Estimating the Occurrence of Amyotrophic Lateral Sclerosis among Gulf War (1990–1991) Veterans Using Capture-Recapture Methods

Abstract

Objective: Using data from a recent report that indicated a 2-fold higher risk of amyotrophic lateral sclerosis (ALS) among veterans of the 1991 Gulf War, we applied capture-recapture methodology to estimate possible under-ascertainment of ALS cases among deployed and non-deployed military personnel who were on active duty during that war. Study Design and Setting: One of the most serious concerns facing field epidemiological investigations is that of case ascertainment bias, particularly when it is differential among the study groups. Capture-recapture methods, however, have promise as an approach to assessing the impact of case ascertainment bias in such studies. To overcome potential limitations of any one approach, three different estimation methods were used: log-linear models, sample coverage, and ecological models, to obtain a comprehensive view of under-ascertainment bias in these populations. Results: All three approaches indicated differential undercount of ALS cases with modest under-ascertainment likely to have occurred among non-deployed military personnel, but little under-ascertainment among the deployed. After correcting the rates for under-ascertainment, the age-adjusted risk of ALS remained elevated among military personnel who had been deployed to S.W. Asia during the 1991 Gulf War, confirming the earlier report. Conclusions: Capture-recapture methods are a useful approach to assessing the magnitude of case ascertainment bias in epidemiological studies from which ascertainment-adjusted estimates of rates and relative risks can be calculated.