|PROBLEMS IN DEFINING A CASE OF PULMONARY
TUBERCULOSIS IN PREVALENCE SURVEYS
|Raj Narain, SS Nair, K Naganna, P Chandrasekhar, G
Ramanatha Rao & Pyare Lal: Bull WHO 1968, 39, 701-29.
Generally there is no acceptable definition of
the term case of pulmonary tuberculosis, although such
a definition is of fundamental importance both in clinical medicine
where results of various chemotherapeutic regimens are compared,
as well as for the comparison of different epidemiological data.
The main purpose of this paper is to focus attention on the difficulties
of defining a case on the basis of bacteriological examination,
X-ray examination and tuberculin test. Data from two successive
prevalence surveys in a random sample of 134 villages in Bangalore
district with a population 70,000 have been utilized to illustrate
some of the difficulties in defining a case of pulmonary
tuberculosis for reporting the prevalence or incidence of the diseases.
The entire population was tuberculin tested with 1 TU RT 23 with
Tween 80 at both rounds and those 5 years of age and older were
examined by 70mm photofluorogram. The sputum specimens (spot and
overnight) were collected from those with any abnormality on X-ray
as recorded by either of the two independent readers. Both the specimens
were examined by fluorescent microscopy and Ziehl-Neelsen technique
and by culture.
Analysis of data has shown that the term a
case of pulmonary tuberculosis does not represent a single
uniform entity, but embraces cases of several types, differing considerably
in their tuberculin sensitivity, results of X-ray and sputum examination,
in the reliability of their diagnosis and mortality experience.
The status of cases found at initial and subsequent surveys showed
changes with time, and such changes show considerable differences
for the various types of cases. It was felt that a single straight-forward
definition of a case was not possible to suit all situations. One
has to use more than one definition. Although theoretically, finding
a single bacillus in sputum should be adequate proof of pulmonary
tuberculosis, it was shown that finding of a few bacilli (3 or less)
was very often due to artifacts and should not be the basis for
a diagnosis. It has also been found that positive radiological findings,
in the absence of bacteriological confirmation, indicate only a
high risk of the disease and not necessarily pulmonary tuberculosis.
Direct microscopy appears to be a consistent index of disease but
in community surveys has the limitation of missing a substantial
proportion of cases and of adding some false ones.
In view of the difficulty of providing a single
definition of a case of tuberculosis, four indices have been suggested.
(1) Cases definitely positive by direct smear; (2) Cases definitely
positive by culture; (3) All cases positive by culture (including
less than twenty colonies); (4) Sputum positive cases which are
radiologically active. Each of these could be used for different
situations. However, it was concluded that, there seems to be no
option but to use more than one definition for assessing the prevalence
and incidence of disease.
|KEY WORDS: CASE-DEFINITION, SURVEY, PREVALENCE,