Skip to main content

Table 3 Correlation between Xpert MTB/RIF semi-quantitative results and AFB smear microscopy results

From: A retrospective study on Xpert MTB/RIF for detection of tuberculosis in a teaching hospital in China

 Prediction of Xpert positiveAFB Smear resultsPrediction of AFB smear positivity
 Total samplesConfirmed TBPPV (95% CI)Negative1+2+3+4+Total smear positiveSensitivity (95% CI)PPV (95% CI)
Xpert resultsPositive high1212100% (75.8–100)32250975.0% (46.8–91.1)100% (70.1–100)
Positive medium1313100% (77.2–100)66010753.9% (29.1–76.8)100% (64.6–100)
Positive low232191.3% (73.2–97.6)21200029.5% (2.7–28.9)100% (34.2–100)
Positive very low171694.1% (73.0–99.0)152000212.5% (3.5–36.0)100% (34.2–100)
Negative72227NPV = 96.3% (94.6–97.4)715*1 a2 #a3 b1 a73.7% (0.7–18.3)14.3% (2.6–51.3)
Total7878995.4% (87.3–98.4)760134912723.6% (15.2–33.8)77.8% (59.2–89.3)
  1. 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
  2. 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)
  3. 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