Clinical performance of the Abbott RealTime Mycobacterium tuberculosis (MTB) PCR on bronchoscopic specimens for diagnosing pulmonary tuberculosis.

PURPOSE: We evaluated the performance of the Abbott RealTime Mycobacterium tuberculosis (MTB) PCR (RT MTB) on bronchoscopic specimens using conventional culture as the reference standard in a low-Tuberculosis (TB) prevalence setting.

METHODS

A total of 6,988 specimens (4,682 bronchial aspirates [BAS] and 2,306 bronchoalveolar lavages [BAL]) from 4,118 patients with suspected pulmonary TB were included. When BAS and BAL specimens from the same bronchoscopy procedure were available, these were mixed 1:1 prior to culture inoculation and PCR testing. Following processing, specimens were inoculated into a Löwenstein-Jensen and a Bactec MGIT 960 tube and incubated at 37 °C for 3 months and at 35 °C for 8 weeks, respectively. RT MTB was performed as indicated by the manufacturer. RT MTB targets the insertion sequence IS6110 and the protein antigen B (PAB) gene, both highly conserved within the Mycobacterium tuberculosis complex. Whole genome Next generation sequencing of clinical MTBC isolates was performed when indicated.

RESULTS

Among the 104 culture-positive specimens, 84 (1.2%) were detected by PCR. Additionally, 16 specimens (0.3% of all samples), from 16 patients, were PCR-positive despite negative culture results. Conversely, 20 specimens (0.4% of all samples), from 19 patients, were culture-positive but not detected by PCR. No significant differences were found between PCR-positive and PCR-negative specimens with respect to the number of IS6110 copies per isolate or PAB gene sequences (P = 0.69). Finally, there were 4,683 specimens (97.4%) from the remaining 4,014 patients tested PCR-negative/Culture-negative. Following the resolution of discrepancies based on clinical grounds the sensitivity and specificity of RT MTB were 83.9% (CI 95%, 76.0-90.0) and 99.9% (CI 95%, 99.9-99.9), respectively. These results exceed the minimum performance requirements defined in the WHO Target Product Profiles for molecular TB diagnostics. Although RT MTB is not a point-of-care test but rather a moderate-complexity automated NAAT, it is recommended by WHO as part of the Abbott RealTime MTB/MTB RIF-INH testing algorithm, in which MTBC detection by RT MTB is followed by reflex testing with the MTB RIF/INH assay for detection of rifampicin and isoniazid resistance.

CONCLUSION

RT MTB shows a good performance on bronchoscopic specimens.