The Indian Journal of Tuberculosis - LRS Institute of Tuberculosis ...

54 A.N. SASHIDHARA AND K. CHAUDHURI
the same. Its standard dose is 1:100 (1 mg in 0.1
ml). OT is now rarely used, except for multipuncture
tuberculin tests.
The PPD is a highly specific, potent, stable and
constant strength tuberculin. It is always prepared
and diluted according to a standard procedure.
Nonetheless, the mixture of polysaccharides varies
in composition and strength from batch to
batch. Besides, on storage, the active principles
get adsorbed to varying extent to the glass/plastic
vial walls. Polysorbate (Tween 80) has been
found to minimise such adsorption, but its addition
leads to comparatively smaller and softer reactions
which are more difficult to read correctly,
and their distributions tend to take a shape different
from those from tuberculin without Tween
80.
There is great merit in using a standard tuberculin,
of pre-determined strength, proved stability
and specificity. The search for a better, purer and
monospecific tuberculin than PPD has been
going on for a long time, without much success so
far.
The attempt to reduce the noted shortcomings
of PPD led to the preparation of very large
batches of PPD, field tested before use to determine
its potency and preserved in deep freeze for
worldwide use for a long time. One large batch
(lot 496.8) was prepared by Seibert et al in 1941,
and was labelled as PPD-S. It was made the international
standard and the United States standard
for calibrating other tuberculins in 1952. The
standard (first) dose of PPD-S is 5 TU (Tuberculin
units), defined as the delayed skin test activity
elicited by 0.0001 mg of PPD-S in 0.1 ml dose.
Another large batch was prepared for WHO in
1952, labelled as RT-23. Its standard dose for a
single or first test (1 TU) is equal to 0.00002 mg
of PPD-S + 0.000008 mg of buffer salts and
0.005% Tween 80. In India, RT-23 is preserved
and diluted for general use by the BCG Vaccine
Laboratory, Guindy (Madras). Most of the research
studies reported from India, therefore, are
with RT-23 with Tween 80 and comparable with
studies reported from elsewhere using the same
tuberculin.
Tuberculin Supplies
The ready-for-use tuberculin is supplied by the
BCG Vaccine Laboratory in amber colour, stop-
pered 100 ml vials; 0.1 ml of the solution contains
1 TU of RT-23. Exposure to light, heat and storage
leads to denaturing and deterioration in the
strength of tuberculin. Therefore, the vials must
be stored in a refrigerator (4°c) and used up, after
shaking, within the recommended time. Unless
these precautions are taken, the results of the
test will not be reliable. However, cooling is not
essential, for short duration, during actual testing;
the vial then needs to be protected against light
only.
The dose related rationale of the test is complex.
The reaction to a particular dose (tuberculin
sensitivity) varies with the biological species, and
in a species, individually as well as with the quality
of the antigen used. Therefore, the dose to be
used for testing is decided on the basis of sensitivity
and specificity of the test. Sensitivity (the condition
being present, what is the probability that
the test will be positive) measures the capability
of the test to correctly pick up truly infected person.
Specificity denotes the error of wrongly labelling
people as being infected (the condition
not being present, what is the probability of the
test response being negative). The epidemiological
situation also influences both these parameters.
Scientific studies done with the standard tuberculin
test have indicated the following :
1. Almost anyone, even infants less than 6
months in age, would react to tuberculin if a
sufficiently high dose is used for testing.
2. Most, but not all, patients of tuberculosis
react to a relatively low dose of tuberculin.
3. Most persons who have no history of tuber
culosis or a known contact with a case of
tuberculosis react to higher doses of tuber
culin.
4. With higher doses, the frequency of reac
tions due most probably to infection with
non-tuberculosis mycobacteria increases.
5. Taking > 6 mm reaction size as indicative
of tuberculous infection, a dose of 0.00001
mg of PPD would elicit 85% response
among patients and none at all in children
with no history of contact. However, with
the dose increased to ten times, 99.7%
among the patients would react and 0.9%
among children with no history of contact.
As a rule, the first or single dose of tuberculin
is selected on the basis of field trials. Higher