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Measuring the burden Surveys of TB infection and disease WPRO Workshop May 2003

Measuring the burden Surveys of TB infection and disease WPRO Workshop May 2003. Exercises# Measuring the burden of tuberculosis.doc. We want to reduce Morbidity and mortality New infections Recurrence of old infections The infector pool. Routine programme data. Notifications

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Measuring the burden Surveys of TB infection and disease WPRO Workshop May 2003

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  1. Measuring the burden Surveys of TB infection and disease WPRO Workshop May 2003

  2. \Exercises\# Measuring the burden of tuberculosis.doc

  3. We want to reduce • Morbidity and mortality • New infections • Recurrence of old infections • The infector pool

  4. Routine programme data • Notifications • Cure rates • Drug resistance • Mortality • Process indicators

  5. But... • Biases • Incompleteness • Recording • Reporting • Analysing • AIDS

  6. Four things we can measure • Prevalence of infection (tuberculin surveys) • Incidence of infection (ARTI) • Incidence of disease • Prevalence of disease • Prevalenc of HIV in TB patients

  7. Measuring the prevalence of infection Tuberculin surveys

  8. Sampling Districts: Probability proportional to size Schools in districts: Random Children in schools: Total sample within schools Sample size Levels of variation Logistical considerations

  9. Tanzania 1983 to 1988 Egypt 1995 to 1997 15 10 5 0 4 2 0 Frequency (%) Frequency (%) BCG BCG No BCG No BCG 0 10 20 30 0 10 20 30 Size of induration (mm) Size of induration (mm) Korea

  10. South Korea 1965 1995 Age (years)

  11. Calculating ARTI(R) from prevalence (P) and age (A) R = 1 – (1 – P)1/A Probability of being infected per year = R Probability of not being infected per year = 1 – R Probability of not being infected after A years = (1 – R)A 1 – P = (1 – R)A 1 – R = (1 – P)1/A

  12. Assumptions The annual risk of infection is constant over time and independent of age. Surveys in school children, aged about 5 to 10 years: estimate of the ARTI is averaged over about five to ten years.

  13. South Korea 10 to 20 years 5 to 10 years

  14. Measuring the incidence of disease

  15. Calculating incidence from the ARTI Styblo showed that for every 1% increase in the annual risk of infection, the incidence of SS+ disease increases by (5010)/100,000/yr. If we multiply the ARTI by 50 this gives us an estimate of the incidence of SS+ TB.

  16. Estimating sample sizes quickly If you count N positives then: N 95% confidence limits  2N Fractional error 2N/N = 2/N Incidence (South Korea, 1990)  40 per 100,000 95% confidence limits   2N = 13 ( 32%) Sample of 800,000 gives 320 positive  11%

  17. South Korea Incidence Survey 1998: Baseline 920,000 had chest radiography 840,000 did not have suspicious CXR 1990: Follow up 790,000 available for CXR 2,500 with suspicious X-rays 2000 had sputa taken 200 smear positive 300 culture positive (smear negative)

  18. South Korea ARTI50 Civil servants

  19. Measuring the prevalence of disease Same problems as for incidence….

  20. South Korea

  21. The Impact of HIV

  22. The incidence of SS+ tuberculosis plotted against the prevalence of HIV for African countries in 2001.

  23. Sub-Saharan Africa

  24. Nairobi

  25. Nairobi

  26. Nairobi

  27. Age and gender specific incidence of tuberculosis in Kisii, Kenya Green: 1991 to 1993; blue: 1994 to 1998; red: 1999 to 2001. HIV prevalence (%) Age-specific prevalence of HIV among men and women in Carletonville, South Africa, in 1998.

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