Mutation in blood coagulation factor V associated with resistance to activated protein C

Activated protein C (APC) is a serine protease with potent anticoagulant properties, which is formed in blood on the endothelium from an inactive precursor. During normal haemostasis, APC limits clot formation by proteolytic inactivation of factors Va and VIIIa (ref. 2). To do this efficiently the enzyme needs a nonenzymatic cofactor, protein S (ref. 3). Recently it was found that the anticoagulant response to APC (APC resistance) was very weak in the plasma of 21% of unselected consecutive patients with thrombosis and about 50% of selected patients with a personal or family history of thrombosis; moreover, 5% of healthy individuals show APC resistance, which is associated with a sevenfold increase in the risk for deep vein thrombosis. Here we demonstrate that the phenotype of APC resistance is associated with heterozygosity or homozygosity for a single point mutation in the factor V gene (at nucleotide position 1,691, G-->A substitution) which predicts the synthesis of a factor V molecule (FV Q506, or FV Leiden) that is not properly inactivated by APC. The allelic frequency of the mutation in the Dutch population is approximately 2% and is at least tenfold higher than that of all other known genetic risk factors for thrombosis (protein C (ref. 8), protein S (ref. 9), antithrombin10 deficiency) together.

Venous thrombosis: a multicausal disease

The risk factors for venous thrombosis differ from those for arterial vascular disease. During the past 5 years, knowledge about the aetiology of venous thrombosis has advanced with the discovery of several factors that contribute to the incidence of thrombosis, particularly the role of coagulation abnormalities. These abnormalities are common in the general population and therefore will be present simultaneously in some individuals. The resultant gene-gene and gene-environment interactions between risk factors are the key to the understanding of why a certain person develops thrombosis at a specific point in time.

Incidence and mortality of venous thrombosis: a population-based study

BACKGROUND

Estimates of the incidence of venous thrombosis (VT) vary, and data on mortality are limited.

OBJECTIVES

We estimated the incidence and mortality of a first VT event in a general population.

METHODS

From the residents of Nord-Trøndelag county in Norway aged 20 years and older (n = 94 194), we identified all cases with an objectively verified diagnosis of VT that occurred between 1995 and 2001. Patients and diagnosis characteristics were retrieved from medical records.

RESULTS

Seven hundred and forty patients were identified with a first diagnosis of VT during 516,405 person-years of follow-up. The incidence rate for all first VT events was 1.43 per 1000 person-years [95% confidence interval (CI): 1.33-1.54], that for deep-vein thrombosis (DVT) was 0.93 per 1000 person-years (95% CI: 0.85-1.02), and that for pulmonary embolism (PE) was 0.50 per 1000 person-years (95% CI: 0.44-0.56). The incidence rates increased exponentially with age, and were slightly higher in women than in men. The 30-day case-fatality rate was higher in patients with PE than in those with DVT [9.7% vs. 4.6%, risk ratio 2.1 (95% CI: 1.2-3.7)]; it was also higher in patients with cancer than in patients without cancer [19.1% vs. 3.6%, risk ratio 3.8 (95% CI 1.6-9.2)]. The risk of dying was highest in the first months subsequent to the VT, after which it gradually approached the mortality rate in the general population.

CONCLUSIONS

This study provides estimates of incidence and mortality of a first VT event in the general population.

A Method to Determine the Optimal Intensity of Oral Anticoagulant Therapy

Oral anticoagulant therapy has been shown to be effective for several indications. The optimal intensity of anticoagulation for each indication, however, is largely unknown. To determine this optimal intensity, randomised clinical trials are conducted in which two target levels of anticoagulation are compared. This approach is inefficient, since the choice of the target levels will be arbitrary. Moreover, the achieved intensity is not taken into account.

We propose a method to determine the optimal achieved intensity of anticoagulation. This method can be applied within a clinical trial as an “efficacy-analysis”, but also on data gathered in day-to-day patient care.

In this method, INR-specific incidence rates of events, either thromboembolic or hemorrhagic, are calculated. The numerator of the incidence rate is based on data on the INR at the time of the event. The denominator consists of the person-time at each INR value, summed over all patients, and is calculated from all INR measurements of all patients during the follow-up interval. This INR-specific person-time is calculated with the assumption of a linear increase or decrease between two consecutive INR determinations. Since the incidence rates may be substratified on covariates, efficient assessment of the effects of other factors (e.g. age, sex, comedication) by multivariate regression analysis becomes possible.

This method allows the determination of the optimal pharmacological effects of anticoagulation, which can form a rational starting point for choosing the target levels in subsequent clinical trials.

Venous thrombosis due to poor anticoagulant response to activated protein C: Leiden Thrombophilia Study

We undertook a population-based case-control study to test the clinical importance of a hereditary abnormality in the coagulation system, characterised by poor anticoagulant response to activated protein C (APC), which is associated with familial thrombophilia. The abnormality was detected in 64 (21%) of 301 unselected consecutive patients younger than 70 years, with a first, objectively confirmed episode of deep-vein thrombosis and without underlying malignant disease. Among 301 healthy control subjects matched for age and sex, the frequency was 5% (14 subjects). Thus, there is a seven-fold increase in risk of deep-vein thrombosis in subjects with a poor response to APC (matched odds ratio 6.6 [95% CI 3.6-12.0]). In addition, there was a clear inverse relation between the degree of response to APC and thrombosis risk. In the families of the patients an autosomal dominant mode of transmission of the abnormality was confirmed. 9 of 10 thrombosis patients with a poor response to APC had 1 parent with a similar poor response, whereas 9 of 10 patients with normal tests had parents with equally normal tests. The abnormality was found in both parents of 1 patient with an extremely poor response to APC; this patient is probably homozygous for the abnormality. We conclude that the poor response to APC is the most important hereditary cause of venous thrombosis. Its high prevalence in a series of unselected patients will make testing of all thrombosis patients for this abnormality worth while.

Role of clotting factor VIII in effect of von Willebrand factor on occurrence of deep-vein thrombosis

To elucidate the roles of the ABO blood group, von Willebrand factor (vWF), and clotting factor VIII in the process of deep-vein thrombosis we undertook a population-based patient-control study in which 301 consecutive patients younger than 70 with a first, objectively diagnosed episode of venous thrombosis and without an underlying malignant disorder were compared with 301 healthy controls matched for age and sex. In univariate analysis, blood group, vWF concentration, and factor VIII concentrations were all related to deep-vein thrombosis. The risk of thrombosis increased with increasing vWF or factor VIII concentration and was higher in subjects of non-O blood groups than in those of group O. In multivariate analysis only factor VIII remained as a risk factor, and the dose-response relation between factor VIII concentration and risk of thrombosis persisted (subjects with factor VIII concentrations above 1500 IU/L had an adjusted odds ratio of 4.8 [95% Cl 2.3-10.0]). By contrast, the adjusted odds ratio for each vWF stratum did not differ from 1, and that for blood group was 1.5 (1.0-2.2). Our findings point to an effect on thrombosis risk of vWF and blood group, the former fully and the latter at least partly mediated through factor VIII. The 25% prevalence of factor VIII concentrations above 1500 IU/L among unselected consecutive thrombosis patients and the high adjusted relative risk for thrombosis lead to the conclusion that high factor VIII concentrations are common and represent a clear increase in risk of thrombosis, similar to the risks conferred by deficiencies of the coagulation-inhibiting proteins and activated protein C resistance.

Hyperhomocysteinemia as a risk factor for deep-vein thrombosis

BACKGROUND

Previous studies have suggested that hyperhomocysteinemia may be a risk factor for venous thrombosis. To assess the risk of venous thrombosis associated with hyperhomocysteinemia, we studied plasma homocysteine levels in patients with a first episode of deep-vein thrombosis and in normal control subjects.

METHODS

We measured plasma homocysteine levels in 269 patients with a first, objectively diagnosed episode of deep-vein thrombosis and in 269 healthy controls matched to the patients according to age and sex. Hyperhomocysteinemia was defined as a plasma homocysteine level above the 95th percentile in the control group (18.5 micromol per liter).

RESULTS

Of the 269 patients, 28 (10 percent) had plasma homocysteine levels above the 95th percentile for the controls, as compared with 13 of the controls (matched odds ratio, 2.5; 95 percent confidence interval, 1.2 to 5.2). The association between elevated homocysteine levels and venous thrombosis was stronger among women than among men and increased with age. The exclusion of subjects with other established risk factors for thrombosis (e.g., a deficiency of protein C, protein S, or antithrombin; resistance to activated protein C; pregnancy or recent childbirth; or oral-contraceptive use) did not materially affect the risk estimates.

CONCLUSIONS

High plasma homocysteine levels are a risk factor for deep-vein thrombosis in the general population.

Thromboembolic and bleeding complications in patients with mechanical heart valve prostheses

BACKGROUND

Patients with mechanical heart valve prostheses may experience valve thrombosis and subsequent systemic embolism for which they are treated with oral anticoagulant therapy. It is essential to know reliable estimates of the risks and benefits of this therapy in order to answer a number of clinical questions rationally. We sought to obtain more precise estimates of the risks and benefits by combining the data from individual studies by using meta-analysis.

METHODS AND RESULTS

We searched for studies in which the incidences were reported of embolic or bleeding complications in patients with mechanical heart valve prostheses. They were collected from the Medline and Current Contents database and by cross-references between 1970 and 1992. Since most studies vary greatly in many respects, we used a number of inclusion criteria, thus selecting comparable studies of acceptable quality only. The influence of antithrombotic therapy, valve position, and valve type was analyzed by univariate and by multivariate analysis with Poisson regression techniques. Forty-six studies were found, including 13,088 patients studied for 53,647 patient-years. We found an incidence of major embolism in the absence of antithrombotic therapy of 4 per 100 patient-years. With antiplatelet therapy this risk was 2.2 per 100 patient-years, and with coumarin therapy it was reduced to 1 per 100 patient-years. This risk varied with the type and the site of the prosthesis. A prosthesis in mitral position increased the risk almost twice as compared with the aortic position. Tilting disc valves and bileaflet valves showed a lower incidence of major embolism than caged ball valves. An incidence of major bleeding was found in patients treated with coumarin derivatives of 1.4 per 100 patient-years. The incidence of bleeding became significantly higher with the addition of antiplatelet therapy, although this did not decrease the risk of thromboembolism any further.

CONCLUSIONS

These data provide a reference for future studies and give adequate risk estimates for clinical decision making.

Increased risk of venous thrombosis in oral-contraceptive users who are carriers of factor V Leiden mutation

We investigated whether the occurrence of venous thrombosis in young women who use oral contraceptives might be explained by the factor V Leiden mutation, which leads to resistance to activated protein C and enhances susceptibility to thrombosis. We compared 155 consecutive premenopausal women, aged 15 to 49, who had developed deep venous thrombosis in the absence of other underlying diseases, with 169 population controls. The risk of thrombosis among users of oral contraceptives was increased 4-fold (relative risk 3.8 [95% CI 2.4-6.0]). The risk of thrombosis among carriers of the mutation compared with non-carriers was increased 8-fold (7.9 [3.2-19.4]). Compared with women who did not use oral contraceptives and were not carriers of the mutation, the risk of thrombosis among those with both risk factors was increased more than 30-fold (34.7 [7.8-154]). Recalculation of population incidences from these relative risks shows that the absolute risk of venous thrombosis in young women who use oral contraceptives is much larger when they carry the factor V Leiden mutation. When a young woman develops thrombosis, her factor V Leiden status should be considered in counselling about her future method of contraception.

Optimal oral anticoagulant therapy in patients with mechanical heart valves

BACKGROUND

The optimal intensity of oral anticoagulant therapy for patients with mechanical heart valves (i.e., the level at which thromboembolic complications are effectively prevented without excessive bleeding) is not known. We attempted to determine the optimal intensity by calculating the incidence of both complications at different levels of anticoagulation.

METHODS

Data were collected on all patients with mechanical heart valves who have been seen at four regional Dutch anticoagulation clinics since 1985. The primary outcome events were episodes of thromboembolism or major bleeding. The intensity-specific incidence of each type of event was calculated as the number of events that occurred at a certain intensity of anticoagulation (expressed in terms of the international normalized ratio [INR]) divided by the number of patient-years during which the INR was at this level in the total patient population.

RESULTS

A total of 1608 patients were followed during 6475 patient-years. Cerebral embolism occurred in 43 patients (0.68 per 100 patient-years) and peripheral embolism in 2 (0.03 per 100 patient-years). Intracranial and spinal bleeding occurred in 36 patients (0.57 per 100 patient-years) and major extracranial bleeding in 128 (2.1 per 100 patient-years). The optimal intensity of anticoagulation, at which the incidence of both complications was lowest, was achieved when the INR was between 2.5 and 4.9.

CONCLUSIONS

The intensity of anticoagulant therapy for patients with prosthetic heart valves is optimal when the INR is between 2.5 and 4.9. To achieve this level of anticoagulation, a target INR of 3.0 to 4.0 is recommended.

High levels of coagulation factor XI as a risk factor for venous thrombosis

BACKGROUND

Factor XI, a component of the intrinsic pathway of coagulation, contributes to the generation of thrombin, which is involved in both the formation of fibrin and protection against fibrinolysis. A deficiency of factor XI is associated with bleeding, but a role of high factor XI levels in thrombosis has not been investigated.

METHODS

We determined factor XI antigen levels in the patients enrolled in the Leiden Thrombophilia Study, a large population-based, case-control study (with a total of 474 patients and 474 controls) designed to estimate the contributions of genetic and acquired factors to the risk of deep venous thrombosis. Odds ratios were calculated as a measure of relative risk.

RESULTS

The age- and sex-adjusted odds ratio for deep venous thrombosis in subjects who had factor XI levels above the 90th percentile, as compared with those who had factor XI levels at or below that value, was 2.2 (95 percent confidence interval, 1.5 to 3.2). There was a dose-response relation between the factor XI level and the risk of venous thrombosis. Adjustment of the odds ratios for other risk factors such as oral-contraceptive use, homocysteine, fibrinogen, factor VIII, female sex, and older age did not alter the result. Also, when we excluded subjects who had known genetic risk factors for thrombosis (e.g., protein C or S deficiency, antithrombin deficiency, the factor V Leiden mutation, or the prothrombin G20210A mutation), the odds ratio remained the same, indicating that the risk of venous thrombosis associated with elevated levels of factor XI was not the result of one of the known risk factors for thrombosis.

CONCLUSIONS

High levels of factor XI are a risk factor for deep venous thrombosis, with a doubling of the risk at levels that are present in 10 percent of the population.

Low-molecular-weight heparin versus standard heparin in general and orthopaedic surgery: a meta-analysis

Low-molecular-weight heparins (LMWHs) have theoretical advantages over standard heparin as postoperative thromboprophylactic agents. We conducted a meta-analysis of studies reported between 1984 and April, 1991, in which LMWHs were compared with standard heparin for postoperative prophylaxis. We included only randomised studies (reported in English, French, or German) in which investigators compared currently recommended doses of the agents and used adequate screening techniques for deep vein thrombosis. For all surgical studies the relative risk (LMWH versus standard heparin) for deep vein thrombosis was 0.74 (95% Cl 0.65-0.86), for pulmonary embolism 0.43 (95% Cl 0.26-0.72), and for major bleeding 0.98 (95% Cl 0.69-1.40). Comparable relative risks were observed for the general and orthopaedic surgery studies separately. When the analysis for the general surgery studies was limited to those of strong methodology, assessed by eight criteria defined in advance, the benefit/risk ratio was less favourable--relative risk for deep vein thrombosis 0.91 (95% Cl 0.68-1.23), for major bleeding 1.32 (95% Cl 0.69-2.56). There is at present no convincing evidence that in general surgery patients LMWHs, compared with standard heparin, generate a clinically important improvement in the benefit to risk ratio. However, LMWHs may be preferable for orthopaedic surgery patients, in view of the larger absolute risk reduction for venous thrombosis.

Incidence of venous thrombosis in a large cohort of 66,329 cancer patients: results of a record linkage study

BACKGROUND

The incidence of venous thrombosis (VT) for cancer patients is increased compared with patients without cancer, but estimations of the incidence for different types of cancer have rarely been made because of the low incidence of various types of cancer. Large registries offer an opportunity to study the risk of VT in large cohorts of cancer patients, which is essential in decisions on prophylactic anti-coagulant treatment.

METHODS

This cohort study estimates the incidence of VT in cancer patients by using record linkage of a Cancer Registry and an Anticoagulation Clinic database in the Netherlands. Cumulative incidences in patients with different types of malignancies were estimated. We calculated relative risks (RRs) in relation to the presence of distant metastases and treatment.

RESULTS

Tumors of the bone, ovary, brain, and pancreas are associated with the highest incidence of VT (37.7, 32.6, 32.1, and 22.7/1000/0.5 year). Patients with distant metastases had a 1.9-fold increased risk [RRadj: 1.9; 95% confidence interval (CI): 1.6-2.3]. Chemotherapy leads to a 2.2-fold increased risk (RR(adj): 2.2; 95% CI: 1.8-2.7) and hormonal therapy leads to a 1.6-fold increased risk (RRadj: 1.6; 95% CI: 1.3-2.1) compared with patients not using these treatment modalities. Patients with radiotherapy or surgery did not have an increased risk.

CONCLUSIONS

We compared the overall incidences of VT in the first half year in our study to the risk of major bleeding as described in the literature. For patients with distant metastases, for several types of cancer, prophylactic anti-thrombotic treatment could be beneficial.

Enhancement by factor V Leiden mutation of risk of deep-vein thrombosis associated with oral contraceptives containing a third-generation progestagen

Recent concern about the safety of combined oral contraceptives (OCs) with third-generation progestagens prompted an examination of data from a population-based case-control study (Leiden Thrombophilia Study). We compared the risk of deep-vein thrombosis (DVT) during use of the newest OCs, containing a third-generation progestagen, with the risk of "older" products. We also investigated the influence of family history of thrombosis, previous pregnancy, age, and the thrombogenic factor V Leiden mutation. We selected 126 women with DVT and 159 controls aged 15-49 (mean age 34.9) and premenopausal and found, as compared with non-users, the highest age-adjusted relative risks to be that for an OC containing desogestrel and 30 micrograms ethinyloestradiol (relative risk [RR] 8.7, 95% CI 3.9-19.3). We found lower relative risks for all other types of OC, ranging from 2.2 to 3.8. In a direct comparison, users of the desogestrel-containing oral contraceptive had a 2.5-fold higher risk (95% CI 1.2-5.2) than users of all other OC types combined. The relative risk for the desogestrel-containing OC was similar among women with and without a family history--ie, preferential prescription because of family history cannot explain our findings. Nor could the excess risk be explained by previous pregnancy, and it was highest in the youngest age categories, where we would expect most new users. The age-adjusted RR for the desogestrel-containing contraceptive was 9.2 (3.9-21.4) among non-carriers of the factor V Leiden mutation and 6.0 (1.9-19.0) among carriers of the mutation. This latter risk is superimposed on the 8-fold increased risk of venous thrombosis for carriers of the factor V Leiden mutation. The risk of carriers using the desogestrel-containing OC as compared with noncarrier non-users will therefore be increased almost 50-fold. Use of low-dose OCs with a third-generation progestagen carries a higher risk of DVT than the previous generation of OCs. The absolute risk of DVT associated with these OCs seems to be especially high among carriers of the factor V Leiden mutation and among women with a family history of thrombosis. However, the higher risk associated with OC with a third-generation progestagen compared with previous generations was also present in women without factor V Leiden and with no family history.

Increased fetal loss in women with heritable thrombophilia

BACKGROUND

A successful outcome of pregnancy requires an efficient uteroplacental vascular system. Since this system may be compromised by disorders of haemostasis associated with a prothrombotic state, we postulated that maternal thrombophilia might be a risk factor for fetal loss. We studied the relation between heritable thrombophilic defects and fetal loss in a cohort of women with factor V Leiden or deficiency of antithrombin, protein C, or protein S.

METHODS

We studied 1384 women enrolled in the European Prospective Cohort on Thrombophilia (EPCOT). Of 843 women with thrombophilia 571 had 1524 pregnancies; of 541 control women 395 had 1019 pregnancies. The controls were partners of male members of the EPCOT cohort or acquaintances of cases. We analysed the frequencies of miscarriage (fetal loss at or before 28 weeks of gestation) and stillbirth (fetal loss after 28 weeks of gestation) jointly and separately.

FINDINGS

The risk of fetal loss was increased in women with thrombophilia (168/571 vs 93/395; odds ratio 1.35 [95% Cl 1.01-1.82]). The odds ratio was higher for stillbirth than for miscarriage (3.6 [1.4-9.4] vs 1.27 [0.94-1.71]). The highest odds ratio for stillbirth was in women with combined defects (14.3 [2.4-86.0]) compared with 5.2 (1.5-18.1) in antithrombin deficiency, 2.3 (0.6-8.3) in protein-C deficiency, 3.3 (1.0-11.3) in protein-S deficiency, and 2.0 (0.5-7.7) with factor V Leiden. The corresponding odds ratios for miscarriage in these subgroups were 0.8 (0.2-3.6), 1.7 (1.0-2.8), 1.4 (0.9-2.2), 1.2 (0.7-1.9), and 0.9 (0.5-1.5). Significantly more pregnancy terminations had been done in women with thrombophilia than in controls (odds ratio 2.9 [1.8-4.8]); this discrepancy was apparent in nine of 11 participating centres and for all thrombophilia subgroups.

INTERPRETATION

Women with familial thrombophilia, especially those with combined defects or antithrombin deficiency, have an increased risk of fetal loss, particularly stillbirth. Our findings have important implications for therapy and provide a rationale for clinical trials of thromboprophylaxis for affected women with recurrent fetal loss.

The venous thrombotic risk of oral contraceptives, effects of oestrogen dose and progestogen type: results of the MEGA case-control study

OBJECTIVE

To assess the thrombotic risk associated with oral contraceptive use with a focus on dose of oestrogen and type of progestogen of oral contraceptives available in the Netherlands.

DESIGN

Population based case-control study.

SETTING

Six participating anticoagulation clinics in the Netherlands (Amersfoort, Amsterdam, The Hague, Leiden, Rotterdam, and Utrecht).

PARTICIPANTS

Premenopausal women <50 years old who were not pregnant, not within four weeks postpartum, and not using a hormone excreting intrauterine device or depot contraceptive. Analysis included 1524 patients and 1760 controls.

MAIN OUTCOME MEASURES

First objectively diagnosed episodes of deep venous thrombosis of the leg or pulmonary embolism. Odds ratios calculated by cross-tabulation with a 95% confidence interval according to Woolf's method; adjusted odds ratios estimated by unconditional logistic regression, standard errors derived from the model.

RESULTS

Currently available oral contraceptives increased the risk of venous thrombosis fivefold compared with non-use (odds ratio 5.0, 95% CI 4.2 to 5.8). The risk clearly differed by type of progestogen and dose of oestrogen. The use of oral contraceptives containing levonorgestrel was associated with an almost fourfold increased risk of venous thrombosis (odds ratio 3.6, 2.9 to 4.6) relative to non-users, whereas the risk of venous thrombosis compared with non-use was increased 5.6-fold for gestodene (5.6, 3.7 to 8.4), 7.3-fold for desogestrel (7.3, 5.3 to 10.0), 6.8-fold for cyproterone acetate (6.8, 4.7 to 10.0), and 6.3-fold for drospirenone (6.3, 2.9 to 13.7). The risk of venous thrombosis was positively associated with oestrogen dose. We confirmed a high risk of venous thrombosis during the first months of oral contraceptive use irrespective of the type of oral contraceptives.

CONCLUSIONS

Currently available oral contraceptives still have a major impact on thrombosis occurrence and many women do not use the safest brands with regard to risk of venous thrombosis.

Pregnancy, the postpartum period and prothrombotic defects: risk of venous thrombosis in the MEGA study

BACKGROUND

Venous thrombosis is one of the leading causes of maternal morbidity and mortality.

OBJECTIVE

In the MEGA study, we evaluated pregnancy and the postpartum period as risk factors for venous thrombosis in 285 patients and 857 control subjects.

PATIENTS/METHODS

Between March 1999 and September 2004, consecutive patients with a first episode of venous thrombosis were included from six anticoagulation clinics. Partners of patients and a random digit dialing group were included as control subjects. Participants completed a questionnaire and DNA was collected.

RESULTS

The risk of venous thrombosis was 5-fold (OR, 4.6; 95% CI, 2.7-7.8) increased during pregnancy and 60-fold (OR, 60.1; 95% CI, 26.5-135.9) increased during the first 3 months after delivery compared with non-pregnant women. A 14-fold increased risk of deep venous thrombosis of the leg was found compared with a 6-fold increased risk of pulmonary embolism. The risk was highest in the third trimester of pregnancy (OR, 8.8; 95% CI, 4.5-17.3) and during the first 6 weeks after delivery (OR, 84.0; 95% CI, 31.7-222.6). The risk of pregnancy-associated venous thrombosis was 52-fold increased in factor V Leiden carriers (OR, 52.2; 95% CI, 12.4-219.5) and 31-fold increased in carriers of the prothrombin 20210A mutation (OR, 30.7; 95% CI, 4.6-203.6) compared with non-pregnant women without the mutation.

CONCLUSION

We found an increased risk of venous thrombosis during pregnancy and the postpartum period, with an especially high risk during the first 6 weeks postpartum. The risk of pregnancy-associated venous thrombosis was highly increased in carriers of factor V Leiden or the prothrombin 20210A mutation.

Coagulation factor activity and clinical bleeding severity in rare bleeding disorders: results from the European Network of Rare Bleeding Disorders

BACKGROUND

The European Network of Rare Bleeding Disorders (EN-RBD) was established to bridge the gap between knowledge and practise in the care of patients with RBDs.

OBJECTIVES

To explore the relationship between coagulation factor activity level and bleeding severity in patients with RBDs.

PATIENTS/METHODS

Cross-sectional study using data from 489 patients registered in the EN-RBD. Coagulation factor activity levels were retrieved. Clinical bleeding episodes were classified into four categories according to severity.

RESULTS

The mean age of patients at data collection was 31 years (range, 7 months to 95 years), with an equal sex distribution. On linear regression analysis, there was a strong association between coagulation factor activity level and clinical bleeding severity for fibrinogen, factor (F) X, FXIII, and combined FV and FVIII deficiencies. A weaker association was present for FV and FVII deficiencies. There was no association between coagulation factor activity level and clinical bleeding severity for FXI. The coagulation factor activity levels that were necessary for patients to remain asymptomatic were: fibrinogen, > 100 mg dL(-1); FV, 12 U dL(-1); combined FV + VIII, 43 U dL(-1); FVII, 25 U dL(-1); FX, 56 U dL(-1) ; FXI, 26 U dL(-1); FXIII, 31 U dL(-1). Moreover, coagulation factor activity levels that corresponded with Grade III bleeding were: undetectable levels for fibrinogen, FV and FXIII, < 15 U dL(-1) for combined FV + VIII; < 8 U dL(-1) for FVI; < 10 U dL(-1) for FX; and < 25 U dL(-1) for FXI.

CONCLUSIONS

There is a heterogeneous association between coagulation factor activity level and clinical bleeding severity in different RBDs. A strong association is only observed in fibrinogen, FX and FXIII deficiencies.

Geographic Distribution of the 20210 G to A Prothrombin Variant

A variant in prothrombin (clotting factor II), a G to A transition at nucleotide position 20210, has recently been shown to be associated with the prothrombin plasma levels and the risk of both venous and arterial thrombosis. The purpose of this study was to investigate the prevalence of carriership of this mutation in various populations.

We combined data from 11 centres in nine countries, where tests for this mutation had been performed in groups representing the general population. We calculated an overall prevalence estimate, by a precision-weighted method, and, since the distribution of the prevalences did not appear homogeneous, by an unweighted average of the prevalences. We examined differences in the prevalences by geographical location and ethnic background as a possible explanation for the heterogeneity.

Among a total of 5527 individuals who had been tested, 111 heterozygous carriers of the 20210A mutation were found. The prevalence estimates varied from 0.7 to 4.0 between the centres. The overall prevalence estimate was 2.0 percent (CI95 1.4-2.6%). The variation around the summary estimate appeared more than was expected by chance alone, and this heterogeneity could be explained by geographic differences. In southern Europe, the prevalence was 3.0 percent (CI95 2.3 to 3.7%), nearly twice as high as the prevalence in northern Europe (1.7%, CI95 1.3 to 2.2%). The prothrombin variant appeared very rare in individuals from Asian and African descent.

The 20210A prothrombin variant is a common abnormality, with a prevalence of carriership between one and four percent. It is more common in southern than in northern Europe. Since this distribution within Europe is very different to that of another prothrombotic mutation (factor V Leiden or factor V R506Q), founder effects are the most likely explanation for the geographical distribution of both mutations.

Is hyperhomocysteinaemia a risk factor for recurrent venous thrombosis?

Several studies have shown a relation between hyperhomocysteinaemia and arterial vascular disease. We looked at the association between hyperhomocysteinaemia and venous thrombosis which could be clinically important as hyperhomocysteinaemia is easily corrected by vitamin supplementation. We studied 185 patients with a history of recurrent venous thrombosis and 220 controls from the general population. Homocysteine concentrations were measured before and 6 h after oral methionine loading. We defined hyperhomocysteinaemia as the homocysteine concentration above the fasting or the postmethionine value found for the 90th percentile of the controls. Of the 185 patients with recurrent thrombosis, 46 (25%) had fasting homocysteine concentrations above the 90th percentile or the controls (odds ratio is 3.1 [1.8-5.5]). After adjustment for age, sex, and menopausal status the odds ratio was 2.0 (1.5-2.7). Similar results were found for the post-methionine value (unadjusted odds ratio 3.1 [1.7-5.5], adjusted 2.6 [1.9-3.5]). Hyperhomocysteinaemia is a common risk factor for recurrent venous thrombosis and can lead to a two-fold or three-fold increase in risk.

Increased risk of venous thrombosis in carriers of hereditary protein C deficiency defect

The relevance of heterozygosity for hereditary protein C deficiency as a risk factor for venous thrombosis has been disputed because heterozygotes without symptoms have been identified among blood donors and relatives of homozygotes. As a result, clinicians do not know whether to offer prophylaxis or not. We have compared thrombosis-free survival in 161 heterozygous and normal members of the families of 24 heterozygotes for protein C deficiency referred from several centres in the Netherlands and with a history of symptoms. We studied the influence of heterozygosity and of putative additional risk factors on the occurrence of thrombotic events noted when a medical history was taken. Protein C activities were measured but a diagnosis of heterozygosity was based on the presence of the specific mutation in one of the protein C genes identified in the proband of the family. We found a significant difference in the thrombosis-free survival of the 77 heterozygotes and 84 normals: by age 45, 50% of heterozygotes and 10% of normal relatives can be expected to have had a manifestation of venous thromboembolism. The presence of such a mutation was clearly associated with an increased risk of venous thrombotic events. Thrombotic events occurred more often in years in which the patient had been immobile for more than a week or had had surgery. Other putative risk factors showed no significant effect in the incidence of thrombotic events. About 50% of all first episodes and 65% of recurrences of venous thromboembolism in the heterozygotes were spontaneous--ie, there was no predisposing event such as surgery or pregnancy. There was no increased risk for arterial occlusions in heterozygotes. We conclude that members of the family of a symptomatic heterozygote proband who are heterozygous for the mutation in the protein C gene have an increased risk of venous thrombotic events compared with their normal family members. For such individuals prophylactic anticoagulation should be considered; the decision will need to be taken on an individual basis.

Oral contraceptives and the risk of myocardial infarction

BACKGROUND

An association between the use of oral contraceptives and the risk of myocardial infarction has been found in some, but not all, studies. We investigated this association, according to the type of progestagen included in third-generation (i.e., desogestrel or gestodene) and second-generation (i.e., levonorgestrel) oral contraceptives, the dose of estrogen, and the presence or absence of prothrombotic mutations

METHODS

In a nationwide, population-based, case-control study, we identified and enrolled 248 women 18 through 49 years of age who had had a first myocardial infarction between 1990 and 1995 and 925 control women who had not had a myocardial infarction and who were matched for age, calendar year of the index event, and area of residence. Subjects supplied information on oral-contraceptive use and major cardiovascular risk factors. An analysis for factor V Leiden and the G20210A mutation in the prothrombin gene was conducted in 217 patients and 763 controls

RESULTS

The odds ratio for myocardial infarction among women who used any type of combined oral contraceptive, as compared with nonusers, was 2.0 (95 percent confidence interval, 1.5 to 2.8). The adjusted odds ratio was 2.5 (95 percent confidence interval, 1.5 to 4.1) among women who used second-generation oral contraceptives and 1.3 (95 percent confidence interval, 0.7 to 2.5) among those who used third-generation oral contraceptives. Among women who used oral contraceptives, the odds ratio was 2.1 (95 percent confidence interval, 1.5 to 3.0) for those without a prothrombotic mutation and 1.9 (95 percent confidence interval, 0.6 to 5.5) for those with a mutation

CONCLUSIONS

The risk of myocardial infarction was increased among women who used second-generation oral contraceptives. The results with respect to the use of third-generation oral contraceptives were inconclusive but suggested that the risk was lower than the risk associated with second-generation oral contraceptives. The risk of myocardial infarction was similar among women who used oral contraceptives whether or not they had a prothrombotic mutation.

Assessment and control for confounding by indication in observational studies

In the evaluation of pharmacologic therapies, the controlled clinical trial is the preferred design. When clinical trial results are not available, the alternative designs are observational epidemiologic studies. A traditional concern about the validity of findings from epidemiologic studies is the possibility of bias from uncontrolled confounding. In studies of pharmacologic therapies, confounding by indication may arise when a drug treatment serves as a marker for a clinical characteristic or medical condition that triggers the use of the treatment and that, at the same time, increases the risk of the outcome under study. Confounding by indication is not conceptually different from confounding by other factors, and the approaches to detect and control for confounding--matching, stratification, restriction, and multivariate adjustment--are the same. Even after adjustment for known risk factors, residual confounding may occur because of measurement error or unmeasured or unknown risk factors. Although residual confounding is difficult to exclude in observational studies, there are limits to what this "unknown" confounding can explain. The degree of confounding depends on the prevalence of the putative confounding factor, the level of its association with the disease, and the level of its association with the exposure. For example, a confounding factor with a prevalence of 20% would have to increase the relative odds of both outcome and exposure by factors of 4 to 5 before the relative risk of 1.57 would be reduced to 1.00. Observational studies have provided important scientific evidence about the risks associated with several risk factors, including drug therapies, and they are often the only option for assessing safety. Understanding the methods to detect and control for confounding makes it possible to assess the plausibility of claims that confounding is an alternative explanation for the findings of particular studies.

Venous thrombosis in the elderly: incidence, risk factors and risk groups

The incidence of venous thrombosis (VT) increases sharply with age: it is very rare in young individuals (<1 per 10,000 per year) but increases to ∼ 1% per year in the elderly, which indicates that aging is one of the strongest and most prevalent risk factor for venous thrombosis. The cause of this steep age gradient is as yet, unexplained. The aim of this review was to provide an overview of studies on the effect of conventional risk factors as well as age-specific risk factors for thrombosis in the elderly. Limited data are available on risk factors for thrombosis in the elderly, i.e. all results are based on small study groups. Results indicate that, of the conventional risk factors, malignant disease, the presence of co-morbidities and the genetic risk factors factor (F)V Leiden and the prothrombin mutation seem to be associated with an increased risk of venous thrombosis. In the elderly, the population attributable risk (PAR) of malignancy is approximately 35%, for co-morbidities a PAR up to 25% is found, and the contribution of genetic risk factors to the thrombosis incidence is estimated to be 7-22%. Age-specific risk factors of thrombosis, i.e. endothelial dysfunction and frailty may be important in the explanation of the increased incidence of VT in the elderly. In conclusion, as aging is a major risk factor for thrombosis, further identification of the risk factors for thrombosis in the elderly is needed to elucidate the age gradient of the incidence of VT and to target preventive measures.