The study was undertaken among 22,957 persons belonging to 55 randomly selected villages of Nelamangala taluk of Bangalore district in 1975, to find out precise estimates of prevalence of bacillary disease. Symptom screening was done by well experienced social investigators, according to a brief interview schedule. Sputum was collected from all above the age of 5 years reporting chest symptoms for seven or more number of days during the previous two months. Within two weeks after symptom questioning, all were tuberculin tested and all 5 years and above were X-rayed. Additional sputum collection was done for those asymptomatics who had abnormal shadows in their chest X-rays.

The overall prevalence rate of culture confirmed bacillary cases by symptom and/or X-ray screening was 0.32 percent. Same prevalence was seen with X-ray alone also. But the overall prevalence rate based on symptom screening alone was 0.21 percent which is significantly lower than that of symptom and/or X-ray screening, or X-ray screening alone. The prevalence rates by age and sex based on symptom screening were about two-thirds that of rate based on X-ray and/or symptom screening. Hence to obtain prevalence rate according to X-ray and/or symptom screening, a correction factor of 1.52 should be applied to the prevalence rates obtained by symptom screening alone. This correction factor is fairly good for most of the age groups. It was also estimated that the cost of surveying the population by symptom screening alone is about half that of surveying the population by X-ray screening.