Table 3. Performance characteristics of the Xpert MTB/RIF test overall, and stratified by smear microscopy status, using MGIT culture as the reference standard.
| Overall | Smear-positive | Smear-negative | ||
| Total N = 6621a | Total N = 87b | Total N = 6534 | ||
| Xpert sensitivity | N/N | 117/187 | 32/33 | 85/154 |
| % (95% CI) | 62.6 (55.2, 69.5) | 97.0 (84.2, 99.9) | 55.2 (47.0, 63.2) | |
| Xpert specificity | N/N | 6407/6434 | 53/54c | 6354/6380 |
| % (95% CI) | 99.6 (99.4, 99.7) | 98.2 (90.1, 100.0) | 99.6 (99.4, 99.7) | |
| Xpert PPV | N/N | 117/144 | 32/33 | 85/111 |
| % (95% CI) | 81.3 (73.9, 87.3) | 97.0 (84.2, 99.9) | 76.6 (67.6, 84.1) | |
| Xpert NPV | N/N | 6407/6477 | 53/54 | 6354/6423 |
| % (95% CI) | 98.9 (98.6, 99.2) | 98.2 (90.1, 100.0) | 98.9 (98.6, 99.2) | |
| Agreement between Xpert and MGIT | N/N | 6524/6621 | 85/87 | 6439/6534 |
| % (95% CI) | 98.5 (98.2, 98.8) | 97.7 (91.9, 99.7) | 98.6 (98.2, 98.8) |
Abbreviations: CI, confidence interval; MGIT, Mycobacteria Growth Indicator Tube;
PPV, positive predictive value; NPV, negative predictive value.
a
Excluded from the analyses were 249 specimens that were contaminated in MGIT culture and 23 specimens that had a final result of failed on Xpert MTB/RIF testing.
b
sensitivity of smear microscopy for detection of MTB, using MGIT as reference standard, was 33/187 (17.6%, [95%CI 12.5, 23.9]) vs. Xpert sensitivity of 62.6% overall (p≤0.001).
c
These 54 specimens were culture-negative for M. tuberculosis. For 11/54, a non-tuberculous mycobacterium was isolated from culture. For 43/54 the culture was negative for any mycobacterial growth; among these 43 specimens, 32 were smear grade scanty.