Academic journal article Bulletin of the World Health Organization

Prevalence of Tuberculous Infection and Incidence of Tuberculosis; a Re-Assessment of the Styblo rule/Prevalence De L'infection Tuberculeuse et Incidence De la Tuberculose : Reevaluation De la Regle De Styblo/Prevalencia De la Infeccion Tuberculosa E Incidencia De Tuberculosis: Reevaluacion De la Regla De Styblo

Academic journal article Bulletin of the World Health Organization

Prevalence of Tuberculous Infection and Incidence of Tuberculosis; a Re-Assessment of the Styblo rule/Prevalence De L'infection Tuberculeuse et Incidence De la Tuberculose : Reevaluation De la Regle De Styblo/Prevalencia De la Infeccion Tuberculosa E Incidencia De Tuberculosis: Reevaluacion De la Regla De Styblo

Article excerpt

Introduction

The formulation of the Millennium Development Goals (MDGs) in 2000 was a landmark step taken by 189 countries in their commitment to "meet the needs of the world's poorest." (1) A projected target within goal number 8 (combat HIV/AIDS, malaria and other diseases) is to "stop the increase and start reversing the incidence of tuberculosis (TB) by 2015". The Stop TB Partnership has translated this goal into targets to be met by national TB control programmes (NTPs). These targets are to detect at least 70% of the incident smear-positive TB cases and to successfully treat 85% of these. (2)

To be able to assess whether the detection target is met, information about the incidence of smear-positive TB in the general population is needed. This information is not readily available, because studies to measure disease incidence are rarely conducted due to the logistical problems and costs involved. (3) Two population surveys assessing prevalence of TB disease must be conducted within a limited time frame and with an adequate surveillance system to monitor TB disease in persons emigrating or dying between the two surveys. Incidence estimates based on prevalence of disease and duration of illness as measured in population-based surveys, or on TB-specific death rates from routine health system or vital registration data, are often imprecise due to measurement errors and misclassifications. (3,4) Using the number of notified TB cases within a country as a proxy for incidence of TB disease is in general not a valid approach. The passive reporting underlying notification data makes them dependent, among others, on NTP performance, the population's access to health care, diagnostic service quality and the social stigma surrounding TB. Notification data can be relied on as a proxy for incidence of TB only when their quality and completeness areas assured through the presence of a well-established surveillance system.

The Styblo rule

A method often used to estimate the incidence of TB disease in the general population is to assess the annual risk of tuberculous infection (ARTI) in the general population and apply the Styblo rule. (5) The ARTI denotes the proportion of persons in a community who become (re-)infected within one year and is estimated in large-scale tuberculin skin test (TST) surveys in the general population. The Styblo rule assumes a fixed mathematical relationship between the incidence of smear-positive TB, the prevalence of smear-positive TB and the ARTI. For the quantification of the rule, Styblo used estimates of the incidence of TB disease derived from three different sources. Directly measured incidence in the general population was used in the Netherlands (routine notifications to the health system were used, but notification was deemed complete). Data on TB mortality were used in the Netherlands and Alaska (United States of America). Data on measured prevalence of disease were used for India and 12 other developing countries. The original data are reproduced in Table 1.

Since only the data from the Netherlands were based on measured disease incidence (complete notification), assumptions were made to arrive at an estimate for the incidence of smear-positive TB from the other data sources. These assumptions were: 1) the mortality is half the disease incidence, and 2) the number of prevalent cases at a given point in time is twice the number of incident cases in one year, assuming a duration of disease of two years. By doing so, Styblo calculated that 50-60 new cases per 100 000 population per year of smear-positive TB corresponded to 1% ARTI (Fig. 1). In the data from 10 of the 13 developing countries, the ratio between the number of new infections (per 100 000 per year), as derived from the ARTI, and the number of prevalent smear-positive TB cases (per 100 000) ranged from 8 to 12.

The data used by Styblo were from an era without established TB control programmes and efficacious TB treatment regimens, and before the emergence of the HIV epidemic. …

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