© 2000 American Heart Association, Inc.
Thrombosis |
Determinants of Population Changes in Fibrinogen and Factor VII Over 6 Years
The Atherosclerosis Risk in Communities (ARIC) Study
From the Division of Epidemiology (A.R.F., M.R.), School of Public Health, University of Minnesota, Minneapolis; the Division of Hematology (K.K.W., N.A.), University of Texas Medical School, Houston; the Collaborative Studies Coordinating Center (L.E.C.), Chapel Hill, NC; and the School of Hygiene and Public Health (F.J.N.), Johns Hopkins University, Baltimore, Md.
Correspondence and reprint requests to Aaron R. Folsom, MD, Division of Epidemiology, School of Public Health, University of Minnesota, Ste 300, 1300 S 2nd St, Minneapolis, MN 55454-1015. E-mail folsom{at}epivax.epi.umn.edu
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Abstract |
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Abstract—Although numerous cross-sectional studies have identified possible determinants of plasma fibrinogen and factor VII levels, few prospective studies exist. We assessed the longitudinal relation of changes in fibrinogen and factor VII over 6 years with changes to other cardiovascular risk factors in a sample of 440 men and 549 women from the Atherosclerosis Risk in Communities (ARIC) study. Fibrinogen increased more in older participants, those with or who developed diabetes, those who at any time smoked, and those whose plasma HDL cholesterol or triglycerides decreased and increased less in female participants who started hormonal replacement therapy. Factor VII coagulant activity increased more in younger participants, women, those who gained greater weight or developed diabetes, those who quit smoking, those in whom plasma triglycerides decreased, and female participants who received hormonal replacement therapy. Thus, our longitudinal data suggest with some exceptions that adverse changes in cardiovascular risk factors are accompanied by increases in plasma levels of fibrinogen and factor VII.
Key Words: fibrinogen • factor VII • diabetes mellitus • studies, prospective
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Introduction |
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Considerable epidemiological evidence implicates plasma fibrinogen1 2 3 4 5 6 7 and possibly factor VIIc2 5 as risk factors for coronary heart disease (CHD). Cross-sectional epidemiological studies and short-term experiments have identified several determinants of fibrinogen and factor VIIc levels.8 9 However, we know of only 1 longitudinal population-based study of long-term changes in hemostatic factors.10 That study showed that changes in fibrinogen were positively associated with changes in cigarette smoking and negatively associated with changes in alcohol consumption but were not associated with weight change. In addition, factor VIIc increased with weight gain but was not independently related to changes in smoking and alcohol intake.10
To further examine potential determinants of long-term changes in fibrinogen and factor VIIc, we repeated these measurements 6 years after baseline in a population-based study, the Atherosclerosis Risk in Communities (ARIC) study.
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Methods |
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Study Population
The ARIC study11 selected population samples from Forsyth County, NC; Jackson, Miss (blacks only); the northwest suburbs of Minneapolis, Minn; and Washington County, Md, and examined a cohort that totaled 15 792 persons between 45 and 64 years of age at recruitment from 1987 through 1989. The study reexamined participants from 1990 through 1992 (93% return rate) and from 1993 through 1995 (86% return rate). We measured hemostatic factors in the entire cohort during the 1987 through 1989 examination and in a stratified sample in the 1993 through 1995 examination. The 1993 through 1995 sample included a random sample of the 12 885 returning participants, with nonmissing values for smoking and diabetes status and oversampling for new diabetics and people who quit smoking. The sample (n=1030) included 122 (20%) of the new diabetics but not cigarette quitters, 17 (26%) of the new diabetics who quit cigarettes, 134 (17%) of the other cigarette quitters, and 757 (7%) of the remaining 1993 through 1995 group.
Measurements
After participants underwent an 8-hour fasting period,
technicians drew blood from an antecubital vein in each participant,
with minimal trauma.. We have published detailed methods for hemostatic
factors.12 13 In brief, the laboratory measured fibrinogen
by the thrombin-time titration method14 with reagents and
calibration materials (Fibriquik) obtained from General
Diagnostics (Organon-Technika Co). Factor VII coagulant
activity (factor VIIc) was measured by
determination of the ability of the testing sample to correct the
clotting time of human factor VII–deficient plasma obtained from
George King Biomedical Inc. The reference material for assays was the
universal coagulation reference plasma (Thromboscreen;
Pacific Hemostasis; Curtin Matheson Scientific, Inc). During the 1987
through 1989 period, the reliability coefficients (method variance plus
intraindividual variance), obtained from repeated testing of
individuals during several weeks, were r=0.72 for
fibrinogen, and r=0.78 for factor VII.15
The Pearson correlation on 815 split, blind, duplicate specimens was
r=0.88 for fibrinogen and r=0.86 for factor
VIIc for the 1987 through 1989 examination, and
on 33 duplicates was r=0.61 for fibrinogen and
r=0.56 for factor VIIc for the 1993
through 1995 examination. During the entire 6-year interval, laboratory
controls were continuously in the established range.
At each examination, we asked participants whether they smoked
cigarettes and, if so, the number of cigarettes smoked per day.
We classified participants as continued nonsmokers, continued smokers,
new smokers, or quitters. We assessed usual alcohol intake and
classified it as an increase, no change, or decrease during the 6
years. We defined diabetes mellitus as a fasting glucose level 
140
mg/dL, nonfasting glucose level 200 mg/dL, or a history of or
treatment for diabetes. We classified participants as having become
diabetic, staying diabetic, or never having had diabetes. Technicians
measured height and weight with participants in scrub suits, and we
computed the body mass index (in kg/m2). We
assessed physical activity by a sport index with the use of a
questionnaire by Baecke et al.16 For analysis, we
categorized changes in BMI and the sport index in tertiles.
The laboratory staff measured plasma total cholesterol and triglycerides by enzymatic methods17 18 and calculated LDL cholesterol.19 HDL cholesterol level was measured after dextran-magnesium precipitation of non-HDL lipoproteins.20 We categorized changes in plasma lipids in tertiles.
Statistical Methods
A total of 549 women and 440 men had fibrinogen and factor
VII measurements taken at baseline and 6 years. To account for the
stratified random sampling design, we conducted analyses using
SUDAAN software21 and weighted each observation for the
inverse of the sampling fraction. We first computed the mean values or
prevalences (in percentage) of risk factors in the 1987 through 1989
and 1993 through 1995 examinations. We then used ANCOVA to relate
changes to adjusted mean fibrinogen (or factor VII) over the 6 years to
changes in other risk factors, with adjustments for age, race, sex, and
initial value of fibrinogen (or factor VIIc). We
adjusted for initial values of fibrinogen (or factor
VIIc) because it is a likely confounder of the
association between risk factor change and change in fibrinogen (factor
VIIc). We computed the adjusted mean change in
both fibrinogen and factor VIIc by category of
risk factor change and by net change (by subtraction) relative to a
specified reference category of risk factor. We present overall
probability value for differences between groups.
We also fit multiple linear regression to calculate the adjusted mean
change of fibrinogen (or factor VIIc) in relation
to multiple other risk factor changes simultaneously. We
included the baseline values of fibrinogen (or factor
VIIc) as covariates. The reported regression
coefficient (
) represents the change in the
hemostatic factor per level of change in the determinant, with
adjustments for the other variables in the model.
We also considered the impact that measurement error of baseline
fibrinogen (or factor VIIc) would have on our
results. In theory, measurement error due to laboratory or
within-person variability could bias the estimates if
baseline fibrinogen (or factor VIIc) were related
to risk factor changes. We corrected for the bias from measurement
error by adapting published methods.22 Results after
correction were similar to the uncorrected results; thus, only the
latter are presented.
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Results |
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The ARIC sample with measurements of fibrinogen and factor VIIc levels in both the 1987 through 1989 and 1993 through 1995 periods included 549 women and 440 men; 25% were blacks and 75% were whites. During the 6 years between examinations, mean alcohol intake in these participants decreased 7 g/wk; BMI increased 1.0 kg/m2; the sports index increased 0.1 unit; LDL cholesterol declined 11 mg/dL; HDL cholesterol increased 0.76 mg/dL; triglycerides increased 13 mg/dL; and among women, hormone replacement therapy increased nearly 20% (Table 1
).
As expected from oversampling and cohort aging, the prevalence of
smoking decreased and of diabetes increased.
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Baseline mean fibrinogen (298 mg/dL) and factor VIIc (118%) levels in this sample assessed twice were similar to the baseline mean fibrinogen (304 mg/dL) and factor VIIc (119%) levels in the entire ARIC cohort (n=15 792). During the 6 years between examinations, mean fibrinogen in the sample increased 26 mg/dL and mean factor VIIc decreased 12%. Spearman correlations for 1987 through 1989 and 1993 through 1995 values were 0.51 for fibrinogen and 0.52 for factor VIIc.
Mean changes in fibrinogen and factor VIIc in
relation to changes in other risk factors are shown in Tables
2 (adjusted for age, race, sex, and
baseline level) and 3
(multivariately). In multivariate
models without biochemical factors (Table 3, model 1),
fibrinogen appeared to increase slightly more in older versus younger
participants, 14 to 29 mg/dL more in those with or who developed
diabetes versus nondiabetics, and 10 to 13 mg/dL more in those who at
any time had smoked versus never-smokers. Conversely, in comparison
with continued smokers, those who quit had a 2 to 3 mg/dL rise in
fibrinogen. In model 1, for women only (not shown in Table 3),
starting hormonal therapy was associated with a 20-mg/dL-greater
decline (P=0.02), continuation of hormonal therapy with a
16-mg/dL-greater decline (P=0.11), and cessation of hormonal
therapy with a 10-mg/dL-greater rise (P=0.66) compared with
women who were not treated with hormonal replacement. Addition of lipid
variables (model 2) did not greatly change the patterns observed in
model 1, except that the association for starting hormone replacement
weakened by approximately one half. Fibrinogen increased more in those
whose HDL cholesterol or triglycerides
decreased.
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With model 1 (Table 3
), factor VIIc
increased more in younger versus older participants, 11% more in women
versus men, 8% per 5 kg/m2 increase in BMI, 8%
more in those who quit smoking, and 6% more in those who developed
diabetes. In model 1, for women only (not shown in Table 3),
starting hormonal therapy was associated with a 7%-greater increase in
factor VIIc (P=0.10), continuation of
hormonal therapy with a 9%-greater increase (P=0.01), and
cessation of hormonal therapy with a 3%-greater increase
(P=0.68) compared with women who were not treated with
hormonal replacement. Addition of lipid variables (model 2)
attenuated the relation between factor VIIc
change and BMI change (and hormonal therapy initiation) by 
40%.
Increases in triglyceride concentrations appeared to
increase factor VIIc.
The relation of fibrinogen to smoking or quitting is shown further in
the Figure. Compared with nonsmokers at
both visits, those who started smoking or recently quit had the most
dramatic rises in fibrinogen concentration. Yet the continued smokers
and longer-term quitters still had slightly greater rises in fibrinogen
than the never-smokers. However, owing to limited sample sizes, none of
these apparent differences were statistically significant.
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Discussion |
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Over the last decade, researchers have discovered that virtually every cardiovascular risk factor is correlated cross-sectionally with plasma fibrinogen concentrations,9 and several are correlated with factor VIIc.8 Clearly, not all of these correlations reflect causal relations. Two types of studies that can help to clarify the true determinants of fibrinogen and factor VIIc include controlled clinical trials and population studies of changes in hemostatic factors. Clinical trials (which should be randomized and controlled because of potential bias, laboratory drift, and regression to the mean) generally focus on short-term changes. Epidemiological studies, such as ours, can examine long-term changes in representative population samples, but without randomization, such studies may exhibit selection bias. For example, people who chose to quit smoking may have uncontrollable differences from other subjects that affect physiological changes.
Over 6 years, we found an overall increase in mean fibrinogen of 26 mg/dL and a decrease in mean factor VIIc of 12%. These changes undoubtedly are partly due to aging and true secular changes. It is nevertheless virtually impossible to rule out some laboratory drift, although blood collection, laboratory methods, and assay control materials were identical for the 1987 through 1989 and 1993 through 1995 periods. As a result, we were uncertain of the absolute level of fibrinogen and factor VIIc change for each individual, but no reason exists to suspect that laboratory drift would have been differential with respect to the correlates we studied. Thus, our measure of net change should have properly ranked individuals and permitted us to assess whether changes in (rank of) fibrinogen and factor VIIc were related to changes in other risk factors.
Our main finding for fibrinogen was that a greater 6-year increase occurred for participants who developed diabetes. Previous studies have established a strong cross-sectional association between diabetes and fibrinogen,9 but no study has explored whether change in fibrinogen level over time also differs according to diabetes status. Diabetes is a prothrombotic condition, and elevated fibrinogen appears to contribute to increased risk of cardiovascular disease in diabetics.23 Fibrinogen appears to be elevated particularly in diabetics with poor metabolic control.24
Fibrinogen increased more in those who reported starting, continuing, or recently quitting smoking than in consistent nonsmokers, but these findings were statistically nonsignificant due, in part, to the small sample sizes. Other than chance occurrence, we are unclear as to why quitting smoking did not reduce fibrinogen in the present study as it did in short-term clinical trials of smoking cessation9 25 and the longitudinal study by Meade et al.10 Lifestyle factors not related to fibrinogen change were alcohol change, BMI change, and sports score change. These findings are consistent with those of most clinical trials9 ; however, Meade et al found longitudinal changes in fibrinogen negatively associated with changes in alcohol consumption.10
We also found that participants who had larger increases in HDL cholesterol and triglycerides seemed to have a greater decline in fibrinogen concentrations. Cross-sectional data also have consistently found HDL cholesterol inversely associated with fibrinogen.9 In the ARIC study, positive cross-sectional univariate association existed between triglycerides and fibrinogen that reversed to a negative association with adjustment for multiple covariates.8 This suggests that no real association may exist between triglycerides and fibrinogen, and that our finding of one may be an artifact of statistical overadjustment for triglyceride correlates such as race, obesity, diabetes, and HDL cholesterol.
Factor VIIc decreased overall, but net increases (or smaller decreases) were apparent in several groups. In particular, factor VIIc increased more in participants with greater weight and triglyceride increases; in those who became diabetic; and in women, particularly those who were treated with hormonal replacement therapy. Clinical trials and longitudinal studies support weight and triglyceride associations with factor VIIc.9 10 Cross-sectional studies support higher factor VIIc levels in women and diabetics,9 but we believe that no previous study has documented greater longitudinal factor VIIc increase in people who develop diabetes or in women. A high proportion of ARIC women were perimenopausal; thus, endogenous hormonal changes could be important. On the other hand, hormonal replacement therapy increases triglyceride concentrations, which probably explains why hormonal replacement also raises factor VIIc.
We observed no change in factor VIIc with changes in alcohol intake or physical activity, consistent with most clinical trials and longitudinal studies,9 and we observed no factor VIIc change with changes in HDL or LDL cholesterol levels. People who started smoking had a net decrease in factor VIIc, and those who quit had an increase. Factor VIIc change has not consistently been associated with smoking changes previously,10 although we have reported an inverse cross-sectional association in men.8 It nevertheless seems paradoxical that smoking, a prothrombotic stimulant, would be associated with lower factor VIIc than is nonsmoking. Because smoking affects body weight and triglyceride levels, our smoking findings may reflect residual effects of these determinants on factor VIIc.
The public health importance of elevated fibrinogen and factor VIIc remains uncertain. Fibrinogen is clearly a marker of increased risk for CHD.1 2 3 4 5 6 7 Factor VIIc is a less-consistent CHD risk marker.2 5 6 Whether lowering fibrinogen or factor VIIc could reduce CHD risk is uncertain, but it nevertheless seems prudent to avoid high levels of these clotting factors. Our finding that smoking, obesity, diabetes, and hypertriglyceridemia may affect these hemostatic factors suggests that unhealthy lifestyles may increase CHD risk, in part, by thrombotic mechanisms. Our findings also provide additional support for control of lifestyle risk factors for cardiovascular health.
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Acknowledgments |
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The ARIC study was funded by contracts N01-HC-55015, N01-HC-55016, N01-HC-55018, N01-HC-55019, N01-HC-55020, N01-HC-55021, N01-HC-55022 from the US National Heart, Lung, and Blood Institute. The authors thank Mark Triputti for programming assistance; Laura Kemmis for manuscript preparation; and the following ARIC staff members for their contributions: Phyllis Johnson, Catherine Paton, James Pankow, and Sharon Pope from the University of North Carolina, Chapel Hill; Melinda Cochran, Shirley Cothern, Amy Haire, Delilah Posey, and Carol Smith from the University of North Carolina, Forsyth County; Bobbie Alliston, Agnes Hayes, Penny Lowery, Stephanie Parker, and Betty Warren from the University of Mississippi Medical Center, Jackson; Collette Cosgriff, Maxine Dammen, Caryl DeYoung, and Jaci Dion from the University of Minnesota, Minneapolis; Patricia Crowley, Tammy Crunkleton, Lily Downs, and Chris Fornwalt from the Johns Hopkins University, Baltimore, Md; Chul Ahn, Ashley Ewing, Harinder Juneja, and Susan Mitterling from the University of Texas Medical School, Houston; Wanda Alexander, Christine Ballantyne, Charles Rhodes, and Andre Surguchov from the Methodist Hospital, Atherosclerosis Clinical Laboratory, Houston, Tex; Carolyn Bell, Delilah Cook, Bob Ellison, and Kathy Joyce from the Bowman-Gray School of Medicine, Ultrasound Reading Center, Winston-Salem, NC; and Margaret Misch, Eunsik Park, Debbie Rubin-Williams, and June Stevens from the ARIC Coordinating Center, University of North Carolina, Chapel Hill.
Received March 15, 1999; accepted August 13, 1999.
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