A study was carried out mainly to find out the prevalence and incidence of sputum negative active pulmonary tuberculosis (suspect cases) among 35,876 persons aged 5 years and above in rural areas of Bangalore district during 1968-72. Two surveys (I & II) at an interval of 3 months, succeeded by a follow up examination of the X-ray abnormals of the earlier surveys, were conducted in the same villages. Examinations at each survey consisted of tuberculin test, X-ray and sputum examinations. X-rays were interpreted individually at the time of each survey by single picture interpretation method and subsequently by Joint Parallel Reading (JPR) method to arrive to a diagnosis. In the JPR method X-ray readings and their comparison was done by a panel of three X-ray readers with full knowledge of age, sex, result of sputum examination and tuberculin test of each person with chest abnormality at any of the three surveys.

On a single picture interpretation the overall prevalence rate of suspect disease was found to be 5.4 per thousand at I survey and 4.59 per thousand at II survey. There was no significant difference in the overall age and sex specific prevalence rates of suspect disease between I & II surveys. Incidence of suspect disease at the end of 3 months was 2.24 per thousand. By JPR method the prevalence rates of suspect disease was 3.2 per thousand at I survey and 3.6 per thousand at II survey. The prevalence rates by single picture method were overestimated to the extent of 38% at I survey and 19% at II survey when compared with those found by JPR method. At I survey prevalence rates on JPR method was significantly lower than by single picture method. This was not so at II survey. Similarly, incidence rate of 0.2 per thousand of suspect disease on JPR was about 1/10th of that found by single picture method.

The incidence of bacteriologically positive cases in 6 months from among suspect cases on JPR was found to be 28%. Majority (76%) of non-tuberculous or inactive tuberculous shadows continued to remain as such after 6 months and about a quarter (23%) became normal. Incidence of bacteriologically positive cases from this group was minimal. Of 19,640 persons with normal X-rays 134 (0.7%) developed new shadows in 3 months; 103 (0.5%) cleared after 2-12 weeks (fleeting shadows). Mis-interpretation of the latter as active tuberculous may falsely boost the estimates of suspect disease to the extent of about 5%.