Epidemiologic Studies of Veterans Exposed to Depleted Uranium Feasibility and Design Issues (2008) / Chapter Skim
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2 Elements of an Epidemiologic Study
2 Elements of an Epidemiologic Study
Pages 9-24
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From page 9... ...
The elements include identification of a relevant study population of adequate size; appropriate assessment and accurate measurement of uranium exposure in the population, including the use of biomarkers when available; an evaluation of long-term health outcomes; adequate followup time; use of reasonable methods for controlling critical confounding factors and minimizing bias; and appropriate statistical analyses. Key issues related to sample size requirements for ensuring adequate statistical power in detecting small effects, the accuracy of exposure measurement, and the need to control critical confounders are addressed in detail.
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From page 10... ...
A previous Institute of Medicine (IOM) report discussed the importance of adequate sample size for studying the health of Gulf War veterans: "sufficient samples sizes for each cohort in the study are crucial to ensure adequate statistical power to find differences as well as to reliably identify the lack of differences between groups" (IOM, 1999)
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From page 11... ...
Detecting smaller associations between DU and health effects would require studies with even greater numbers of exposed subjects to have sufficient statistical power. The committee acknowledges the difficulty of detecting such a small relative risk in epidemiologic studies.
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From page 12... ...
Renal Function The committee considered the minimal sample size required to detect a statistically significant difference in mean serum creatinine concentration, a biomarker for renal injury, between DU-exposed and -unexposed subjects. Again, the committee set the type I error at 5% and the power at 80%.
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From page 13... ...
That might make it possible to conduct a high-quality study of the association between DU exposure and renal function, as measured by serum creatinine concentration, as long as accurate exposure assessment and outcome assessment are available. If such a study is undertaken, the committee advises the investigators to inflate the targeted sample sizes above to compensate for exposure-classification issues, attrition of subjects by dropout, other losses to followup, and deaths over the 20-year followup period.
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From page 14... ...
Unexposed Total Sample 1.10 37,117 148,466 185,583 1.25 6,356 25,423 31,779 1.50 1,760 7,038 8,798 2.00 523 2,092 2,615 exposure classification, anticipated attrition of subjects by dropout, other losses to followup, and death over the 20-year followup period. As in the above examples, the committee assumed a ratio of 4:1 unexposed to exposed subjects.
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From page 15... ...
For example, it can be difficult to communicate to a person that the finding of a statistically significant difference in mean urinary uranium concentration does not necessarily imply important clinical toxicity. Statistical significance gives an indication of the probability that observed differences between groups are due to chance.
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From page 16... ...
The advantages of using film badges include the ability to link a specific dose to a particular exposure incident and a specific dose to an individual worker. In addition to measuring radiation exposure, other types of personal monitoring devices can be used to assess chemical exposure.
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From page 17... ...
A previous Gulf War committee (IOM, 1999) recommended two options for improving response rates: encouraging veteran participation in organizing, designing, and implementing the study; and offering incentives for study participants.
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From page 18... ...
Bias related to self-reporting of exposure and bias related to exposure misclassification may compromise the results of an exposure assessment. Other common sources of information bias are the inability of study subjects to recall accurately the circumstances of their exposure (recall bias)
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From page 19... ...
Adequate Followup Period An adequate followup period is necessary to allow sufficient time after exposure for relevant health outcomes to occur. That is particularly true for outcomes that have long latent periods, as do most cancers and renal diseases.
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From page 20... ...
In principle, the influence of confounding factors on the assessment of the relationship between DU exposure and health outcomes can be overcome to some extent through study designs and data-analysis schemes that address such three-way relationships. One approach to controlling for the influence of confounding factors is a study design that uses algorithms for matching study subjects according to the confounding factors, for example, ensuring that the DU-exposed and -unexposed study groups have nearly identical patterns of smoking.
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From page 21... ...
ASSESSING THE STRENGTH OF THE EVIDENCE A well-designed epidemiologic study that includes accurate exposure assessment and outcome assessment can be used to evaluate exposure to DU and reach conclusions about the potential for adverse health outcomes. Previous IOM reports, including Gulf War and Health, Volume 1: Depleted Uranium, Pyridostigmine Bromide, Sarin, Vaccines (IOM, 2000)
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From page 22... ...
It can test hypotheses about whether an exposure to a specific agent is related to the development of a health effect and can examine multiple health effects that may be associated with exposure to a given agent. A cohort study starts by classifying study participants according to whether they have been exposed to the agent under study, in this case DU.
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From page 23... ...
The committee estimated the minimal sample size required to detect statistically significant differences in risk of lung cancer and in renal malfunction between DU-exposed and -unexposed samples. REFERENCES ACS (American Cancer Society)
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