1. Surveys of TB infection and disease
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 Korea Frequency (%)
BCG
No BCG
Size of induration (mm) Size of induration (mm)
Frequency (%) 4 2 15 10 5
Tanzania 1983 to 1988
Egypt 1995 to 1997
Korea

10. South Korea
1965
1995
Age (years)

11. from prevalence (P) and age (A)
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.