Factor V Leiden and G20210A prothrombin mutation and the risk of subclavian vein thrombosis in patients with breast cancer and a central venous catheter

BACKGROUND

To analyze the influence of the prothrombotic gene mutation factor V G1691A (factor V Leiden) and prothrombin G20210A on the risk of a first episode of catheter-related deep venous thrombosis (DVT) in a group of patients with breast cancer treated with chemotherapy.

PATIENTS AND METHODS

Between January 1999 and February 2001, the occurrence of a first symptomatic DVT was investigated in a cohort of 300 consecutive patients with locally advanced or metastatic breast cancer treated at a single institution with fluorouracil-based chemotherapy, administered continuously through a totally implanted access port. A nested case-control study included 25 women (cases) with catheter-related DVT and 50 controls without DVT matched with cases for age, identical chemotherapy, stage of disease and prognostic features. The G1691A factor V and G20210A prothrombin mutation genotypes were analyzed.

RESULTS

Five cases [20%; 95% confidence interval (CI) 9% to 39%)] and two controls (4%; 95% CI 1% to 14%) were heterozygous carriers of G1691A factor V (P = 0.04). The age-adjusted odds ratio for catheter-related DVT was 6.1 (95% CI 1.1-34.3). Only one patient (case) had the G20210A prothrombin gene mutation. Time from start of chemotherapy infusion to DVT was not significantly different between patients with (median 31 days) and without (median 43 days) G1691A factor V mutation (P = 0.6).

CONCLUSIONS

Factor V Leiden carriers with locally advanced or metastatic breast cancer have an increased risk of developing catheter-related DVT during chemotherapy.

Patients With Type 3 Severe von Willebrand Disease Are Not Protected Against Atherosclerosis

Background— The results of a number of studies in pigs and mice suggest that absence of von Willebrand factor (vWF) protects against the development of atherosclerosis. We studied whether patients with a complete deficiency of vWF (type 3 von Willebrand disease [vWD]) develop fewer atherosclerotic vessel wall changes than healthy controls.

Methods and Results— This study included 47 individuals with type 3 vWD and 84 healthy controls. Early atherosclerotic changes were assessed by measuring the thickness of the intima-media in the carotid and femoral arteries by B-mode ultrasonography. Advanced atherosclerotic changes were quantified by summing the maximal thickness of atherosclerotic plaques in the carotid and femoral arteries and were expressed as a plaque score. Established risk factors were determined to adjust for possible differences between the groups. We found no substantial difference in intima-media thickness between vWD patients and controls (adjusted difference for carotid artery 0.007 mm, 95% CI −0.022 to 0.036 mm; femoral artery 0.069 mm, 95% CI −0.056 to 0.19 mm). Similar proportions of patients and controls had atherosclerotic plaques (19% and 17%, respectively). No difference was found in the plaque score between groups (adjusted difference −0.22 mm, 95% CI −0.69 to 0.26). Among vWD patients, we found no effect of treatment with vWF concentrates on intima-media thickness or plaque score.

Conclusions— The results of this study indicate that vWF does not play a substantial role in human atherogenesis.

Arterial intima-media thickness and its relationship with cardiovascular disease and atherosclerosis: a possible contribution of medium-sized arteries

The intima-media thickness (IMT) of the arterial wall, measured by B-mode ultrasonography, has been related to cardiovascular disease and atherosclerosis in many studies, most of which have considered carotid and common femoral arteries. No study has related the IMT of medium-sized arteries (e.g. brachial) to the atherosclerotic process. The aim of this study was to evaluate whether more peripheral arteries (e.g. brachial, superficial femoral and popliteal) represent a good model in studies of arterial wall IMT, as well as carotid artery. Twenty-six male patients with demonstrated coronary artery disease (median age 63 yrs; range 41-70) and twenty-four male controls (median age 62 yrs; range 53-74) were studied. The far-wall IMT of the common carotid, brachial, superficial femoral and popliteal arteries was measured by high-resolution B-mode ultrasonography. Its relationship with ischemic cardiovascular disease and the presence of atherosclerotic plaques in all the arteries was also evaluated. Mean IMT values were higher in patients than controls, more so in superficial femoral artery (IMT difference 0.08 mm [95% confidence interval 0.02 - 0.14]) and popliteal artery (0.08 [0.01-0.15]) than in common carotid artery (0.02 [-0.07-0.11]) or brachial artery (0.01[-0.01-0.03]). The difference did not change after adjustment for age. The prevalence of plaques was consistently higher in patients than controls, except for brachial artery, in which no plaque was found either in patients and controls. In all the arteries except brachial we found an association between increase in IMT and number of plaques. Age was strongly related to the presence of plaques in the carotid artery, and less markedly in superficial femoral and popliteal arteries. The brachial artery does not seem a good model to study atherosclerosis by ultrasound measurements of arterial IMT, whereas superficial femoral and popliteal arteries might be chosen for these studies besides carotid artery.

Low plasma levels of vitamin B(6) are independently associated with a heightened risk of deep-vein thrombosis

BACKGROUND

Elevated plasma levels of total homocysteine (tHcy) before and after an oral methionine load (PML) are associated with an elevated risk of deep-vein thrombosis (DVT). We investigated whether plasma levels of B vitamins that are involved in Hcy metabolism are associated with an elevated risk of DVT.

METHODS AND RESULTS

We compared 397 cases with previous DVT with 585 matched healthy controls. The plasma levels of folate, vitamin B(12), vitamin B(6,), and fasting and PML tHcy were measured. The ORs for DVT associated with high (>95th percentile) fasting levels and PML increases of tHcy were 2.1 (95% CI, 1.2 to 3.4) and 2.4 (95% CI, 1.5 to 3.9) after adjustment for established risk factors for DVT. Fasting plasma levels and PML increases in tHcy correlated negatively with vitamin levels. The crude OR for folate levels in the lowest quartile compared with the highest was 1.5 (95% CI, 1.1 to 2.1), and that for B(6) levels in the lowest and second quartiles compared with the highest was 1.5 (95% CI, 1.0 to 2.1). However, after adjustment for established risk factors and fasting and PML tHcy, the ORs for B(6) levels in the lowest and second quartiles only remained statistically significant (lowest quartile: OR, 1.8; 95% CI, 1.2 to 2.8; second quartile, OR, 1.9; 95% CI, 1.3 to 2.9).

CONCLUSIONS

High fasting and PML tHcy and low vitamin B(6) plasma levels are associated with an elevated risk for DVT independently of established risk factors for DVT. The association of low vitamin B(6) levels with the risk for DVT is independent of fasting and PML tHcy levels.

A 23bp insertion in the endothelial protein C receptor (EPCR) gene impairs EPCR function

EPCR is a type I transmembrane protein, highly expressed on the endothelium of large vessels, that binds protein C and augments its activation. In this study, a 23bp insertion in the EPCR gene was found in 4/198 survivors of myocardial infarction and 3/194 patients with deep vein thrombosis. The EPCR gene with the insertion predicts a protein that lacks part of the extracellular domain, the transmembrane domain and the cytoplasmic tail. Expression studies showed that the truncated protein is not localized on the cell surface, cannot be secreted in the culture medium, and does not bind activated protein C. Since protein C activation depends on the concentration of EPCR, patients with the EPCR insertion could have a diminished protein C activation capacity. Further clinical studies of adequate samples size are necessary to establish whether or not the EPCR insertion predisposes to the development of thrombotic events.

Risk of pregnancy-related venous thrombosis in carriers of severe inherited thrombophilia

Homozygous carriers of factor V Leiden have an approximately 80-fold increased risk of venous thrombosis. Also double heterozygous carriers of both the factor V Leiden and the prothrombin gene mutations are at high thrombotic risk. The magnitude of the risk of venous thrombosis in pregnant women with the two severe thrombophilic conditions has not been estimated so far. We performed a multicenter retrospective family study in women with homozygous factor V Leiden, double heterozygous factor V Leiden and the prothrombin gene mutation, and women with normal coagulation. Only relatives of index patients with thrombosis formed the study cohort. Fifteen homozygous and 39 double heterozygous women were compared to 182 women with normal coagulation. Venous thrombosis occurred in 3 of 19, 2 of 50 and 1 of 221 pregnancies, respectively. One thrombotic episode occurred in the third trimester, the remaining 5 in the postpartum. The prevalence of venous thrombosis was 15.8% (95% CI 3.4-39.6) for homozygotes. 4.0% (95% CI 0.5-13.7) for double heterozygotes and 0.5% for women with normal coagulation. The relative risk of pregnancy-related venous thrombosis was 41.3 (95% CI 4.1-419.7) for homozygous and 9.2 (95% CI 0.8-103.2) for double heterozygous carriers. In conclusion, homozygous carriers of factor V Leiden and, to a lesser extent, double heterozygous carriers of factor V Leiden and of the prothrombin mutation have an increased risk of venous thrombosis during pregnancy, particularly high during the postpartum period. On the basis of these findings we recommend that these women receive anticoagulant prophylaxis at least in the postpartum, that should perhaps be extended to the whole pregnancy in homozygous carriers.

Identification of four novel polymorphisms in the Aalpha and gamma fibrinogen genes and analysis of association with plasma levels of the protein

Four novel polymorphisms were identified in the fibrinogen gene cluster. Three of them were localized in the promoter regions of the Aalpha-chain (alpha -128 C/G, alpha -58 G/A) or the gamma-chain (gamma -239 A/G) gene, while the remaining one was identified in intron 9 of the gamma-chain gene (gamma 7792 C/T). Genotype distributions for these polymorphisms were analyzed in 200 healthy Italian individuals and were in Hardy-Weinberg equilibrium. Since high levels of plasma fibrinogen have been associated with an increased risk of cardiovascular disease and genetic variations have been evaluated as thrombotic risk predictors, we analyzed their role in determining the plasma levels of this protein. Owing to the low frequency of the rare allele of alpha -128 C/G and gamma -239 A/G polymorphisms, association with plasma fibrinogen levels was investigated for only alpha -58 G/A and gamma 7792 C/T. We also investigated in the same population two previously identified polymorphisms in the fibrinogen gene cluster (alpha TaqI and beta -455 G/A) chosen for their widely studied association with plasma fibrinogen levels. In the multivariate linear regression analysis, no statistically significant association with plasma fibrinogen levels was found.

Increased glucose-dependent insulinotropic polypeptide (GIP) secretion in acromegaly

OBJECTIVE

Acromegaly is often associated with fasting and postprandial hyperinsulinemia, and the mechanisms involved are only partly understood. Hypersecretion of incretins such as glucose-dependent insulinotropic polypeptide (GIP) could play a role in determining hyperinsulinemia in acromegaly, but the available data are inconsistent. The aim of this study was to characterize the fasting and postprandial pattern of plasma GIP and insulin in a group of acromegalic patients.

DESIGN AND METHODS

Eleven non-diabetic patients with newly diagnosed acromegaly and 11 sex- and age-matched healthy subjects were studied. Blood samples were taken at regular intervals in fasting conditions and for 3 h after a standard solid-liquid meal for growth hormone (GH), GIP and insulin measurements.

RESULTS

Not only insulin, but also fasting and postprandial GIP levels were significantly higher in the patients with acromegaly than the healthy subjects (P<0.01). In the former group fasting GIP levels and the integrated GIP response to the meal correlated significantly with GH basal levels (r=0.83, P<0.01 and r=0.65, P<0.05, respectively). Moreover, multivariate linear regression analysis showed that the presence of acromegalic status was associated with higher fasting and postprandial GIP levels independently of sex, age, fasting and postprandial plasma glucose and insulin levels, and the occurrence of normal or impaired glucose tolerance.

CONCLUSION

This study provides evidence that in patients with acromegaly fasting and postprandial GIP levels are abnormally high. GIP hypersecretion in turn might play a role in the pathogenesis of hyperinsulinemia that characterizes acromegaly.

Alterations of haemostatic and fibrinolytic markers in adult patients with growth hormone deficiency and with acromegaly

Alterations of coagulation and fibrinolytic systems might contribute to the increased cardiovascular and cerebrovascular mortality observed in patients with both chronic growth hormone (GH) excess (acromegaly) and deficiency (GHD). However, contrasting results have been so far reported. To assess the importance of GH in modulating haemostatic system, several haemostatic variables in patients with GHD and acromegaly were measured. Twenty-four adult patients with GHD (8 childhood- and 16 adult-onset; age: 41+/-12 years, insulin like growth factor-I, IGF-I: 6.7+/-4 nmol/L), 10 non-diabetic acromegalic patients (age: 39+/-15 years; IGF-I: 109+/-37 nmol/L) and 64 healthy volunteers age- and sex-matched with cases were studied. The plasma levels of tissue-type plasminogen activator antigen (t-PA), prothrombin fragment 1+2 (F1+2) and thrombin-antithrombin complex (TAT) were measured by ELISA. Plasminogen activator inhibitor type I (PAI-1) was measured by an immunoactivity assay and fibrinogen by von Clauss method. GH levels were measured by IFMA and IGF-I by RIA. GHD patients had higher PAI-1 (12.7+/-16.7 vs 4.8+/-5.3 U/ml, p<0.01), fibrinogen (363+/-104 vs 291+/-71 mg/dL, p< 0.05) and TAT levels (6.8+/-9 vs 3.6+/-2.8 ng/ml, p<0.05) than controls. Taking the 95th pecentile of the normal distribution in the control group as the cut-off point for normal plasma levels of the haemostatic variables, high PAI levels were found in 25% of patients with GHD (P<0.01), while high fibrinogen and TAT levels were observed in 21% (P<0.05). The alterations were mostly present in patients with adult-onset GHD, with the exception of hyperfibrinogenaemia which was equally present in adult- and childhood-onset patients. Acromegalic patients had higher mean fibrinogen levels than controls (398+/-111 vs 291+/-71 mg/dL, p< 0.05), 40% having hyperfibrinogenaemia (P<0.01, vs controls). They also had t-PA levels lower than controls and GHD. No correlations between hormonal and haemostatic variables were found. Body mass index and waist to hip ratio correlated positively with PAI-1 levels in GHD patients only. In conclusion, this study shows that several abnormalities of coagulation variables (increased PAI-1. fibrinogen and TAT levels) are present in patients with GHD, while only hyperfibrinogenaemia is found in patients with acromegaly. These changes do not appear to be directly related to IGF-I levels or to the degree of GH deficiency/excess. However, these abnormalities may be an additional trigger for the development of coronary heart disease and thromboembolic complications mostly in patients with GHD.

The risk of venous thromboembolism in family members with mutations in the genes of factor V or prothrombin or both

Factor V Leiden and the G20210A mutation in the prothrombin gene are the most frequent abnormalities associated with venous thromboembolism. It is unknown whether the risks due to the presence of either mutation are of the same magnitude. We compared the prevalence and incidence rate of venous thromboembolism in relatives with either mutation or both. The finding of different rates might influence the strategies for primary prevention of thrombosis in carriers of these mutations. The study population included 1076 relatives of probands with the prothrombin gene mutation, factor V Leiden or both who underwent screening for inherited thrombophilia and were found to be carriers of single mutations or double mutations or who were non-carriers. The prevalence of venous thromboembolism was 5.7% in relatives with the prothrombin gene mutation, 7.8% in those with factor V Leiden, 17.1% in those with both mutations and 2.5% in non-carriers. Annual incidences of thrombosis were 0.13% [95% confidence interval (CI) 0.06-0.24], 0.19% (0.13-0.25), 0.42% (0.15-0.83) and 0.066% (0.03-0.11), respectively, and the relative risk of thrombosis was two times higher in carriers of the prothrombin gene mutation, three times higher in those with factor V Leiden and six times higher in double carriers than in non-carriers. The incidence of venous thromboembolism in carriers of the prothrombin gene mutation is slightly lower than that observed in carriers of factor V Leiden, whereas in carriers of both mutations it is two or three times higher. These findings suggest that lifelong primary anticoagulant prophylaxis of venous thromboembolism is not needed in asymptomatic carriers of single or double mutations. Anticoagulant prophylaxis seems to be indicated only when transient risk factors for thrombosis coexist with mutations.

Acquired von Willebrand syndrome: data from an international registry

The acquired von Willebrand syndrome (AvWS) is a rare bleeding disorder with laboratory findings similar to those of congenital von Willebrand disease (vWD). Despite the numerous cases reported in the literature until 1999 (n = 266), large studies on AvWS are not available. Moreover, diagnosis of AvWS has been difficult and treatment empirical. These considerations prompted us to organize an international registry. A questionnaire, devised to collect specific information on AvWS, was sent to all the members of the International Society on Thrombosis and Haemostasis (ISTH), who were invited to respond if they had diagnosed cases with the AvWS cases. 156 members answered the questionnaire and 54 of them sent information on 211 AvWS cases from 50 centers. Data were compared with those already published in the literature and 25 cases already described or not correctly diagnosed were excluded. The 186 AvWS cases that qualified for the registry were associated with lymphoproliferative (48%) and myeloproliferative disorders (15%), neoplasia (5%), immunological (2%), cardiovascular (21%) and miscellaneous disorders (9%). Ristocetin cofactor activity (vWF:RCo) or collagen binding activity (vWF:CBA) were usually low in AvWS (median values 20 U/dL, range 3-150), while factor VIII coagulant activity was sometimes normal (median 25 U/dL, range 3-191). FVIII/vWF inhibiting activities were present in only a minority of cases (16%). Bleeding episodes in AvWS were mostly of mucocutaneous type (68%) and were managed by DDAVP (32%), FVIII/vWF concentrates (37%), intravenous immunoglobulins (33%), plasmapheresis (19%), corticosteroids (19%) and immunosuppressive or chemotherapic agents (35%). Based upon the data of this international registry, it appears that AvWS is especially frequent in lympho- or myeloproliferative and cardiovascular diseases. Therefore, AvWS should be suspected and searched with the appropriate laboratory tests especially when excessive bleeding occurs in patients with these disorders. On the basis of the information provided by this registry guidelines for diagnosis and management of the AvWS are given.

A novel polymorphism in intron 1a of the human factor VII gene (G73A): study of a healthy Italian population and of 190 young survivors of myocardial infarction

We have identified a novel polymorphism located in intron 1a of the human factor VII gene, caused by the nucleotide change G to A at position + 73. In a population of 128 healthy individuals from northern Italy, the variant A73 allele had a frequency of 0.21, whereas the frequency of the previously reported 10 bp insertion allele located at -323 in the promoter region was 0.17 and that of the Q353 allele in the catalytic region of the factor VII gene was 0. 20. In 75% of the healthy individuals, the A73 allele was present together with the 10 bp insertion and the Q353 alleles, indicating a strong linkage disequilibrium. The concomitant presence of A73 with both the 10 bp and the Q353 alleles was associated with the lowest factor VII levels, measured as coagulant activity, activated factor VII and factor VII antigen. The G73A polymorphism was also evaluated in 190 survivors of myocardial infarction who had experienced the event before the age of 45 years and in 179 individuals with a negative exercise test matched with patients for sex, age and geographical origin. Patients carrying the A73 allele associated with lower factor VII levels tended to have a lower risk of myocardial infarction (adjusted odds ratio 0.54; 95% confidence intervals 0.29-0.99). In conclusion, we found a novel variant allele in intron 1a of the human factor VII gene that is often associated in healthy individuals with the 10 bp and Q353 alleles in the promoter and catalytic region of the same gene. This intronic mutation, alone or in association with other factor VII gene polymorphisms, might confer protection against myocardial infarction in the young.

Coinheritance of the HR2 haplotype in the factor V gene confers an increased risk of venous thromboembolism to carriers of factor V R506Q (factor V Leiden)

With the aim of establishing whether the HR2 haplotype in factor V affects the risk of venous thromboembolism, a retrospective multicenter cohort study was performed in 810 family members identified through 174 probands who suffered from at least 1 episode of deep vein thrombosis and/or pulmonary embolism and had an inherited defect associated with thrombophilia (antithrombin, protein C, or protein S deficiency; factor V R506Q or prothrombin G20210A). Fifty-eight percent (468/810) of the family members had an inherited defect and 10% (47/468) were symptomatic. The HR2 haplotype was found in association with factor V R506Q more frequently in family members with venous thromboembolism (18%) than in those without (8%). Double heterozygosity for factor V R506Q and HR2 conferred a 3- to 4-fold increase in the relative risk of venous thromboembolism compared with factor V R506Q alone. The median age at first event was lower when the 2 defects were associated (46 v 52 years). No increase in risk of venous thromboembolism could be demonstrated when the HR2 haplotype was associated with inherited thrombophilic defects other than factor V R506Q. Because both factor V R506Q and the HR2 haplotype are very frequent, the effect of their coinheritance on the risk of venous thromboembolism might represent a clinically relevant issue, and screening for HR2 in carriers of factor V R506Q should be considered.

Evaluation of the PFA-100 system in the diagnosis and therapeutic monitoring of patients with von Willebrand disease

We have evaluated platelet function at high shear with the PFA-100 system in different subtypes of von Willebrand disease (vWD), before and after the intravenous infusions of desmopressin or a factor-VIII/von Willebrand factor (vWF) concentrate. Closure times with the PFA-100 system were determined for both the collagen/ADP and the collagen/epinephrine cartridges in 52 patients with vWD (9 type 1 "platelet normal", 5 type 1 "platelet-discordant", 8 type 1 "platelet-low", 6 type 2A, 9 type 2B, 6 type 2M Vicenza. 6 type 3 and 3 acquired vWD) and 40 controls. Measurements were repeated 1 and 4 h after the i.v. infusion of desmopressin (0.3 microg/Kg) in 26 patients with types 1, type 2M Vicenza or type 2A vWD, or of a factorVIII/vWF concentrate (Alphanate HT, 60 U/Kg) in 4 patients with type 3 vWD. At all time points, vWF plasma levels and the bleeding time (Symplate II) were also determined. Baseline closure times were longer in vWD patients than in controls with both the collagen/ADP and the collagen/ epinephrine cartridges. The sensitivity of the PFA-100 system (88% and 87% with the two cartridges) was higher than that of the bleeding time (65%). Treatment with desmopressin normalized the closure times in patients with type 1 "platelet-normal" or type 2M Vicenza vWD, had no significant effects in patients with type 1 "platelet-low", type 1 "platelet-discordant" or type 2A vWD. Infusion of a factorVIII/vWF concentrate in patients with type 3 vWD slightly shortened their prolonged closure times. In general, changes in PFA-100 were paralleled by shortenings of the bleeding times and increases in plasma vWF levels. The PFA-100 test reflects vWF-dependent platelet function under high shear stress and could be useful in the diagnosis and therapeutic monitoring of patients with vWD.

Risk of venous thromboembolism and clinical manifestations in carriers of antithrombin, protein C, protein S deficiency, or activated protein C resistance: a multicenter collaborative family study

Deficiencies of antithrombin (AT), protein C (PC) or protein S (PS), and activated protein C resistance (APCR) are very well-established coagulation defects predisposing to venous thromboembolism (VTE). We performed a retrospective cohort family study to assess the risk for VTE in individuals with AT, PC, or PS deficiency, or APCR. Five hundred thirteen relatives from 9 Italian centers were selected from 233 families in which the proband had had at least 1 episode of VTE. We calculated the incidence of VTE in the whole cohort and in the subgroups after stratification by age, sex, and defect. The overall incidence of VTE (per 100 patient-years) in the group of relatives was 0.52. It was 1.07 for AT, 0.54 for PC, 0.50 for PS, 0.30 for APCR, and 0.67 in the group with a double defect. The incidence was associated with age, but not with sex. The mean age at onset was between 30 and 40 years for all the coagulation defects. Women had the peak of incidence in the age range of 21 to 40 years, earlier than men. The lifetime risk for VTE was 4.4 for AT versus APCR, 2.6 for AT versus PS, 2.2 for AT versus PC, 1.9 for PC versus APCR, and 1.6 for PS versus APCR. AT deficiency seems to have a higher risk for VTE than the other genetic defects. There is a relation between age and occurrence of thrombosis for both men and women. The latter had the peak of incidence earlier than the former.

Interaction between the G20210A mutation of the prothrombin gene and oral contraceptive use in deep vein thrombosis

Single-point mutations in the gene coding for prothrombin (factor II:A20210) or factor V (factor V:A1691) are associated with an increased risk of venous thromboembolism. The use of oral contraceptives is also a strong and independent risk factor for the disease, and the interaction between factor V:A1691 and oral contraceptives greatly increases the risk. No information is available about the interaction between oral contraceptives and mutant prothrombin. We investigated 148 women with a first, objectively confirmed episode of deep vein thrombosis and 277 healthy women as controls. Fourteen patients (9.4%) were carriers of factor II:A20210, 24 (16.2%) of factor V:A1691, and 4 (2.7%) of both defects. Among controls, the prevalence was 2.5% for either factor II:A20210 or factor V:A1691, and there was no carrier of both the mutations. The relative risk of thrombosis was 6-fold for factor II:A20210 and 9-fold for factor V:A1691. The most prevalent circumstantial risk factor in patients and the only one observed in controls was oral contraceptive use, which per se conferred a 6-fold increased risk of thrombosis. The risk increased to 16.3 and 20.0 when women with factor II:A20210 or factor V:A1691 who used oral contraceptives were compared with noncarriers and nonusers. These figures indicate a multiplicative interaction between the genetic risk factors and oral contraceptives. No difference in the type of oral contraceptives was observed between patients and controls, those of third generation being the most frequently used (73% and 80%). We conclude that carriers of the prothrombin mutation who use oral contraceptives have a markedly increased risk of deep vein thrombosis, much higher than the risk conferred by either factor alone.