As we stated in our article, our analysis does not support withholding isoniazid preventive therapy for all tuberculin reactors and converters living in areas of high isoniazid resistance. The analysis does demonstrate, however, that human immunodeficiency virus—seronegative tuberculin reactors aged 20 to 34 years living in areas of high isoniazid resistance derive minimal net benefit from isoniazid preventive therapy.
Twenty-year follow-up was used because it is the long-est period of follow-up documented in the literature. One can assume that the protective effect of isoniazid is life-long, but, to our knowledge, there are no data in the literature to support such an assumption. Decision analysis yields useful information only when the probabilities used in the analysis are based on available data.
Interpreting the improvement in life expectancy provided by isoniazid preventive therapy can be made easier by comparing it with that provided by other preventive health interventions, as we described. The