# Introduction

Estimates of the burden of disease caused by TB and measured in terms of incidence, prevalence and mortality are produced annually by WHO using information gathered through surveillance systems (case notifications and death registrations), special studies (including surveys of the prevalence of disease), mortality surveys, surveys of under-reporting of detected TB and in-depth analysis of surveillance data, expert opinion and consultations with countries. This document provides case definitions and describes the methods used in Global TB Report 2015 to derive TB incidence, prevalence and mortality.

# Definitions

Incidence is defined as the number of new and recurrent (relapse) episodes of TB (all forms) occurring in a given year. Recurrent episodes are defined as a new episode of TB in people who have had TB in the past and for whom there was bacteriological confirmation of cure and/or documentation that treatment was completed. In the remainder of this technical document, relapse cases are referred to as recurrent cases because the term is more useful when explaining the estimation of TB incidence. Recurrent cases may be true relapses or a new episode of TB caused by reinfection. In current case definitions, both relapse cases and patients who require a change in treatment are called retreatment cases. However, people with a continuing episode of TB that requires a treatment change are prevalent cases, not incident cases.

Prevalence is defined as the number of TB cases (all forms) at a given point in time.

Mortality from TB is defined as the number of deaths caused by TB in HIV-negative people occurring in a given year, according to the latest revision of the International classification of diseases (ICD-10). TB deaths among HIV-positive people are classified as HIV deaths in ICD-10. For this reason, estimates of deaths from TB in HIV-positive people are presented separately from those in HIV-negative people.

The case fatality rate is the risk of death from TB among people with active TB disease.

The case notification rate refers to new and recurrent episodes of TB notified to WHO for a given year. The case notification rate for new and recurrent TB is important in the estimation of TB incidence. In some countries, however, information on treatment history may be missing for some cases. Patients reported in the unknown history category are considered incident TB episodes (new or recurrent).

Regional analyses are generally undertaken for the six WHO regions (that is, the African Region, the Region of the Americas, the Eastern Mediterranean Region, the European Region, the South-East Asia Region and the Western Pacific Region). For analyses related to MDR-TB, nine epidemiological regions were defined (Figure \ref{fig:epiregions}). These were African countries with high HIV prevalence, African countries with low HIV prevalence, Central Europe, Eastern Europe, high-income countries, Latin America, the Eastern Mediterranean Region (excluding high-income countries), the South-East Asia Region (excluding high-income countries) and the Western Pacific Region (excluding high-income countries).

\label{fig:epiregions}Nine epidemiological regions used for analyses related to MDR-TB.

The source of population estimates needed to calculate various TB indicators was the 2015 revision of the World Population Prospects, which is produced by the United Nations Population Division (UNPD, http://esa.un.org/unpd/wpp/). The UNPD estimates sometimes differ from those made by countries.

# Short historical background

Historically, a major source of data to derive incidence estimates were results from tuberculin surveys conducted in children(Styblo 1985). Early studies showed the following relationship between the annual risk of infection denoted $$\lambda$$ and the incidence of smear positive TB denoted $$I^+$$: one smear positive case infects on average 10 individuals per year for a period of 2 years and an annual risk of infection of 1% corresponds approximately to an incidence of 50 s