Can easy TB diagnosis keep infectious TB low where HIV is common?
HIV made new TB cases about twenty times more common, but because HIV-positive people got sick and were diagnosed quickly, most undiagnosed infectious TB found by screening was in HIV-negative workers.
Source
Epidemiology of tuberculosis in a high HIV prevalence population provided with enhanced diagnosis of symptomatic disease
Study at a glance
- Design
- Cohort — Two-year prospective workforce cohort (nested in a cluster-randomised HIV testing trial) measuring incident TB, followed by a cross-sectional TB culture screening survey of all remaining employees.
- N
- N=6440 · 6,440 employees of 22 Harare workplaces in the incidence cohort; 4,668 of them (874 HIV positive) were screened in the end-of-study prevalence survey.
- Population
- Adult employees (mostly male, middle-aged manual and factory workers) of small and medium enterprises in Harare, Zimbabwe, with 19% HIV prevalence and no routine antiretroviral therapy.
- Outcome
- TB incidence per 1,000 person-years, point prevalence of undiagnosed TB, risk factors for each, and estimated duration of infectious disease before diagnosis by HIV status.
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What they did
Researchers enrolled the workforces of 22 companies in Harare and gave their clinics a trained nurse who investigated anyone with a persistent cough using sputum smears and cultures. They counted new TB cases over two years by HIV status, then screened every remaining employee with symptom questions and sputum culture to find TB that had not yet been diagnosed. Comparing prevalence with incidence let them estimate how long people had infectious TB before diagnosis.
What they found
New culture-positive TB occurred at 25.3 per 1,000 person-years in HIV-positive workers versus 1.3 in HIV-negative workers, an adjusted rate ratio of about 19. Yet at the end-of-study screen, smear-positive TB prevalence was only 1.3 per 1,000, lower than most African surveys, and many cases found were subclinical. HIV-positive TB was diagnosed much sooner (about 12 weeks of culture positivity versus 108 weeks for HIV-negative TB), so HIV accounted for 78% of incident but only 14% of prevalent culture-positive TB.
The limits
What it doesn't show
It is one workplace population that was mainly employed, male and middle-aged, so a healthy-worker effect may make prevalence look lower than in the general population. Only 27 prevalent cases were found, so risk-factor estimates and duration estimates are imprecise. Some incident cases may have been missed, and the effects of isoniazid preventive therapy and HIV care offered through the parent trial were not analysed. It also relied on culture and sensitive microscopy, which many programmes lack, so results may not hold where diagnosis is harder to reach.
Key terms
- Incidence
- The rate of new cases arising in a population over a period of time, here per 1,000 person-years.
- Point prevalence
- The proportion of a population that has the disease at a single moment, including undiagnosed cases.
- Population-attributable fraction (PAF)
- The share of cases in a population that would not occur if a given risk factor were removed, assuming a causal link.
- Subclinical TB
- Active TB detectable by culture in someone who does not yet report symptoms.
- DOTS
- The WHO TB control strategy built on diagnosing people who come forward with symptoms and treating them under direct observation.
- Passive case finding
- Detecting disease only when patients present themselves with symptoms, rather than actively screening everyone.
Flashcards
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Quiz yourself
Roughly how much higher was the rate of new culture-positive TB in HIV-positive workers?
Common questions
If HIV raises TB incidence so much, why didn't it raise prevalence as much?
HIV-related TB progresses quickly and causes symptoms early, so people seek care and are diagnosed within weeks. Prevalence reflects both how often disease starts and how long it lasts before treatment, and the short duration offsets the high incidence.
Why does prevalence matter for transmission?
People with undiagnosed infectious TB are the ones spreading it, so keeping the pool of prevalent smear-positive cases small should reduce transmission even if new cases keep arising.
What is the practical takeaway for TB control?
Case-finding programmes should not focus only on HIV-positive people, because most undiagnosed infectious TB here was in HIV-negative workers who had been ill for much longer.
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