title: Factors affecting the clinical value of microscopy for acid-fast bacilli.
abstract: 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%.
mesh_terms: D000814:Aniline Compounds; D001576:Benzophenoneidum; D001826:Body Fluids / Q000032:analysis; D003951:Diagnostic Errors; D006801:Humans; D008853:Microscopy; D009161:Mycobacterium / Q000302:isolation & purification*; D009164:Mycobacterium Infections / Q000175:diagnosis*; D009169:Mycobacterium tuberculosis / Q000302:isolation & purification; D013194:Staining and Labeling; D014397:Tuberculosis, Pulmonary / Q000175:diagnosis
chemical_list: D000814:Aniline Compounds; D001576:Benzophenoneidum