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PROBLEMS CONNECTED WITH ESTIMATION OF THE INCIDENCE
OF TUBERCULOSIS INFECTION |
Raj Narain, SS Nair, P Chandrasekhar & G Ramanatha
Rao: Indian J TB 1965, 13, 5-23. |
The incidence of infection with mycobacterium tuberculosis
is an index of the risk of infection to which a community is exposed.
An accurate estimation of incidence rate is of considerable importance
in understanding the epidemiology of tuberculosis in organising
control measures. A new method of estimating incidence of infection
is discussed. The material from 3 studies of National TB Institute
has been utilized. Study I: is a part of a survey of a random
sample of 134 villages. No previous tuberculin testing or BCG vaccination
had been carried out in the area, but each person was examined for
BCG scar in order to exclude persons vaccinated probably from other
areas. After a complete census, a Mantoux test with 1 TU of PPD
RT 23 with Tween 80 given on two occasions (Round I and II). Those
with reaction of 13mm or less at Round I were offered a test with
20 TU with Tween 80 within a week of 1 TU test. The interval between
the rounds was about 18 months. From the analysis of the data from
the first 50 villages for which complete information for both rounds
was available, it was seen that there was a general increase in
the size of reactions elicited in the second round. Study 2:
tuberculin testing was carried out with 1 TU and 20 TU among
selected control groups which provided the data regarding
the enhancing of tuberculin allergy seen in repeat tuberculin
tests. Study 3: in the course of the longitudinal survey
reader assessments were carried out periodically to judge
the standards of the tuberculin test readers. Inter & intra-reader comparisons were made. The findings have been used to estimate
the magnitude of reader variation. The data was also used to study
variations in the technique of testing and reading.
It was estimated that on an average inter &
intra-reader variations between the rounds were unlikely to exceed
6mm or more in more than 5% of the observations. The reading errors
have an equal chance of being positive or negative except at extreme
ends of the distribution where zero readings of Round I can only
show an increase, and the very large reactions had a greater chance
of showing only a decrease at a subsequent round. The study mainly
concerns with the problems of estimating the incidence of tuberculous
infection in a community. Calculations based on age-specific prevalence
rates or on rates of tuberculin conversion or both subject to gross
error, leading to unreliable epidemiological conclusions. For estimating
the newly infected, a new approach has been suggested based on the
drawing of a curve for the distribution of differences in reaction
size from one round of tuberculin testing to another. It is assumed
that if new infection causes a distinct rise in the degree of tuberculin
sensitivity which is greater than the combined rise due to enhancement
and reader variation, the distribution of differences between the
rounds should indicate the newly infected. It is shown that the
newly infected probably constitute a homogeneous group with an increase
in mean reaction size of about 24mm and standard deviation of 4mm.
Accordingly, 98% of the newly infected show an increase in reaction
size of 16mm or more.
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KEY WORDS: RISK OF INFECTION, TUBERCULIN ALLERGY, ENHANCEMENT,
INCIDENCE, INFECTION. |