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PubMedDate not supplied

Oral contraceptive use and venous thromboembolism: a consideration of the impact of bias and confounding factors on epidemiological studies.

Recent epidemiological studies have reported an approximately two-fold, significant, increased odds ratio for venous thromboembolism in users of third-generation oral contraceptives (OCs) compared to users of second-generation OCs. However, in each study, this association is of borderline statistical significance, and the studies do not indicate an increase in the absolute risk of venous thromboembolism for users of third-generation OCs compared with the data from prior studies primarily involving second-generation products. The data derived from these recent studies show a lower reported incidence of venous thromboembolism with the use of second-generation OCs over time compared to the prior studies, while the incidence of venous thromboembolism for use of third-generation OCs is approximately equal to that previously reported for second-generation products. Generally, evidence has shown that the risk of venous thromboembolism and the impact on hemostatic parameters are reduced with declining estrogen dose. In addition, there is no evidence of a clinically significant effect of the OC progestogen doses on hemostatic parameters. These inconsistencies point to factors other than a causal relationship to explain the higher risk of venous thromboembolism in users of third-generation OCs. An examination of factors not included in the recent studies has identified at least three potential biases that should be considered: prescription bias, a 'healthy-user' effect and referral bias. Available data on prescription bias come from marketing research surveys, databases of prescribing patterns, epidemiological databases and the recent studies. These data indicate that the recently introduced third-generation OCs have been more extensively used by younger women, for shorter periods of time, and by women with risk factors for venous thromboembolism that have the second-generation OCs. There is also evidence for a healthy-user effect, whereby women who were most susceptible for venous thromboembolism have left the cohort of users of second-generation OCs as a result of a venous thromboembolism during pregnancy or early OC use, and thus were possibly not included in these recent studies. The women remaining in the cohort, who are included in the recent studies, are less susceptible to venous thromboembolic events than are those making up the cohort of women using third-generation OCs. In addition, there are indications that referral bias has occurred because women with risk factors for venous thromboembolism (who disproportionately receive third-generation OCs) are more likely to be referred to a hospital for investigation of possible symptoms of venous thromboembolism, and thus users of third-generation OCs have the potential to be over-represented among cases. These biases act spuriously to increase the observed odds ratio for the first-time occurrence of venous thromboembolism amongst users of third-generation OCs, when compared to users of second-generation OCs. None of the recent epidemiological studies have been able to adjust for the three major biases. These biases are likely to contribute to, or even totally account, for the small increased risk of venous thromboembolism observed for users of third-generation OCs when compared to that for users of second-generation OCs.

Bias↗
PubMed1992

Two critical confounding factors in periodontal epidemiology.

The prevalence and severity of periodontitis is considerably lower than was previously estimated. The available epidemiological methods are based on the premise that loss of periodontal attachment is a unique sign of periodontitis. However, additional factors resulting in loss of periodontal attachment confound data obtained from modern studies. The physiological process of continuous tooth eruption results in loss of periodontal attachment over a lifetime, at an approximate rate of 0.1 mm/year. Anthropological and clinical evidence for continuous tooth eruption is reviewed. The position of the alveolar crest appears to be stable; loss of periodontal attachment occurs as the teeth erupt and less of the root is embedded in bone. Retrograde periodontitis caused by pulpal disease is a known cause of severe, localised destruction of periodontal tissues. Its signs and symptoms include periodontal pocket formation, purulent inflammatory exudates, angular bone loss, swelling and bleeding of the gingival tissues and increased tooth mobility. Anthropological, experimental and clinical evidence for retrograde periodontitis is discussed. Misdiagnosis of angular (hemiseptal), furcal and other forms of localised periodontal damage as periodontitis has probably resulted because of the inability to determine the quality of pulpal health and the assumption of a gingivitis-periodontitis continuum. New epidemiological methods need to be developed in which physiological and all pathological conditions which affect the periodontal attachment are recognised and appropriately categorised.

Confounding Factors, Epidemiologic↗
PubMed2005

[Smoking as potential confounder in german epidemiological studies: standardization of assessment, quantification, and analysis].

Tobacco smoking is one of the most frequently examined risk factors in epidemiological studies due to the high frequency of smoking in the population and its high risk potential. This paper discusses the issue of standardization of the assessment of tobacco exposure in German epidemiological studies based on a suggestion of the working group "Epidemiological methods" of the German Society of Epidemiology (DGEpi), the German Society of Social Medicine and Prevention (DGSMP), and German Society of Medical Informatics, Biometry and Epidemiology (GMDS) together with the working group "Epidemiology in the workplace" of the DGEpi and the German Society of Occupational and Environmental Medicine (DGAUM). The main element is a short questionnaire for German adults for the assessment of tobacco smoking in epidemiological studies where smoking is a potential confounder. In addition, information on the consideration of intermittent periods of non-smoking, a very short questionnaire for medical examinations in the workplace or non-responder interviews as well as the quantification and statistical modelling of smoking are presented. A brief discussion of scientific problems and questionnaires related to the exposure to environmental tobacco smoke is given.

Bias↗
PubMed1997

Examination of "early mortality exclusion" as an approach to control for confounding by occult disease in epidemiologic studies of mortality risk factors.

Methods for the estimation of the effects of chronic disease risk factors on mortality continue to be an area that generates confusion and controversy. In response to the frequently observed U- or J-shaped relations between risk factors and mortality, some authors suggest that subjects dying during the first k years of follow-up (where k is some positive number less than the total length of follow-up) be excluded from statistical analyses. By excluded, the authors mean completely removed from the data set. The rationale is that persons dying during the first k years are likely to have a preexisting occult disease that confounds the relation between the risk factor under study and mortality. Excluding persons dying during the first k years of follow-up purportedly reduces this confounding. However, the authors are aware of no demonstration that this procedure effectively accomplishes its goal. They show that excluding subjects who die during the first k years of follow-up does not necessarily lead to a reduction in bias in the estimated effect of a risk factor on mortality when this relation is confounded by the presence of occult disease. Moreover, it is possible for such exclusion to exacerbate the confounding due to preexisting disease. Thus, excluding subjects dying during the first k years of follow-up is not necessarily an effective strategy for dealing with confounding due to occult disease. Investigators are encouraged to pursue alternative methods.

Confounding Factors, Epidemiologic↗
PubMed2001

Hypothesis: gonadal hormones act as confounders in epidemiological studies of the associations between some behavioural risk factors and some pathological conditions.

There are grounds for suspecting that, to varying degrees, smoking, alcohol consumption, oral contraceptive use, vasectomy and induced abortion are markers for high steroid hormone levels. So in epidemiological studies, false inferences may be drawn that these markers (treated as risk factors) have causal or exacerbating effects on diseases which are truly partially caused by high levels of hormones (e.g. probably prostatic cancer and breast cancer). Analogously, such studies of conditions which are truly partially caused by low levels of hormones (e.g. bone fractures, poor sperm quality, and perhaps testicular cancer and rheumatoid arthritis) may yield spurious suggestions of an ameliorative effect. The results of epidemiological studies of the above five "risk factors" for the above six pathologies are-in many cases-in striking disarray. I suggest that this is, at least partially, because of this form of confounding. The point may be tested by contrasting the hormone levels of people who self-select for smoking, vasectomy, etc., at the time that self-selection is made with those of appropriately selected control subjects.

Abortion, Induced↗
PubMed2005

Controlling for continuous confounding factors: non- and semiparametric approaches.

Confounding is one of the major types of bias encountered in observational epidemiologic surveys designed to study the relation between an exposure factor and a health event. A common way to remove confounding bias during the statistical analysis phase is to adjust for the confounders in a regression model. If a confounding factor is assessed as a continuous variable, it is necessary to define how the variable is entered into the regression model. In the case of logistic regression, we illustrate through simulation that coding by a binary variable or a categorical variable with broad categories may lead to substantial residual confounding. Specific approaches can be used to define a coding method that limits residual confounding. Among these, we briefly present nonparametric approaches and describe in detail several semiparametric approaches (generalised partial linear models, spline regression and fractional polynomials). These can be used to estimate the relation between a continuous factor and the health event of interest by a smooth non pre-specified function. In semiparametric models, the effect of certain covariates is coded by a parametric function, whereas the coding of one or two continuous variables is represented by a nonparametric function. These models can be used in exploratory analyses to describe dose-effect relations between the confounder and the health event, and thus help to define a relevant coding for the confounder.

Abortion, Spontaneous↗
PubMed1998

Exposure to environmental tobacco smoke and risk factors for heart disease among never smokers in the Third National Health and Nutrition Examination Survey.

The relative risk of coronary artery disease among never smokers exposed to environmental tobacco smoke (ETS) versus never smokers not exposed to ETS is approximately 1.2 based on more than a dozen epidemiologic studies. Most of these studies have controlled for the major heart disease risk factors, but residual or uncontrolled confounding remains a possible explanation for the epidemiologic findings. The authors studied 3,338 never-smoking adults aged 17 years or older, who are representative of all US never smokers, in the 1988-1991 Third National Health and Nutrition Examination Survey (NHANES III) to determine whether selected risk factors for heart disease differ between ETS-exposed and -nonexposed persons. Both self-reported ETS exposure (at home and at work) and serum cotinine levels were available, the latter reflecting recent ETS exposure. After adjustments were made for age, sex, race, and education among adults aged 17 years or older, no significant differences were found between the ETS exposed and the nonexposed for any of 13 cardiovascular risk factors with the exception of dietary carotene, which was lower among the exposed. On the other hand, significant positive linear trends were found between serum cotinine and two risk factors (body mass index and alcohol consumption), and significant inverse trends were found with dietary carotene. There were also few differences between exposed and nonexposed never smokers among adults aged 40 years or older, who are most at risk of heart disease. In this group, however, there was an inverse linear trend between serum cotinine and high density lipoprotein cholesterol (p < 0.001). This finding could result from ETS exposure rather than be an indication of confounding; a similar inverse trend was found for children, confirming other results in the literature. Overall, these data suggest little potential for confounding by the heart disease risk factors studied here when ETS exposure is determined by self-report.

Adolescent↗
PubMed1995

Poppers, Kaposi's sarcoma, and HIV infection: empirical example of a strong confounding effect?

Are there empirical examples of strong confounding effects? Textbooks usually show examples of weak confounding or use hypothetical examples of strong confounding to illustrate the paradoxical consequences of not separating out the effect of the studied exposure from that of second factor acting as a confounder. HIV infection is a candidate strong confounder of the spuriously high association reported between consumption of poppers, a sexual stimulant, and risk of Kaposi's sarcoma in the early phase of the AIDS epidemic. To examine this hypothesis, assumptions must be made on the prevalence of HIV infection among cases of Kaposi's sarcoma and on the prevalence of heavy popper consumption according to HIV infection in cases and controls. Results show that HIV infection may have confounded the poppers-Kaposi's sarcoma association. However, it cannot be ruled out that HIV did not qualify as a confounder because it was either an intermediate variable or an effect modifier of the association between popper inhalation and Kaposi's sarcoma. This example provides a basis to discuss the mechanism by which confounding occurs as well as the practical importance of confounding in epidemiologic research.

Confounding Factors, Epidemiologic↗
PubMed2005

Three Mile Island epidemiologic radiation dose assessment revisited: 25 years after the accident.

Over the past 25 years, public health concerns following the Three Mile Island (TMI) accident prompted several epidemiologic investigations in the vicinity of TMI. One of these studies is ongoing. This commentary suggests that the major source of radiation exposure to the population has been ignored as a potential confounding factor or effect modifying factor in previous and ongoing TMI epidemiologic studies that explore whether or not TMI accidental plant radiation releases caused an increase in lung cancer in the community around TMI. The commentary also documents the observation that the counties around TMI have the highest regional radon potential in the United States and concludes that radon progeny exposure should be included as part of the overall radiation dose assessment in future studies of radiation-induced lung cancer resulting from the TMI accident.

Body Burden↗
PubMed2004

Vignettes of the history of epidemiology: Three firsts by Janet Elizabeth Lane-Claypon.

In 1912, Janet Elizabeth Lane-Claypon, a British medical scientist 35 years of age who had already contributed substantial research findings in the fields of reproductive physiology and the bacteriology and biochemistry of milk, reported the results of a retrospective cohort study of weight gain during the first year of life among 204 infants fed boiled cows' milk compared with 300 infants fed human breast milk. The results of her investigation revealed that, up to the age of 208 days, breastfed infants gained more weight than infants fed boiled cows' milk. After that time period, weight gain was equal in the two groups. Lane-Claypon described, discussed, and analyzed her data for the possibility that her findings were due to sampling variation or confounding, and she used Student's t test to evaluate observed differences in weight gain in small subsets of the study population. As far as is known, this was the first use of the retrospective (historical) cohort design and the t test in an epidemiologic study.

Confounding Factors, Epidemiologic↗
PubMed2003

Cancer risks in men who had children with different partners from the Swedish Family-Cancer Database.

We used the nation-wide Swedish Family-Cancer Database to analyse cancer risks in men who had had children with more than one woman. Cancer cases were retrieved from the Swedish Cancer Registry from years 1961-1998. A total of 2.9 million men and 298,134 cancer cases were covered. For men having children with two, three or more women, increasing risk trends were shown for upper aerodigestive tract, lung, urinary bladder and oesophageal cancers. Decreasing trends were observed for tumours of the colon, skin (squamous cell and melanoma), nervous system and endocrine glands and against myeloma and non-Hodgkin's lymphoma. The present results indicated that men who had had children with multiple women showed an excess of smoking- and alcohol consumption-related cancers. The decreased risks for colon cancer, non-Hodgkin's lymphoma and melanoma were possibly related to lifestyle factors connected with economic deprivation, less obesity and physical fitness. These ill-defined protected factors may be a challenge to epidemiological studies.

Aged↗