Skip directly to local search Skip directly to A to Z list Skip directly to navigation Skip directly to site content Skip directly to page options
CDC Home

HuGENet™ Case Study

middle age woman golfing, dna strand, older hands crossed, blood detail

Factor V Leiden and Venous Thrombosis

 

Investigators at Leiden University Hospital in the Netherlands were first to identify the factor V G1691A variant, which has since become known as factor V Leiden (FVL).  In several Swedish and Dutch populations, the prevalence of FVL was 2-7%, or about 10-fold higher than all previously identified genetic risk factors for thrombosis combined.  The prevalence of activated protein C resistance was 20%-60% in selected groups of patients with venous thrombosis, depending on selection criteria.

In 1994, Vandenbroucke et al. conducted a case-control study to examine whether FVL might be a causative factor in venous thrombosis occurring as an uncommon but serious complication of oral contraceptive (OC) use.  Cases were women aged 15-49 years who experienced a first episode of venous thrombosis during 1988-1993 and who did not have cancer.  They were enrolled 6-19 months after the acute event from clinics that monitor nearly all anticoagulation therapy in three well-defined geographic areas in the Netherlands.  Controls were women in the same age group who were friends or acquaintances of cases or partners of other clinic patients.   Current OC use was defined as use within 30 days before the thrombosis event (cases) or index date (controls).  Women who were postmenopausal or who had been pregnant within the prior 30 days were excluded.  The results are summarized in the following table.

Factor V genotype and current oral contraceptive (OC) use among women with venous thrombosis and controls*

Factor V
genotype H
OC use Cases Controls Total
G/G
-
+
36
84
100
63
136
147
G/A
-
+
8
22
4
2
12
24
A/A
-
+
2
3
0
0
2
3
Total
155
169
324

*Adapted from Vandenbroucke et al., 1994.
HG=normal allele, A=factor V Leiden (FVL) allele. 

 

Question 1: What was the prevalence of the FVL genotype (at least one A allele) among controls?  Among cases?

[view answer]

 

line

 

Because there were no homozygotes among controls, the investigators combined data for the G/A and A/A genotypes.  They calculated the following relative risks of venous thrombosis (estimated as odds ratios):

  • 7.9 (95% CI 3.2-19.4) for FVL
  • 7.0 (95% CI 2.1-23.5) for FVL among OC nonusers
  • 3.8 (95% CI 2.4-  6.0) for OC use. 

The investigators also reported the results of a stratified analysis to examine the effect of OC use according to FVL genotype.

Factor V Leiden (FVL) and oral contraceptive (OC) use among women with venous  thrombosis and controls* FVL†
+ -
    Cases Controls Cases Controls
OC use +
25
2
84
63
-
10
4
36
100
OR (95% CI)  
5.0 (0.8-31.8)
3.7 (2.2-6.1)

 

 *Adapted from Vandenbroucke et al., 1994. 
  † “+” denotes G/A or A/A genotype; “-” denotes G/G. 

They concluded that the relative risk of thrombosis in OC users was similar regardless of FVL genotype and not different from the overall relative risk associated with OC use-that is, 5.0 symboll, double wave 3.7 symbol, double wave 3.8.

Question 2: Do you agree that the relative risk of venous thrombosis in OC users was similar regardless of factor V genotype?

[view answer]

 

line

The investigators reasoned that, because FVL did not appear to modify the effect of OC use on risk of venous thrombosis, a reasonable estimate of the joint effect was:  OR FVL x OR OC use symbol, double wave 7 x 4 symbol, double wave 30.  The odds ratio calculated from the data, comparing women who had FVL and used OCs with those who had neither risk factor, was 34.7 (95% CI 7.8 – 154). 

Another way to present these data is in a “two-by-four table.”[i]  This format lends itself to calculating measures of association for genetic and environmental exposures and the joint exposure, consistently using persons with neither exposure as the reference group.

 Factor V Leiden (FVL) and oral contraceptive (OC) use among women with venous  thrombosis and controls*
FVL † OC use Cases Controls OR 95%CI
+ +
25
2
34.7
(7.8-310.0)
+ -
10
4
6.9
(1.8-  31.8)
- +
84
63
3.7
(1.2-    6.3)
- -
36
100
ref
 
Total  
155
169
   

*adapted from Botto and Khoury, 2001
† “+” denotes G/A or A/A genotype; “-” denotes G/G.

The two-by-four table is also convenient for assessing gene-environment interaction.  Two common statistical models of interaction are:

  1. additive, where ORge = ORg + ORe – 1, and
  2. multiplicative, where ORge = ORg x ORe

where g denotes genotype and e denotes the environmental factor.  Inequality in either statement may be interpreted as statistical evidence of interaction. 

Of course, biologic models of interaction can be more complicated, depending on the number of genetic loci involved, the dose of the environmental exposure, and the interplay of their effects at the molecular level.

Question 3: What evidence do these data provide for or against interaction between FVL and OC use in venous thrombosis?

[view answer]

 

line

The case-only study[ii] is a nontraditional study design that has been suggested for evaluating gene-environment interaction where only case data are available (e.g., from a case series or registry), or where sample sizes are too small for stratified analysis.  Under a multiplicative model of interaction where genotype and exposure are independent in the population,

ORcase only   = ORge / ( ORe x ORg) = 1 

where ORcase only  measures the association between the genotype and the exposure among cases.  A departure from unity indicates the presence of gene-environment interaction. 

We can analyze the case data from the study by Vandenbroucke et al. in the manner of a case-only study by constructing a two-by-two table:

 Association between factor V Leiden (FVL) and oral contraceptive (OC) use in women with venous thrombosis
    OC use    
    + - OR 95% CI
FVL* +
25
10
1.1
0.5-2.5
-
84
36
   

* “+” denotes G/A or A/A genotype; “-” denotes G/G.

Question 4: How does the result of this case-only analysis compare with results of the case-control analysis?

[view answer]

 

line

From their analysis, Vandenbroucke et al. concluded that “the combined effect of these risk factors seems close to a multiplication of the separate relative risks.  In terms of absolute effect, however, this means that the risk of venous thrombosis among women who use OCs is much greater when they carry the factor V Leiden mutation.”

The investigators estimated the incidence (absolute risk) of first venous thrombosis using data from the Leiden clinic, which had a geographic source population of 109,824 women aged 15-49 years (according to census data).  In all, 117 cases of venous thrombosis occurred in this geographic area during the 5 years of the study.  Thus, the estimated incidence of venous thrombosis among 15- to 49-year-old women was:

  • 117 / (109,824  x 5) = 2.1 per 10,000 person-years 

The 155 cases from all three areas were assumed to have arisen from

  • 155 / 2.1 / 10,000 » 740,000 person-years 

Total person-years were apportioned to the four exposure groups according to the distribution of exposures among controls.  The results served as approximate denominators for calculating incidence.  For example, women without FVL who did not use OCs accounted for 59% of the control group and 

  • 0.59 x 740,000 = 437,870 person-years.

 

Question 5: What biases could be introduced by using this approach to estimate person-years at risk for calculating incidence?

[view answer]

 

line

 Estimated population incidence of first venous thrombosis in women aged 15-49 years,  according to  presence of FVL and use of oral contraceptives (OCs)*
Factor V Genotype † OC use Cases Person-years (py) per 10,000 py
G/G
no
36
437,870
0.8
 
yes
84
275,858
3.0
G/A or A/A
no
10
17,515
5.7
 
yes
25
8,757
28.5

*adapted from Vandenbroucke et al., 1994 
†G=normal allele, A=FVL allele  

Question 6: What is the risk difference associated with OC use in women without FVL?  In women with FVL?

[view answer]

 

line

Clearly, a woman who uses OCs is at much higher risk for venous thrombosis if she has FVL, raising the question whether women should be screened for FVL before taking OCs.   The investigators addressed this issue in a second article. 

Vandenbroucke JP, van der Meer FJM, Helmerhorst FM, Rosendaal FR.  Factor V Leiden: should we screen OC users and pregnant women?  BMJ 1996;313:1127-1130.

The authors considered the “worst case” outcome, death from pulmonary embolism.  A U.S. study estimated that the case fatality rate for venous thrombosis was 2% in persons aged <40 years. 

Estimated population incidence of pulmonary embolism in women aged 15-49 years with FVL:

  • 5.7 x 0.02 / 10,000 py = 0.11 / 10,000 py in OC non-users
  • 28.5 x 0.02 / 10,000 py = 0.57 / 10,000 py in OC users

 

Question 7: How many women with FVL would have to avoid OCs to prevent one death from pulmonary embolism?  How many women would have to be screened to find this many women with FVL?

[view answer]

 

line

The authors concluded that about 400,000 women would have to be screened-and 20,000 carriers of FVL would have to avoid OCs-to prevent one death from pulmonary embolism each year.

Question 8: What are other potential costs and benefits to consider when deciding whether to screen young women for FVL to prevent venous thrombosis?

[view answer]

 

line

The authors discussed several issues that introduce uncertainty in this kind of analysis:  imperfect estimates of  FVL prevalence and incidence of venous thrombosis, issues related to screening test performance, costs associated with alternative forms of contraception, and morbidity costs associated with venous thrombosis (including risks associated with anticoagulant therapy). 

They concluded that routine screening for FVL when prescribing OCs was “unlikely to stand the competition for resources with other medical screening and therapeutic interventions.”  However, they advised taking a family history of venous thrombosis to help identify families with a “thrombophilic tendency.”  FVL will be found in half of these families, who may also share one of the rarer thrombophilic gene variants, or perhaps other sources of genetic susceptibility that have yet to be identified.

line

[i] Botto LD, Khoury MJ.  Commentary:  facing the challenge of gene-environment interaction:  the two-by-four table and beyond.  Am J Epidemiol 2001;1016-1020.
[ii] Khoury MJ, Flanders WD.  Nontraditional epidemiologic approaches in the analysis of gene-environment interaction:  case-control studies without controls.  Am J Epidemiol 1996;144:207-213.

line

View all answers.

 

line

Question 1 - Answer
The prevalence of FVL was 22.6% in cases and 3.6% in controls. While 3.2% of cases were homozygous for FVL, there were no homozygotes among controls. (The expected number among controls is 0.05 based on Hardy-Weinberg equilibrium,* corresponding to prevalence of 3.2x10-4 , or 1 in 3117.) Prevalence estimates for both groups are similar to those from previous studies—some of which were based on the same study population.

*p2=163/169=0.964497, p=0.9820881, q=0.0179118, 2pq=0.0351821, q2=3.2x10-4

Question 2 - Answer
Odds ratios of 5.0, 3.7, and 3.8 obtained in this study are essentially the same, given wide confidence intervals.   However, two additional points should be considered:

  1. Data for FVL heterozygotes and homozygotes were combined in this analysis.  No odds ratio could be calculated for homozygotes because there were only five among cases and none among controls.  FVL homozygotes could be at higher risk of DVT than heterozygotes

  2. Clearly, both OC use and FVL are risk factors for DVT.  When deciding whether to prescribe OCs, clinicians are more concerned with absolute risk than with relative risk.

Question 3 - Answer
In these data,

  1. Rgesymbol double wave, not equal > Rg + Re – 1, because 30 > 7 + 4 - 1, but
  2. Rgesymbol, double wave  >Rg x Re, because 30symbol double wave, not equal > 7 x 4

These findings indicate that there is more than additive interaction; the effects of these factors are approximately multiplicative.  From a clinical standpoint, the relevant finding shows that women with FVL who use OCs are at much higher risk for venous thrombosis than are other OC users. 

Question 4 - Answer
The case-only analysis yields OR case only  symbol, double wave  1, suggesting that no interaction exists between FVL and OC use in venous thrombosis under a multiplicative model.  Additive or other models of interaction are not excluded by this result. One way to check that genotype and environmental exposure are independent is to compute OR control-only =0.79 in this study.

The case-only design does not yield estimates of the effect of  genotype or environmental exposures, or their joint effects.

Question 5 - Answer
Although the authors refer to this as a population-based study, only the cases can really be considered population-based.  The controls were recruited among friends and acquaintances of cases (60%), or partners of other patients at the anticoagulation clinics.  Friends may share the same method of birth control, which would tend to inflate the estimated person-years at risk in OC users and thus diminish the estimated absolute risk in this group. (However, the authors noted that prevalence of OC use in this study was similar to that in local population surveys.) 

Current OC use was defined as use within 30 days before thrombosis (or index date in controls).  Apportioning person-years at risk based on current exposure among controls assumes that OC use patterns were unchanged throughout the course of the study.  Subsequent studies have shown that risk of venous thrombosis is greatest during the first year of OC use.  Therefore, relevant information for assessing the effect of OC exposures should  ideally include dates of  initiation and duration of use. 

Cases and controls were matched by age, which is critical, because the risk of venous thrombosis increases dramatically with age.  However, the analysis was done in unmatched fashion.

Question 6 - Answer
Without FVL:  3.0 – 0.8 =   2.2

With FVL:     28.5 – 5.7 = 22.8

The joint effect of the two factors is more than additive.

Question 7 - Answer
Excess deaths associated with OC use in women with FVL: (0.57 – 0.11) / 10,000 = 0.46 death / 10,000 py

Number of women with FVL denied OCs: 10,000 py / 0.46 deaths = 21,739 py / 1 death

Number needed to screen to find 20,000 with FVL: 20,000 / 0.05 prevalence of FVL symbol, double wave 400,000

  • NNS = NNT/prevalence = (1/risk difference)/prevalence = 1/4.6x10-5/0.05 symbol, double wave 400,000

NOTE:  Non-use of OCs by 20,000 women for 1 year is in theory equivalent to nonuse by 10,000 women for 2 years; however, the risk for thrombosis appears to be highest in the first year.

Question 8 - Answer

  • Substantial uncertainty is inherent in all of the quantities used in this calculation—conclusions might change as parameters are varied.
  • Costs and reliability of the screening test would have to be considered, along with costs of incorrect results (misclassification).
  • Avoiding OCs could lead to use of other, potentially less desirable or less effective contraceptive measures, which are associated with other personal costs as well as consequences of unwanted pregnancies.
  • Even when not fatal, venous thrombosis causes morbidity, including morbidity resulting from anticoagulant therapy.
  • Screening in other groups (e.g., women who have had a previous venous thrombosis) might have different costs and benefits.
 

Contact Us:
  • Centers for Disease Control and Prevention
    1600 Clifton Rd. Atlanta, GA 30333 USA
    800-CDC-INFO (800-232-4636)
  • Additional information for Public Health Genomics is available on our contact page.
USA.gov: The U.S. Government's Official Web PortalDepartment of Health and Human Services
Centers for Disease Control and Prevention   1600 Clifton Road Atlanta, GA 30329-4027, USA
800-CDC-INFO (800-232-4636) TTY: (888) 232-6348 - Contact CDC–INFO
A-Z Index
  1. A
  2. B
  3. C
  4. D
  5. E
  6. F
  7. G
  8. H
  9. I
  10. J
  11. K
  12. L
  13. M
  14. N
  15. O
  16. P
  17. Q
  18. R
  19. S
  20. T
  21. U
  22. V
  23. W
  24. X
  25. Y
  26. Z
  27. #