were asked to provide their medical records, as well as those of their children (up to age 17), their present partner, and any previous partner. These hard copy records were duplicated and exist as part of AFHS data assets. In addition, questionnaires were administered eliciting information on education; employment; income; marital and fertility history; child and family health; health habits; recreation, leisure, and physical activities; toxic substances exposure; military experience; and wartime herbicide exposure. In all, 2,758 subjects participated in at least one cycle exam.

In addition, reproductive data were gathered and coded on 9,921 conceptions and 8,100 live births. The mortality component of the study has followed for nearly 25 years more than 20,000 Vietnam War-era veterans who served in Southeast Asia.

More than 86,000 biologic specimens have been collected over the course of the study; approximately half of these are serum. Blood was collected in all six cycles and serum stored; stored semen is available only from Cycle 1 (1982), and urine is available only from Cycles 1–3 (1982, 1985, and 1987). In the last (sixth) cycle, whole blood was also stored. Chapters 3 and 4 detail the data and specimens collected over the course the study.

Participation rates in the morbidity study have been relatively high, particularly among the Ranch Hands. Of the 1,208 Ranch Hands eligible at the time of the baseline examination, 1,045 (87 percent) took part. Although participation has dropped over time—from 85 percent at Cycle 2 to 74 percent at Cycle 6—the relatively high participation rates suggest that there may not be substantial selection biases in the cohort. Selection bias would exist, for example, if subjects with a health outcome differentially dropped out by exposure status. Among refusal subjects at Cycle 6, 5.4 percent of Ranch Hand refusals cited health reasons as their barrier to participation. Among comparison subjects, 5.1 percent of original comparisons and 5.4 percent of replacement comparisons refused to participate due to health reasons (AFHS, 2005). Passive refusal (non-response to invitation) rates were higher for comparison subjects versus Ranch Hands. The committee notes that the rate of attrition over time is high enough to potentially introduce other sources of bias, and that the introduction of such biases could compromise the internal validity of the study. However, it also notes that AFHS participation rates are comparable to those commonly observed in other longitudinal epidemiologic studies.

All told, the AFHS has more data points and a higher rate of follow-up than the Framingham Heart Study, according to a USAF representative (RHAC, 2004). Approximately $143 million has been spent or allocated for collecting, managing, and analyzing these materials.

Although several other epidemiologic studies of Vietnam veterans have been conducted, few have longitudinal data or have collected and analyzed any biologic samples. Two ongoing studies—the VA Normative Aging Study (Bell et al., 1972; Bossé and Spiro, 1995; MAVERIC, 2001) and the Vietnam Era Twin

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