The study of epidemiological situation of a communicable disease like Tuberculosis (TB) is of paramount importance for formulating the disease control strategies. It also provides an insight into the impact of the control programmes on the disease situation in the community.

The study of epidemiological situation of tuberculosis has always been a big challenge. Tuberculosis has been known to be a major public health problem in India for a long time. However, it was only in 1955-58 that the nation wide study conducted by ICMR during 1955-58 provided for the first time, an insight into the enormity of suffering caused by TB in India. The survey findings revealed the prevalence of sputum positive PTB to be about 400 per 100,000 population. The prevalence was found to be similar in rural and urban areas. Considering the country's population, there were an estimated 1.5 million infectious cases at that time spreading infection in the community. About 80% of these cases lived in villages. In fact, before the aforesaid survey, TB was generally believed to be an urban disease. The inputs from the study were of great value in formulating the National Tuberculosis Program (NTP) which was launched all over the country in the year 1962.

All subsequent disease prevalence studies have been conducted in limited geographical areas. The methodology also varied in terms of the age group studied, the screening and diagnostic methods as well as defining TB cases. The disease prevalence studies require significant resources and the sample sizes required are often large. Such studies are also time consuming and are therefore difficult to conduct. For the same reasons, the studies of disease incidence are even more difficult because of the need to subject the same study population to repeated surveys at periodic intervals. Therefore, very few incidence studies have been conducted in the country. The routine surveillance data compiled from case finding reports obtained from health facilities under TB control programmes also do not reflect on the epidemiological situation of tuberculosis.

The epidemiological parameter that is widely used to assess the TB situation in the community is the Annual Risk of Tuberculous Infection (ARTI). The studies to estimate the ARTI are relatively inexpensive and require simpler technology. The ARTI indicates the probability of an uninfected person getting infected or re-infected with tubercle bacilli during the course of one year. It reflects the overall impact of various factors influencing the transmission of the tubercle bacilli like the load of infectious cases in the community and the efficiency of public health measures to control tuberculosis. The ARTI is the first epidemiological parameter to be affected following a change in the situation of tuberculosis in the community. However, most of the studies on the estimation of ARTI were localized to pockets in the southern parts of the country in the vicinity of the two pillars of strength on TB research - NTI and TRC.

It can be concluded from the above that the information on the prevailing epidemiological situation of TB was lacking for most parts of the country. Studying the prevalence of tuberculosis disease across the country was also not an easy proposition. The decision to conduct a nation wide tuberculin survey to estimate ARTI in different parts of the country was taken in this background.

From an operational perspective, the country was stratified into four zones namely north, south, east and west, each having about a fourth of the country's population (Map 1). The survey was designed to estimate the prevalence of infection and compute average Annual Risk of Infection in each of these four zones. The survey was conducted at a time when the TB control activities in the country were in the midst of being revitalized by adopting DOTS strategy under RTNCP.