93%). Those data, however, were available only for the 45.2% who signed consent forms that allowed researchers to verify records.
The concerns relating to symptom self-reporting are best addressed through medical evaluations in which the veterans’ reports of symptoms can be further explored with a health professional. This was done by VA researchers (for example, Eisen et al., 2005) and by several other investigators with the resources to conduct medical evaluations (for example, Sim et al., 2003). The committee acknowledges that many medical conditions, such as chronic fatigue syndrome (CFS) and fibromyalgia, are diagnosed solely on the basis of symptom reports, but where possible these symptoms should be evaluated by a medical professional.
Another limitation of most major cohort studies is self-reporting of exposures, often years after the exposure had occurred. Self-reporting of exposures, as with self-reporting of symptoms, introduces the possibility of recall bias, the tendency for participants who are symptomatic to overestimate (or underestimate) their exposures compared with those who are not symptomatic. Indeed, a major study from the United Kingdom found that Gulf War veterans with more symptoms were likely to report more exposures than those not deployed to the gulf (Unwin et al., 1999). Exposures often cannot be validated by objective means, may have occurred years earlier, and might have been perceived rather than actual (Fricker et al., 2000). For example, the high sensitivity of chemical-warfare monitors used in the Persian Gulf War to warn of impending attacks led to many false alarms, which might have been perceived by veterans as actual exposures. Enhanced recordkeeping and monitoring of environmental exposures during and after the Gulf War would have averted this exposure reporting problem. Indeed, many expert panels have recommended improved recordkeeping and environmental monitoring in future deployments (for example, IOM, 1999; NRC, 2000a,b,c).
Other limitations of the body of evidence are that studies might be too narrow in their assessment of health status, the measurement instruments might have been too insensitive to detect health problems that affect deployed veterans, and the period of investigation has been too brief to detect health outcomes that have a long latency or require many years to progress to the point where disability, hospitalization, or death occurs. Virtually all US studies are cross-sectional, and this limits the opportunity to learn about symptom duration and chronicity, latency of onset (especially for health conditions with a long latency, such as cancer), and prognosis.
This chapter describes the major cohort studies and their derivative studies used by the Update committee to provide the evidence for the conclusions presented in Chapter 4. For each separately assembled cohort, the reference study is described first, followed by a summary of any derivative studies cited in Chapter 4 or Appendix A. The majority of the cohort studies are population-based, although some military-unit-based studies are described later in the chapter. Table 3-1 lists the reference and derivative studies of each cohort cited in this chapter and Chapter 4.
The committee did not identify any new major cohort studies for this report. Furthermore, not all derivative studies for a particular cohort study are included in this chapter or in Chapter 4. In many cases, the derivative studies identified by the committee from the literature searches were highly specialized; for example, they reported on family issues, pathologic changes in a subgroup of veterans, or treatment outcomes. Such studies are not included in this chapter or in Chapter 4.