Norbert Heinrich, M.D. Medical Center of the University of Munich heinrich@lrz.uni-muenchen.de (et al.)
Boehme et al. report encouraging results on the use of an automated molecular test for Mycobacterium tuberculosis and resistance to rifampin (Xpert MTB/RIF). However, the population of patients with clinical tuberculosis who have negative cultures still poses a problem of interpretation, which was not discussed in the article. Among study patients whose samples were culture-negative but who had symptoms of tuberculosis, 29.3% had positive results on the automated test; these patients made up 4.3% of the total number of automated test–positive patients.
In such patients, tuberculosis that was detected by the automated test may have had a false negative culture because of low bacillary load or overgrowth, but the possibility of false positivity cannot be excluded. Furthermore, 23 patients with nontuberculous mycobacteria in culture were excluded from the analysis. In our site in Tanzania and in other African locations, nontuberculous mycobacteria are frequently found in culture, so the capability of the automated test to discriminate between tuberculosis and nontuberculous
mycobacteria, for which preliminary results have been encouraging, would be of great interest.
More effort should be made in future studies to elaborate on these two groups, thus clearing an
uncertainty regarding the performance of the automated test.
http://www.nejm.org/doi/pdf/10.1056/NEJMc1011919
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