Generally there is no acceptable definition of the term “case of pulmonary tuberculosis”, although such a definition is of fundamental importance both in clinical medicine where results of various chemotherapeutic regimens are compared, as well as for the comparison of different epidemiological data. The main purpose of this paper is to focus attention on the difficulties of defining a case on the basis of bacteriological examination, X-ray examination and tuberculin test. Data from two successive prevalence surveys in a random sample of 134 villages in Bangalore district with a population 70,000 have been utilized to illustrate some of the difficulties in defining a “case” of pulmonary tuberculosis for reporting the prevalence or incidence of the diseases. The entire population was tuberculin tested with 1 TU RT 23 with Tween 80 at both rounds and those 5 years of age and older were examined by 70mm photofluorogram. The sputum specimens (spot and overnight) were collected from those with any abnormality on X-ray as recorded by either of the two independent readers. Both the specimens were examined by fluorescent microscopy and Ziehl-Neelsen technique and by culture.

Analysis of data has shown that the term “a case of pulmonary tuberculosis” does not represent a single uniform entity, but embraces cases of several types, differing considerably in their tuberculin sensitivity, results of X-ray and sputum examination, in the reliability of their diagnosis and mortality experience. The status of cases found at initial and subsequent surveys showed changes with time, and such changes show considerable differences for the various types of cases. It was felt that a single straight-forward definition of a case was not possible to suit all situations. One has to use more than one definition. Although theoretically, finding a single bacillus in sputum should be adequate proof of pulmonary tuberculosis, it was shown that finding of a few bacilli (3 or less) was very often due to artifacts and should not be the basis for a diagnosis. It has also been found that positive radiological findings, in the absence of bacteriological confirmation, indicate only a high risk of the disease and not necessarily pulmonary tuberculosis. Direct microscopy appears to be a consistent index of disease but in community surveys has the limitation of missing a substantial proportion of cases and of adding some false ones.

In view of the difficulty of providing a single definition of a case of tuberculosis, four indices have been suggested. (1) Cases definitely positive by direct smear; (2) Cases definitely positive by culture; (3) All cases positive by culture (including less than twenty colonies); (4) Sputum positive cases which are radiologically active. Each of these could be used for different situations. However, it was concluded that, there seems to be no option but to use more than one definition for assessing the prevalence and incidence of disease.