Breaking the law: tuberculosis disobeys Styblo's rule

Abstract

In 1985 Karel Styblo derived one of the guiding rules of tuberculosis (TB) epidemiology. Bringing together data from 16 countries he proposed that an annual incidence of 50 sputum-smear-positive TB cases in a population of 100 000 generates an annual risk of infection of 1%. This rule of thumb quickly became an established and cherished part of the epidemiological canon because it provided a way to estimate albeit indirectly an important but elusive quantity (disease incidence) from a comparatively simple measurement procedure (risk of infection via tuberculin surveys). Though the rule has never formally been viewed as anything better than approximate its theoretical and practical underpinnings have been steadily eroded by the last two decades of epidemiological change. In this issue of the Bulletin van Leth et al. present yet another analysis suggesting that TB disobeys Styblos rule. The rule can no longer be trusted as a method for estimating TB incidence. To understand why the rule has become inapplicable we need to go back to first principles. Working with the limited data available at the time Styblo deduced that deaths per year incidence per year and prevalence of smear-positive TB were held in the ratio 1:2:4. That is in the pre-drug era a smear-positive TB case remained infectious for an average of 2 years and the case fatality rate was 50%. These observations allowed him to estimate smear-positive incidence from prevalence and mortality data and compare these incidence estimates with the measured risk of infection. (excerpt)