The persistence of tuberculosis in the age of DOTS: reassessing the effect of case detection
David W Dowdy & Richard E Chaisson
Volume 87, Number 4, April 2009, 296-304
Table 1. Parameter values for a compartmental difference-equation model of a TB epidemic1
| Parameter | Value | Sensitivity range | Reference | |
|---|---|---|---|---|
| TB dynamics | ||||
| Secondary TB infections per smear-positive person-yeara (No.) | 11.9 | 6–15 | ||
| Proportion of incident TB cases that are smear-positive | ||||
| If HIV– | 0.45 | 0.3–0.6 | ||
| If HIV+ | 0.35 | 0.2–0.5 | ||
| Infectivity of patients with pulmonary smear-negative TB (relative to that of smear‑positive TB patients) | 0.22 | 0.1–0.3 | ||
| Proportion of smear-negative TB cases that are extrapulmonary (i.e. non-infectious) | 0.33 | 0.2–0.5 | ||
| Proportion of new TB infections progressing to active TB within 1 year | ||||
| If HIV– | 0.14 | 0.05–0.25 | ||
| If HIV+ | 0.25 | 0.1–0.4 | ||
| Efficacy of latent infection in preventing reinfection | ||||
| If HIV– | 0.72 | 0.6–1.0 | ||
| If HIV+ | 0.25 | 0–0.5 | ||
| Yearly rate of endogenous reactivation (%) | ||||
| If HIV– | 0.11 | 0.05–0.25 | ||
| If HIV+ | 4.86 | 2.0–10.0 | ||
| Yearly spontaneous conversions from smear-negative to smear-positive (%) | 2.0 | 1.0–3.0 | ||
| Mortality | ||||
| Mean life expectancy without HIV infection (years)b | 55 | 45–70 | ||
| Mean life expectancy after HIV infection (years) | 11 | 8–14 | ||
| Untreated TB patients who will die (%) | ||||
| If HIV–, smear-positive | 50 | 40–70 | ||
| If HIV+ | 100 | 80–100 | ||
| Mean duration of untreated TB disease (years) | ||||
| If HIV–, smear-positive | 2.0 | 1.0–3.0 | ||
| If HIV+ | 1.0 | 0.5–2.0 | ||
| Mortality in smear-negative TB patients (relative to smear-positive TB patients) | ||||
| If HIV– | 0.33 | 0.2–0.4 | ||
| If HIV+ | 1.0 | 0.75–1.25 | ||
| TB diagnosis and treatment | ||||
| Mean duration of successful diagnostic attempt (weeks)c | 5 | 2–10 | ||
| Diagnostic sensitivity for smear-negative TB patients (relative to that for smear-positive TB patients)d | 0.67 | 0.5–1.0 | ||
| Treatment success rate | 0.85 | 0.7–0.9 | Assumed | |
| Proportion of TB patients registered as deceasede | 0.07 | 0.02–0.10 | ||
| Annual relapse rate among patients treated unsuccessfullyf | 0.065 | 0.05–0.15 | ||
CDR, case detection rate; TB, tuberculosis.
a The source gives an annual rate of TB infection of 2.0% in a population representative of sub-Saharan Africa (51% CDR for smear-positive TB, 45% overall CDR, and 22% HIV infection prevalence among incident TB cases).
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b Representative of the projected South African population in 2007.
c The source assumes 3 weeks from presentation to diagnosis
32 plus 2 weeks of therapy to render the patient non-infectious and at baseline mortality risk.
33
d The source assumes that a sputum smear is obtained from 63% of suspected TB cases and that others receive a combination of chest X-ray (with 76% sensitivity) and a trial of broad-spectrum antibiotics (with 60% sensitivity).
e The source gives data for the WHO African Region in 2006.
f Calculated in the source as the rate of TB recurrence after default, minus the rate of confirmed reinfection, in two urban populations near Cape Town, South Africa.