Gulf War and Health Volume 2: Insecticides and Solvents (2003) / Chapter Skim
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2. Identifying and Evaluating the Literature
2. Identifying and Evaluating the Literature
Pages 17-38
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From page 17... ...
Similarly, studies of occupations with exposure to multiple agents (for example, farmers, agncultural workers) that did not address specific agents were excluded, as were studies of short-term outcomes.
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From page 18... ...
Accordingly, the criteria for each category express a decree of confirl~nre based on the extent to which sources of error were reduced. Sup f icien t Evid en ce of a Ca usa ~ R ela tionsh ip O ~ Evidence from available studies is sufficient to conclude that a causal relationship exists between exposure to a specific agent and a specific health outcome in humans, and the evidence is supported by experimental data.
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From page 19... ...
Limited/Suggestive Evidence of an Association Evidence from available studies suggests an association between exposure to a specific agent and a specific health outcome in human studies, but the body of evidence is limited by the inability to rule out chance and bias, including confounding, with confidence. For example, at least one high-quality study reports a positive association that is sufficiently free of bias, including adequate control for confounding.
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From page 20... ...
Epidemiologic studies can establish statistical associations between exposure to specific agents and health effects, and associations are generally estimated by using relative risks or odds ratios. To conclude that an association exists, it is necessary for exposure to an agent to occur with the health outcome more frequently than expected by chance alone.
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From page 21... ...
Epidemiologic Study Designs Ecologic Studies In ecologic studies exposure to specific agents and disease are measured in populations as a whole. The data are presented as averages or rates within populations, and multiple populations are examined.
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From page 22... ...
The weakness of such study designs is their inability to measure multiple exposures. Retrospective cohort studies often focus on mortality rather than incidence because of the relative ease of determining the vital status of subjects in the past and the availability of death certificates to detennine the cause of death.
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From page 23... ...
The cases may respond to questions about past biologic or chemical exposures differently from controls because the cases have already developed the disease. For example, they may overreport being exposed to specific agents in an attempt to "explain" their disease or might underreport such exposures.
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From page 24... ...
, computed by dividing the risk or rate of developing the disease or condition over the followup period in the exposed group by the risk or rate in the unexposed group. An relative risk greater than ~ suggests that exposed subjects are more Iikelv to rlF~v~lon the n~~tr~rn ~ ~ Hi "~ ~ r ~~ than unexposed subjects, that is, it suggests a positive association between exposure to the putative agent and the disease.
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From page 25... ...
Graphic assessment of the relation is helpful in this regard. Assessing the Validity of Findings ,,' ~ _ ~ As described above, the goal of observational epidemiologic studies is to examine associations between exposures to particular agents and health outcomes in a population.
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From page 26... ...
l Epically, ap value of less than 0.05 is taken to be indicative that such a result would be "unlikely" if no true association existed and consequently provides evidence of a real association. A relative risk close to ~ indicates that there is little appreciable difference in risk (rates)
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From page 27... ...
Selection' Bias Selection bias can occur in the recruitment of study subjects to a cohort. For example, in a retrospective cohort study, when the exposed and unexposed groups are selected differentially on outcome, the assembled cohort can differ from the target population with respect to the association between exposure to the agent under study and disease outcome.
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From page 28... ...
Recall bias is of special concern in case-control studies: a differential likelihood of exposure reporting between study groups can be related to disease status rather than to the actual biologic or chemical exposure. The assumption is that cases would be more likely to report exposure to specific agents and to report it more fully.
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From page 29... ...
can be used with some realistic values of the proportion of workers smoking at varying intensities and the relative risk of developing lung cancer to obtain a series of expected relative risks under the assumption that there is no effect of exposure. For example, assume that there is a cohort of subjects exposed to organic solvents and the end point is incidence from lung cancer.
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From page 30... ...
Because exposure estimation is complex, we cover here only some of the major points that influence the interpretation of results of epidemiologic studies4. The discussion highlights many of the challenges that the committee members faced as they assessed a study's findings and drew conclusions about the strength of a reported association between a specific agent and a health outcome.
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From page 31... ...
. Brent study designs; the main ones are retrospective cohort studies and population-based case-control studies.
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From page 32... ...
The above description represents the gold standard by which exposures to various agents are attributed in retrospective cohort studies. Prospective cohort studies can use the same approach but accuracy can be improved if a detailed measurement program is included.
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From page 33... ...
Information on a health outcome can be obtained in many ways in observational studies, including direct questioning of subjects about symptoms before diagnosis, clinical examination of subjects, medical-record review, and access to vital-statistics registries to determine diagnoses or causes of death of study subjects (Rothman and Greenland, 1998~. There are a number of issues related to the use of each of those methods, including the reliability of self-reported symptoms, the accuracy of medical records, and variability in the degree to which death certificates correctly specify causes of death.
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From page 34... ...
It is thought that mechanistic understanding will make the use of experimental data easier and more reliable for predicting health outcomes in humans. Toxicologic studies have several important advantages over epidemiologic studies.
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From page 35... ...
A fairly large body of evidence supports the use of animal toxicity results to predict effects in humans. But there are still substantial uncertainties regarding the interpretation and predictive value of animal data.
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From page 36... ...
In the present case, however, the goal is not prevention of risk, but rather the use of the best available data to categorize evidence for a relationship between a chemical exposure and the occurrence of an adverse health outcome in humans. Here, precautionary policies have no substantial role (at least not the same way that they have in regulation)
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From page 37... ...
Recommendations for exposure assessment. American Journal of Industrial Medicine 26~3~:313-326.
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From page 38... ...
1997. Invited commentary: How would we know a Gulf War syndrome if we saw one?
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