TABLE 4.
Xpert MTB/RIF performance classified by specimen appearance, centrifuged specimens, and othera
| Specimen category and group | No. (%) of specimens tested | No. of specimens classified by: |
No. of specimens with definitive results confirmed by reference culture | % sensitivity (95% CI) | % specificity (95% CI) | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| MGIT culture |
Xpert MTB/RIF |
|||||||||
| Positive | Negative | Contaminated | Positiveb | Negative | Error | |||||
| Appearance | ||||||||||
| Clear (watery) | 292 (25) | 62 | 201 | 29 | 51 | 240 | 1 | 262 | 48 (36 to 61) | 93 (89 to 96) |
| Bloody | 10 (0.9) | 0 | 9 | 1 | 1 | 9 | 0 | |||
| Thick | 255 (22) | 62 | 131 | 62 | 92 | 162 | 1 | 192 | 87 (76 to 94) | 85 (77 to 90) |
| Trace of blood | 618 (53) | 153 | 433 | 32 | 116 | 498 | 4 | 582 | 52 (44 to 60) | 93 (91 to 96) |
| Centrifuge treatment | ||||||||||
| Not centrifuged | 1,004 (850) | 241 | 673 | 90 | 206 | 792 | 6 | 908 | 57 (50 to 63) | 94 (91 to 95) |
| Centrifuged on receipt | 171 (14.6) | 36 | 101 | 34 | 54 | 117 | 0 | 137 | 75 (58 to 89) | 81 (72 to 88) |
| Treatment with sheep blood agar followed by NALC-NaOHc | ||||||||||
| Aspirate | 8 (22) | 5 | 3 | 1 | 7 | |||||
| Fluid | 12 (33) | 2 | 8 | 2 | 2 | 10 | ||||
| Pus | 8 (22) | 3 | 2 | 3 | 4 | 4 | ||||
| Tissue | 8 (22) | 5 | 3 | 8 | ||||||
a
A total of 1,175 specimens were tested. A total of 124 samples (10.5% of 1,175 samples), including 9 aspirate samples, 1 bone sample, 1 CSF sample, 5 urine samples, 72 fluid samples, 27 pus samples, and 9 tissue biopsy samples, were classified as contaminated by MGIT culture testing. Xpert MTB/RIF classified 35 (28%) of these as positive and 89 (72%) of these as negative.
b
Data include all RIF results (RIF sensitive, RIF resistant, and RIF indeterminate).
c
A total of 36 samples were subjected to 24 h of bacterial growth on sheep blood agar followed by NALC-NaOH treatment.