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Direct estimates of the burden of disease Surveys of infection and disease

Direct estimates of the burden of disease Surveys of infection and disease AFRO Workshop August 2003. We want to reduce Morbidity and mortality New infections Recurrence of old infections The infector pool. Routine programme data. Notifications Cure rates Drug resistance Mortality

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Direct estimates of the burden of disease Surveys of infection and disease

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  1. Direct estimates of the burden of disease Surveys of infection and disease AFRO Workshop August 2003

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

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

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

  5. 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

  6. Measuring the prevalence of infection Tuberculin surveys

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

  8. 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

  9. South Korea 1965 1995 Age (years)

  10. 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

  11. 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.

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

  13. Measuring the incidence of disease

  14. 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.

  15. 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%

  16. 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)

  17. South Korea ARTI50 Civil servants

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

  19. South Korea

  20. Exercises 1. Open the CD 2. Go to the directory Exercises 3. Open the file # Notes and examples.doc 4. Do exercises 8, 10, 2

  21. Can we use the prevalence of HIV in TB patients as a monitoring tool?

  22. Sub-Saharan Africa

  23. Kenya

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