Factors affecting the clinical value of microscopy for acid-fast bacilli

Rev Infect Dis. 1984 Mar-Apr;6(2):214-22. doi: 10.1093/clinids/6.2.214.


In order to assess the clinical value of microscopy for acid-fast bacilli (AFB), the results of 3,207 clinical specimens submitted for mycobacterial smear and culture were analyzed. Mycobacteria grew from 176 (5.5%) of the specimens, 95 (54%) of which were Mycobacterium tuberculosis. Although the overall sensitivity of the smear was low (33%), 65% of respiratory specimens yielding M. tuberculosis had positive AFB smears. Furthermore, 96% of patients with pulmonary tuberculosis from whom more than one specimen was processed had at least a single positive AFB smear. Smear sensitivity correlated well with quantitative growth; 89% of specimens yielding greater than or equal to 50 colonies per slant were smear positive. Specificity of the AFB smear was high; 89% of smear-positive specimens had positive cultures. After the results from culture-negative patients known to have active tuberculosis were eliminated from the analysis, the specificity of a positive smear rose to 98.3%. When the results of all specimens from each patient were considered in toto, the AFB smear had a predictive value of greater than or equal to 96%.

Publication types

  • Review

MeSH terms

  • Aniline Compounds*
  • Benzophenoneidum*
  • Body Fluids / analysis
  • Diagnostic Errors
  • Humans
  • Microscopy
  • Mycobacterium / isolation & purification*
  • Mycobacterium Infections / diagnosis*
  • Mycobacterium tuberculosis / isolation & purification
  • Staining and Labeling
  • Tuberculosis, Pulmonary / diagnosis


  • Aniline Compounds
  • Benzophenoneidum