TABLE B-2. Range of Actual Transmission Rates for Observed Skin Test Conversion Rate

Conversion Rate

Actual Transmission Rate (%)









data from populations where a positive tuberculin reaction had a very high likelihood of indicating true infection.

If we apply these interventions to populations with a lesser likelihood of being truly infected, two phenomena will occur. The first is that the effectiveness of the intervention will appear to be increased. As the number of persons with false-positive reactions increases, the likelihood of true infection in the population decreases, and the likelihood of disease developing subsequent to the intervention decreases in like proportion. Second, the true effectiveness of the intervention actually decreases. As the proportion of people with the condition to be prevented decreases, there is less to prevent and the number of cases actually prevented falls. This actual decrease in true benefit results in a rise in the cost-benefit and risk-benefit ratios.


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