It has been postulated that BCG scar disappears in a good number of children and some of the vaccinated children will get included in the non- vaccinated group and cause difficulty in interpreting the results of tuberculin test. It was decided to analyse information on BCG scar status in the younger population of a rural community in 3 taluks of Bangalore district with an objective to find out whether disappearance of BCG scar is dependent on the age of the child, size of post-vaccination induration at initial survey and tuberculin sensitivity status of children in whom BCG scar has disappeared, in comparison with children in whom the BCG scar has not disappeared. In all, 1095 children aged 0 to 14 years were found with BCG scar in 119 randomly selected villages during an epidemiological survey done in 1961 at the time of intake. Following two groups of children were studied for disappearance of the scar. Of them, a) 796 children who had BCG scar at the first survey, and whose BCG scar status was available at 4th survey, b) 299 who showed no BCG scar at first survey but were found with BCG scar at 2nd survey and whose BCG scar status was available at 4th survey.

Of the BCG scars recorded at intake, 26.4% and 32.5% disappeared subsequently during three and a half and five year periods respectively. The waning of BCG scars was independent of age of the child and tuberculin sensitivity status at intake. Tuberculin sensitivity status in children in whom scar had disappeared was the same as that found in children in whom scar had persisted at intake and after five years. The misclassification of children, in whom scars have disappeared, as unvaccinated leads to a difficulty in interpreting the results of tuberculin test done for the purpose of computation of the Annual Risk of Infection. Further, the extent of misclassification increases in proportion with the increase in BCG coverage of the population. This finding justifies the practice of identifying the demarcation level on the basis of the distribution of tuberculin induration sizes for classifying the infected persons in a population in each survey.

Mass miniature radiography (MMR) is the usual tool for population screening in tuberculosis case prevalence surveys. However, this facility is not available at most centres in India. An attempt was made to study the feasibility of carrying out sputum positive case prevalence survey in a population by introducing methodological variation in the screening, in order to select those eligible for sputum test without resorting to the customary use of MMR for the purpose. The study was carried out in Bangalore rural district during 1984-1986. The area was the same as for six earlier prevalence surveys conducted since 1961. The population aged up to 44 years was tuberculin tested. Persons with test induration size of = 10 mm were eligible for sputum examination, besides all those aged over 45 years were eligible. It was observed that 78.4% of the registered population (29400) in the age group 10 years and above were required to undergo sputum examination by the present method of screening leading to a very high work load of sputum examination necessitating deployment of additional sputum cultures. Thus, the purpose of pre selection for sputum examination was hardly fulfilled. Further, a high contamination rate was observed. The changed screening procedure in this survey made comparison with the earlier data difficult.

The overall prevalence rate of cases was 438/100,000 in persons aged 10 years and above, while smear positive prevalence rate was 68/100,000. The observed prevalence rate was similar to earlier surveys, while smear positive prevalence rate was much lower. In conclusion, the screening methodology was found to be operationally unfeasible, ineffective and counterproductive to complicate the survey procedure in the quest for simplicity.