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Late night thoughts on tuberculin surveys
Brian Williams with acknowledgements to
Philippe Glaziou
Cairo, October 2009
Karel Styblo: 1985
If the annual risk of TB infection
is 1% the annual incidence of
TB disease is 50/100k/yr.
If the prevalence of TB in
children aged A is P then the
annual risk of TB infection is
ARI = 1-(1-P)1/A
We can use the prevalence in
children to estimate the
incidence in adults
First the good news…
Frequency distribution of induration sizes in Cape Town
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
1955 1965 1975 1985 1995
AR
TI in
So
uth
Ko
rea
South Korea
Halving every
ln(2)/0.08 ≈ 10 years
Rate of decline = 7.8%/yr
0
100
200
300
400
500
600
700
800
900
1960 1965 1970 1975 1980 1985 1990 1995
Pre
va
len
ce
SS
+ T
B/1
00
kSouth Korea
Rate of decline = 8.0%/yr
0
100
200
300
1960 1970 1980 1990
SS
+ T
B in
cid
en
ce
/10
0k
ARTI×50
Civil servants
South Korea
Rate of decline = 8%/yr
Now the bad news…
South Korea Survey of
Incidence among Civil Servants
1998: Baseline
920,000 had chest radiography
1990: Follow up
790,000 available for CXR
2,500 with suspicious X-rays
2000 had sputa taken
300 culture positive
200 smear positive
Styblo’s argument assumptions (before chemotherapy)
Each prevalent SS+ case infects 10 people per year
Disease duration is 2 years
Each incident SS+ case infects 20 other people
ARI = 20××××Incidence
Incidence = ARI/20
Incidence××××100k = (ARI××××100)/100××××100k/20
= (ARI/100)××××50
1% ARI corresponds to and incidence of 50/100k
ARI is independent of age. Average over last ten years
Low prevalence
As the incidence declines, the
prevalence of infection declines and it
gets much harder to measure it.
Changes in distribution patterns of
induration size in Korea
Environmental mycobacteria
Prevalence of environmental
mycobacteria seems to be very high
around the equator and declines at very
high and very low latitudes
Frequency distribution of induration sizes in Somalia.
Red: fitted; black: environmental; green: TB
Dependence on testing day
(Somalia 2006)
Proportion positive when the induration was read on
three of four days after the challenge
Dependence on BCG
(Somalia 2006)
Proportion positive with and without a BCG scar
The way forward?
Tuberculin skin test responses in
houehold contacts of active TB cases979 children, median age 7yr, Istanbul
0
10
20
30
40
50
60
70
80
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31
ELISpot -ve
ELISpot +ve
Source: Bakir et al 2006
Thank you
0 5 10 15 20 25
Induration size (mm.)
Fre
qu
en
cy (
%)
10
8
6
4
2
0
Annual risk of tuberclous infection in the northern zone of India
Chadha, V.K. et al. Bull. WHO (2003)
Styblo assumptions (cont)
• An infected case develops TB with probably 0.1 over his lifetime
• The probability for TB to be smear positive is 0.5
• Therefore, 1 smear positive generates: 10 x 2 x 0.1 x 0.5 = 1 new smear positive case over 1 transmission cycle and TB is stably endemic
Origin of Styblo’s ruleBull IUAT vol 60, 1985
Ratio basedon
Risk of infection(%)
Ratio ofrisk:incidence
Netherlands 1921-38 Mortality 2.7 – 6.0 38
Netherlands 1951-76 Incidence 0.038 – 0.4 37
Developingcountries
1956-61 Prevalence 2.0 – 8.0 40-60
Alaskaeskimos
1948-51 Mortality 25 52
Indialongitudinalstudy
1961-68 Prevalence 1.5 53
Indiapreventiontrial
1969-71 Prevalence 4.1 51
Changes in Styblo ratio in Chennai, S. India
0
50
100
K P E Th Ti TiT All
Blocks in Chingelput district
Inc
ide
nc
e S
+C
+/A
RT
I
1968-71
1973-78
Source: IJTLD 5, 142 (2001)
Assumptions (cont)
• 2 year duration of infectiousness: not necessarily true, duration may decrease with increasing programme performance
• 10 cases infected per year: depends on living conditions, number of household members, etc, and may decline over time
• TB is in a steady state: untrue in most countries
High sensitivity to assumptions on
distributions in Somalia
Inconsistent patterns
(Somalia 2006)
In summary
• Interpretation of data can be problematic
• Diagnostic value of PPD test difficult to predict
• Uncertainty about the relationship between prevalence of infection and incidence
• Uncertainty about time changes in the distribution of determinants of induration size
• Difficult to interpret time-changes in ARI
Diverging time-changes between
SS(+) prevalence and ARI in China
Source: Bulletin of the World Health Organization 2008;86:20–26.
Diverging time-changes between SS(+)
prevalence and ARI in the Philippines
Source: Bulletin of the World Health Organization 2008;86:20–26.
Ethical considerations
• No (or little) benefit to surveyed individuals
• Most often done in children
• Ethical requirements:
– Clearance from ethical committee
– Informed consent from parents
– Informed consent from children
Sampling
Typically multistage with clusters defined as schools or classes within schools→ need to account for sampling design when computing age-specific prevalence of infection→ is the sample self-weighed (each selected child should represent the same number of children)?→ missing values (e.g. Malawi, 21%)
0.0
0.2
0.4
0.6
0.8
1.0
0 5 10 15 20 25 30
0
100
200
300
400
500
600
0 5 10 15 20 25 30
Induration (mm.)
Nu
mb
er/
mm
.Can tuberculin surveys measure risk of infection?
Data from Somalia 2006
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