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 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)

  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