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 ≥50 coloniesper 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 ≥96%.