Table 3.
Correlation between Xpert MTB/RIF semi-quantitative results and AFB smear microscopy results
| Prediction of Xpert positive | AFB Smear results | Prediction of AFB smear positivity | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Total samples | Confirmed TB | PPV (95% CI) | Negative | 1+ | 2+ | 3+ | 4+ | Total smear positive | Sensitivity (95% CI) | PPV (95% CI) | ||
| Xpert results | Positive high | 12 | 12 | 100% (75.8–100) | 3 | 2 | 2 | 5 | 0 | 9 | 75.0% (46.8–91.1) | 100% (70.1–100) |
| Positive medium | 13 | 13 | 100% (77.2–100) | 6 | 6 | 0 | 1 | 0 | 7 | 53.9% (29.1–76.8) | 100% (64.6–100) | |
| Positive low | 23 | 21 | 91.3% (73.2–97.6) | 21 | 2 | 0 | 0 | 0 | 2 | 9.5% (2.7–28.9) | 100% (34.2–100) | |
| Positive very low | 17 | 16 | 94.1% (73.0–99.0) | 15 | 2 | 0 | 0 | 0 | 2 | 12.5% (3.5–36.0) | 100% (34.2–100) | |
| Negative | 722 | 27 | NPV = 96.3% (94.6–97.4) | 715* | 1 a | 2 #a | 3 b | 1 a | 7 | 3.7% (0.7–18.3) | 14.3% (2.6–51.3) | |
| Total | 787 | 89 | 95.4% (87.3–98.4) | 760 | 13 | 4 | 9 | 1 | 27 | 23.6% (15.2–33.8) | 77.8% (59.2–89.3) | |
Abbreviation: CI confidence interval; PPV Positive predictive value; NPV Negative predictive value. *, include 26 false negative Xpert results; #, include 1 false negative Xpert result (1 true positive AFB smear result); a, include 1 false positive AFB smear result; b, include 3 false positive AFB smear results
Xpert MTB/RIF semi-quantitative results defined by the manufacturer as follows: positive very low (cycle threshold, CT > 28), low (22 < CT≤ 28), medium (16 < CT≤ 22) or high (CT ≤ 16)
AFB smear microscopy results were graded according to American Thoracic Society scale: Negative, 0 AFB/300 fields; Positive 1+, 1–9 AFB/100 fields; Positive 2+, 1–9 AFB/10 fields; Positive 3+, 1–9 AFB/field; Positive 4+, > 9 AFB/field