Of all the available methods for the diagnosis of pulmonary tuberculosis, bacteriological examination is the most reliable. Diagnosis of pulmonary tuberculosis is chiefly done by sputum microscopy and culture. This paper discusses sputum microscopy from various points of view. Sputum, which forms the basis of bacteriological diagnosis, is a variable source material. Type of specimen, its quality, quantity, bacterial content and viability of organism considerably influence the sensitivity and the specificity of the methods; and these in turn would vary under different diagnostic situations. One of the reasons for the observed variations could be the different criteria adopted for examination; another might be due to the observed range of diagnostic situations varying from an epidemiological survey situation at the one extreme to the other where cases seek treatment in a comparatively backward community with poor tuberculosis diagnostic services.

In epidemiological community survey (ICMR 1968), it has been found that culture positives that were also smear positives varied from 73% to 87%, whereas among patients attending rural general health institutions for diagnosis, about 82% of the infectious cases found by culture could also be discovered by microscopy of single spot specimens (Rao, 1966). Sikand (1965) from New Delhi Tuberculosis Centre, could get 67% of culture positives by microscopy, whereas Mitchison (1967) found that 35% were smear positive among the sputum positive patients reporting for the first time. In the longitudinal epidemiological study carried out in the Bangalore rural area, it was found that about 40%-48% were positive by both direct smear and culture and the rest by culture only. Reasons for these variations could be (i) different criteria adopted for examination (ii) different situations from where the sputum specimens were collected (iii) sensitivity and specificity of sputum microscopy technique adopted and (iv)the experience of the trained technician. It was observed that over diagnosis by the trained auxiliary staff in the general health institutions (1.9%) compares favourably with the over diagnosis of 1.3% by experienced technicians indicating simplicity of smear examination. Besides these aspects, other factors like the quality of sputum smear, time spent on examination, type of sputum specimen, the use of multiple smears, etc., also influence the results. The cost of bacteriological examination have also been studied, and the cost ratio between microscopy and culture have been worked out to be 1:6.6. Cost can become an important factor in deciding the suitability of bacteriological methods for diagnosis of pulmonary tuberculosis in various countries and in different diagnostic situations.