The annual risk of tuberculosis infection used as predictor of the future incidence of smear-positive cases

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.(ABSTRACT TRUNCATED AT 250 WORDS)