline was the 19761978 examination, for 277 it was the 19811983 examination, and for 1810 it was the 19911994 examination. We terminated any further follow-up on 31 December 1999 because this was the last date on which complete diagnostic information on end points was obtained. Seventy-six participants emigrated during follow-up and were therefore censored at the emigration date. Four participants who could not be traced were censored at the date on which they were lost. Consequently, follow-up is more than 99% complete. Median follow-up time was 23 years (range, 0.04 to 23 years). We gathered information on incident cases of deep venous thrombosis (International Classification of Diseases, 8th revision [ICD-8], codes 451.00, 451.08, 451.09, 451.90, 451.92, 671.01671.09 and ICD, 10th revision [ICD-10], codes I80.1, I80.2, I80.3, O22.3, O87.1) and pulmonary embolism (ICD-8 codes 450.99, 673.99 and ICD-10 codes I26.0, I26.9, O88.2) until 31 December 1999 from the Danish National Hospital Discharge Register and from the Danish National Register of Causes of Death. We classified venous thromboembolic events as secondary if they occurred 1) in a participant with a history of cancer (n = 27) or with incident cancer within 5 years of the thromboembolic event (n = 12), 2) during a hospitalization for a cause other than venous thromboembolism (n = 72), 3) within 12 months of hospitalization for fracture of the lower extremities or for a cerebrovascular event (n = 6), 4) during pregnancy or puerperium (n = 0), 5) during use of oral contraceptives (n = 0), or 6) during postmenopausal hormone replacement therapy (n = 11). We classified all other events as primary thromboembolic events. In analyses on isolated deep venous thrombosis, we excluded participants with pulmonary embolism. Venous thromboembolic events obtained by requesting hospital records on individuals registered with a venous thromboembolic event from 1980 through 2000 from the Danish National Hospital Discharge Register were validated by others: Among the 176 medical records from North Jutland County with a discharge ICD code for venous thromboembolism, 72% of cases met objective diagnostic criteria (Bjerregaard Larsen T. Personal communication). The diagnostic criteria used were ultrasonography or venography in the case of deep venous thrombosis and ventilationperfusion scintigraphy or pulmonary angiography in the case of pulmonary embolism. All hospitals in Denmark report to the Danish Hospital Discharge Register. Likewise, all death certificates in Denmark are registered in the Danish Register of Causes of Death. The physicians attending patients from the Copenhagen City Heart Study did not use a diagnostic protocol defined specifically by the Copenhagen City Heart Study but rather used Danish standard diagnostic practices, which included venography for the diagnosis of deep venous thrombosis and ventilationperfusion scintigraphy for the diagnosis of pulmonary embolism in the period 19761999. The Danish ethics committee for the City of Copenhagen and Frederiksberg approved the study (#100.2039/91). All participants gave written informed consent. Statistical Analysis We analyzed data using the Stata statistical software package, version 8.0 (Stata Corp., College Station, Texas). We made 2-group comparisons using the Pearson chi-square test, Student t-test, or MannWhitney U-test. A 2-sided P value less than 0.05 was considered statistically significant. We present plots of cumulative incidence (NelsonAalen estimate) as a function of age, and we tested differences between factor V Leiden genotypes for significance by using the log-rank test. When left truncation (that is, delayed entry) was used with age as the time scale, Cox proportional hazards models estimated hazard ratios for venous thromboembolism. This means that differences in age are automatically adjusted for. The final Cox regression model included the following covariates, which were forced into all models: factor V Leiden genotype, sex, body mass index (<25, 25 to 30, and >30 kg/m2), smoking status (smoker or nonsmoker), previous myocardial infarction, leisure time physical activity (<2 hours per week, 2 to 4 hours of light exercise per week, 2 to 4 hours of demanding exercise per week, or >4 hours of exercise per week), use of oral contraceptives, use of postmenopausal hormone replacement therapy, menopausal status, and year of entry. We constructed 95% CIs using bootstrap estimation (10000 replications) with the 2.5th and 97.5th percentiles of the generated hazard ratio distribution as the lower and upper limits. The baseline value for each covariate, as well as values from subsequent examinations, was used. Interaction between factor V Leiden genotype and other covariates in the models was tested for statistical significance by using the likelihood ratio test to compare the model with and without the 2-factor interaction term. Proportionality of hazards over time for the individual covariate was assessed by plotting ln(ln(survival)) versus ln(analysis time). Suspicion of nonparallel lines was further tested by using Schoenfeld residuals. We detected no violations of the proportional hazards assumption. To investigate whether factor V Leiden increases the risk for deep venous thrombosis rather than for pulmonary embolism and also increases the risk for primary thromboembolism rather than for secondary events, we performed Cox regression on participants with venous thromboembolism. The end points in these models were deep venous thrombosis or primary events, and the same covariates