Model inputs: cohort probabilities, diagnostic parameters and costs
| Value | Range | Reference | |
|---|---|---|---|
| Probability parameters | |||
| Underlying prevalence of TB among those with a cough of at least 2 weeks' duration | 5% | 1–10% | 24 |
| Prevalence of HIV among those with active TB | 48% | 40–60% | 1 |
| Prevalence of HIV among those with cough but no active TB | 7.2% | 2–20% | 1 |
| Probability of HIV serostatus awareness (among those with HIV/TB) | 91% | 80–98% | 1 |
| Probability of HIV serostatus awareness (among those with HIV and a cough but no active TB) | 63% | 45–80% | 1 |
| Probability that referral for Xpert is completed | 85% | 65–96% | 19 |
| Probability of no treatment after positive Xpert result | 0% | 0–10% | Assumption |
| Probability that a patient with TB missed by screening does not return for future diagnosis and treatment | 20% | 11–30% | 25 |
| Probability of empiric treatment among patients with TB testing negative | 17% | 8–25% | 26 |
| Monthly probability of TB spontaneously resolving (HIV+) | 0% | 0–1% | 27 |
| Monthly probability of TB spontaneously resolving (HIV−) | 2% | 1–6% | 23 |
| Probability of TB cure on first-line treatment | 77% | 65–85% | 1 |
| Monthly probability of death from untreated TB | 1% | 0.5–3% | 1 |
| Probability of TB death on first-line treatment | 6% | 2–15% | 1 |
| Probability of loss to follow-up on first-line treatment | 17% | * | 1 |
| Diagnostic parameters | |||
| Sensitivity of clinical diagnosis† | 44% | 26–63% | 26 |
| Sensitivity of CRP (10 mg/L as a positive screen) | 98% | 80–100% | 28 and C Yoon et al. Under review |
| Sensitivity of CXR (‘any abnormality’ as a positive screen) | 98% | 72–99% | 29 |
| Sensitivity of Xpert | 89% | 75–95% | 30 |
| Specificity of clinical diagnosis† | 87% | 81–92% | 26 |
| Specificity of CRP (10 mg/L as a positive screen) | 59% | 50–68% | 28 and C Yoon et al. Under review |
| Specificity of CXR (‘any abnormality’ as a positive screen) | 67% | 50–80% | 31 |
| Specificity of Xpert | 99% | 98–99% | 30 |
| Cost parameters | |||
| CRP test | US$3 | US$2–US$6 | Field data |
| CXR | US$5 | US$3–US$10 | 32 |
| Xpert | US$23.58 | US$18–US$28 | 26 |
| First-line TB therapy (category 1—total) | US$201 | US$156–US$243 | 26 |
| Effectiveness parameters | |||
| Discounted life expectancy, HIV-positive | 13.75 years | 9–19 | Assumption |
| Discounted life expectancy, HIV-negative | 17.87 years | 13–23 | Assumption |
*Set equal to (1-probability of cure-probability of death).
†Probability that an individual with active TB will be referred for Xpert testing in the absence of a triage test.
CRP, C reactive protein; CXR, chest X-ray; TB, tuberculosis.