toman's tuberculosis case detection, treatment and monitoring

5. How reliable is smear microscopy?
K. Toman 1
To assess the reliability of smear microscopy quantitatively, answers are needed to the
following questions:
1. What is the probability of finding acid-fast bacilli (AFB) in smears prepared from
specimens containing few, some, or many bacilli?
2. What is the probability of reporting a (false-)positive result for smears from specimens
without tubercle bacilli?
3. What is the frequency of agreement between microscopists or laboratories reporting
the results for smears prepared from the same specimens?
Table 1 under “How many bacilli are present in a sputum specimen found positive by
smear microscopy?” (page 11) supplies part of the answer to the first question. The
figures in that table are derived from experimental findings and have been extrapolated
on the assumption that bacilli are evenly distributed throughout specimens.
Since the bacillary content varies from one sample to another, however, such measurements
must be performed on a large number of specimens, taking the results of
culture as a yardstick (1). In several studies (2, 3), the bacillary counts of smears were
compared with the number of colonies grown on cultures prepared from the same
specimen.
In a cooperative study by eight laboratories, it was confirmed that colony counts
for samples taken from the same specimen varied from one sample to the next,
although these variations were minimal. 2 The disparity of colony counts between
samples from different specimens was due mainly to the variation in the concentration
of bacilli in these specimens. It was concluded, therefore, that there is a positive
correlation between the concentration of culturable bacilli in the specimens, the
number of AFB in the corresponding smears, and the probability of their being identified
by microscopy. The results (Table 2) show that the chance of finding AFB in a
1 Deceased.
2 David HL et al. Sensitivity and specificity of acid-fast microscopy. Atlanta, GA, United States Department
of Health, Education and Welfare, Centers for Disease Control (unpublished document prepared for the
WHO Expert Committee on Tuberculosis, Geneva, 1973).
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