Abstract
It has been estimated that in underdeveloped countries every 1% annual risk of tuberculosis infection results in an incidence of 50 per 100,000 smear-positive cases. However, workers had theoretically speculated that the incidence would be about one-third less in countries with good medical facilities. I decided to test the latter theory in the city of Riyadh (capital of Saudi Arabia) which enjoys a high standard of living, easy access to medical facilities, and free treatment. I calculated the annual risk of infection (using the standard tables developed by the TSRU of the International Union Against Tuberculosis) from a nationwide survey of tuberculin reactivity that was conducted in 1987-1988. The annual risk was found to be 0.5% in the whole of Saudi Arabia as well as the city of Riyadh. Accordingly, the incidence of smear-positive cases should have been 25 per 100,000 in an underdeveloped community (0.5 x 50 = 25). However, the actual incidence of smear-positive cases in Riyadh was found to be only 15.2 per 100,000 in 1991-1992. This supports the theoretical speculation that the incidence, as predicted from annual risk of infection, is about one-third less in wealthy communities with adequate access to medical care. If this finding is confirmed, it could have two applications: First, the annual risk of infection could be used to predict the future incidence of smear-positive cases in high risk groups in wealthy countries, like immigrants from the third world, and foresee in advance what impact they will have on active tuberculosis.