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Nucleic Acid Amplification for TB Diagnosis: Helpful but Not Definitive
There is no laboratory gold standard for diagnosing active pulmonary tuberculosis: Because both acid-fast smear and culture can be misleading, overall clinical suspicion remains quite important. Now that nucleic acid amplification (NAA) techniques are available, has this picture changed?
Based on clinical, laboratory, and radiological criteria, 338 adults suspected of having pulmonary tuberculosis were sorted into 2 groups: 72 patients with active disease and 266 without. An expert panel of consultants arbitrated any ambiguous cases. All subjects also provided 1 to 6 morning sputum samples for NAA analysis.
Sensitivity and specificity of NAA were 83 percent and 97 percent, respectively, compared with 60 percent and 92 percent for acid-fast smear, and 90 percent and 99.6 percent for culture. When clinicians were asked to quantitate their pretest suspicion of tuberculosis on a scale of 0 to 100, the NAA test appeared to be most useful in patients with intermediate scores (25 to 75). Among these patients, the NAA test had a positive predictive value of 100 percent and negative predictive value of 91 percent (in contrast to 30 percent and 71 percent for acid-fast smears in the same group).
Comment: Two NAA kits for diagnosing tuberculosis in clinical specimens are FDA approved, including the one used in this study. The test is rapid and clearly helpful in some cases, but, as with the other tests for TB, its results must be interpreted in light of clinical suspicion. The manufacturer of the TB NAA kits used here supported this study.
— A Zuger
Published in Journal Watch General Medicine February 11, 2000
Citation(s):
Catanzaro A et al. The role of clinical suspicion in evaluating a new diagnostic test for active tuberculosis: Results of a multicenter prospective trial. JAMA 2000 Feb 2 283 639-645.
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