Abstract:
BACKGROUND: In many resource poor settings only sputum microscopy is employed for
the diagnosis of HIV-associated pulmonary tuberculosis; sputum culture may not be
available.
METHODS: We determined the diagnostic accuracy of sputum microscopy for active
case finding of HIV-associated pulmonary tuberculosis using TB culture as the
reference standard.
RESULTS: 2216 potential subjects screened for a TB vaccine trial submitted 9454
expectorated sputum specimens: 212 (2.2%) were sputum culture positive for
Mycobacterium tuberculosis (MTB), 31 (0.3%) for non-tuberculous mycobacteria, and
79 (0.8%) were contaminated. The overall sensitivity of sputum microscopy was
61.8% (131/212) and specificity 99.7% (9108/9132). Sputum microscopy sensitivity
varied from 22.6% in specimens with < 20 colony forming units (CFU)/specimen to
94.2% in patients with > 100 CFU/specimen plus confluent growth. The incremental
diagnostic value for sputum microscopy was 92.1%, 1.8% and 7.1% for the first,
second and third specimens, respectively. The positive predictive value and
negative predictive values for sputum microscopy were 84.5% and 99.1%,
respectively. The likelihood ratio (LR) of a positive sputum microscopy was 235.1
(95% CI 155.8 - 354.8), while the LR of a negative test was 0.38 (95CI 0.32 -
0.45). The 212 positive sputum cultures for MTB represented 103 patients; sputum
microscopy was positive for 57 (55.3%) of 103 patients.
CONCLUSION: Sputum microscopy on 3 expectorated sputum specimens will only detect
55% of culture positive HIV-infected patients in active screening for pulmonary
tuberculosis. Sensitivity is higher in patients with greater numbers of CFUs in
the sputum. Culture is required for active case finding of HIV- associated
pulmonary tuberculosis.
