Table 2. Performance results of the proposed diagnostic algorithm with different ACR score thresholds, T.
| ACR threshold T | Selected for Xpert (%) | Sensitivity (%) | Specificity (%) | PPV (%) | NPV (%) | CSS ($) | CNTBC ($) | Throughput per day |
|---|---|---|---|---|---|---|---|---|
| 0 | 100 | 78.9 | 98.1 | 90.3 | 95.4 | 13.09 | 90.70 | 45 |
| 22 | 90 | 76.1 | 98.4 | 91.5 | 94.8 | 13.23 | 95.09 | 50 |
| 36 | 80 | 76.1 | 99.1 | 94.7 | 94.8 | 11.91 | 85.64 | 56 |
| 47 | 70 | 73.2 | 99.1 | 94.5 | 94.3 | 10.63 | 79.36 | 64 |
| 60 | 60 | 70.4* | 99.1 | 94.3 | 93.7 | 9.32 | 72.33 | 75 |
| 73 | 50 | 70.4* | 99.1 | 94.3 | 93.7 | 7.97 | 61.86 | 90 |
| 85 | 40 | 67.6* | 99.7 | 98.0 | 93.2 | 6.72 | 54.34 | 113 |
| 94 | 30 | 63.4* | 99.7 | 97.8 | 92.4 | 5.34 | 46.04 | 150 |
| 97 | 20 | 49.3* | 99.7 | 97.2 | 89.8 | 4.09 | 45.36 | 225 |
The first row matches a scenario where all subjects undergo Xpert and no ACR. Throughputs are based on an Xpert capacity of 45/day and an ACR capacity of 300/day. Costs for Xpert and ACR are $13.09 and $1.46 respectively.
ACR, automated chest radiography; CNTBC, cost per notified TB case; CSS, cost per screened subject; NPV, negative predictive value; PPV, positive predictive value; Xpert, Xpert MTB/RIF.
*Sensitivity significantly different from Xpert standalone, T = 0. (p < 0.05 considered significant).