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Tuberculin
Skin Test (TST)
The TST is widely used as a supportive second line
test to identify patients actively infected with tuberculosis.
It has remained more or less unchanged for the last
60 years and has been in existence for more than 100
years. There are three types of tuberculin skin test
but the most common is the Mantoux test.
The skin test works by injecting Purified Protein Derivative
(PPD) into the skin. PPD is a collection of mixed proteins
and other materials filtered from killed M. tuberculosis
cultures. The test works on the basis that if the body
has been exposed to infection with TB it will recognise
the proteins and mount an immune response to it. This
response would take the form of a lump, swelling or
blister at the site of injection. If there is a lump
(called an induration) then this may mean that the person
is infected. Unfortunately, the skin test has a poor
sensitivity (the ability to detect infection if it is
present). This sensitivity falls further if the person
being tested has had the BCG vaccination earlier in
life or if they have a depressed immune system (immunocompromised)
due to other illness or medical treatment.
TB skin test is also known as Mantoux
test. It is the most widely used test in which
graded doses of tuberculin are injected intradermally
on the forearm using a tuberculin syringe.
Koch's tuberculin was an impure extract of boiled culture
of tubercle bacilli. In 1934, Siebert made a simple
protein precipitate of the old tuberculin (one prepared
by Koch) and named it as purified protein derivative
(PPD).
During 1970, it was recognized that PPD in solution
adheres to glass to the extent that 20% of its potency
can be lost in 30 minutes and 80% in 24 hours. This
can be prevented by addition of Tween 80.
Procedure:
0.1 ml of the 5 TU of PPD is injected intradermally
on the forearm. On examination after 48-72 hours a positive
reaction is indicated by erythema and in duration of
> 10 mm size. Erythema(Redness) alone is not taken
as positive reaction.
Tuberculin skin testing is used as an aid in diagnosing
active infection in infants and young children, to measure
the prevalence of infection in a community and to select
susceptible or high risk patients for BCG vaccination.
All persons with prior infection with tubercle bacilli
will exhibit a positive response.
Some of the common problems doctors and nurses encounter
with the skin test are described below:
False positives
As the active ingredient used in the skin test contains
a whole series of proteins that are shared with the
BCG vaccine and other mycobacterium common in the environment,
the skin test is commonly falsely positive in people
who have had no exposure to TB. Common causes of false
positive results are prior BCG vaccination and infection
with other types of bacteria that are similar to TB.
As a large percentage of the world's population is BCG
vaccinated this causes considerable problems; it is
currently estimated that almost one third of people
positive to the TST do not actually have TB infection.
False negatives
The sensitivity of the skin test is estimated to be
only around 70% in known active TB cases; so the test
misses up to 30% of the people who are infected. This
sensitivity decreases to as low as 30% in the immunocompromised
(which means the error rate can climb to 70% in these
people). This makes it very difficult for a doctor to
be able to make the right medical decisions because
the reliability of the result is so poor.
Subjectivity and variability
The skin test is difficult to administer correctly
as small variations in the way it is done vary the amount
of PPD delivered into the skin and thus the resulting
size of the reaction. Furthermore, the measurement of
the reaction is highly subjective; the variations in
diagnosis based on different clinicians reading the
same bump in different ways is well documented.
Boosting
A common problem in those people who are regularly
screened for TB infection using the skin test (e.g.
Healthcare Workers) is that they start to become immunised
to PPD by the repeated administrations of it. This is
called 'boosting' and results in a false positive reaction
to the skin test.
Convenience & Resources
The skin test is not patient friendly as it can result
in painful blistering at the injection site and result
in a scar. It also requires two patient visits - one
to inject the tuberculin and one to read the induration
(although it is estimated that a third of people never
return to have the test read). This is inconvenient
for the clinician and patient alike
Sputum
Smear Microscopy (SSM)
The simplest laboratory test is the examination of
sputum (matter thrown up from the lungs) for the detection
of a certain type of bacteria. It is cheap and is performed
in minutes. This test is based on the principle of Ziehl
Neelsen diagnostic technique of direct smear microscopy
of sputum. The unique properties of bacterial cell wall
of Mycobacterium tuberculosis allows it to retain the
primary stain even after exposure to strong acid solutions,
they are called acid-fast. In the Ziehl Neelsen staining
procedure, using carbol fuschsin and methylene blue,
the acid-fast organisms appear red.
Learn More on Acid Fast Direct Microscopy
Acid-Fast
Direct Smear Microscopy- A Laboratory Training Program
TECHNICAL GUIDE on Sputum Examination for Tuberculosis
by Direct Microscopy in Low Income Countries, Fifth
edition 2000
International
Union Against Tuberculosis and Lung Disease
However, the WHO estimates that it only identifies 35%
of patients with active TB. As the test is based on
sputum, it has particular difficulty in detecting non-pulmonary
TB. This test will also identify certain types of bacteria
that are not M. tuberculosis and so it cannot always
distinguish between TB and other infections. Despite
these shortcomings, it is still the front line tool
for active TB diagnosis, partly because the more definitive
culture techniques take longer and partly because it
can help determine if a person is infectious. It is
argued that as long as bacteria are found in the sputum
the patient can continue to pass on the disease to other
people. Sputum smear is therefore one method that can
be used to monitor an active TB patient's response to
treatment.
Chest
X-ray
Chest x-rays are used to check for lung abnormalities
in people who have symptoms of TB disease, but the chest
X-ray cannot confirm that a person has active TB, especially
if the infection is not in the lungs as in 40% of all
cases of active TB. The chest X-ray also has a poor
ability to detect infection in the early stages of disease,
the damage to the lungs may not yet have become sufficiently
marked to be detectable by chest X-ray and thus people
who have active TB can be missed. Further, scarring
in the lungs remains after a previous TB disease (even
if the patient is completely cured) and therefore it
is difficult to distinguish past cured TB from current
active disease.
Polymerase
Chain Reaction (PCR)
This technique detects the presence of DNA-type (genetic)
material from bacteria by effectively amplifying the
measurable amount. Polymerase chain reaction (PCR),
is a relatively new development in active TB testing.
Even though PCR techniques can magnify even the smallest
amounts of genetic material, the sample used still has
to contain a certain number of TB bacteria and this
is not always possible, particularly with non-pulmonary
TB where sensitivity can be as low as 60%. To increase
the number of bacteria, and hence improve the sensitivity
of the test, the laboratory will often culture the sample,
to allow the bacteria to multiply, before carrying out
the PCR test. This can take several days or weeks. The
test is also relatively complicated to run in the laboratory,
is prone to cross contamination and can be expensive.
The main use of PCR is not to diagnose TB per se ,
but to rule out other types of infection in a sputum
smear positive patient, before culture results are known.
Culture
Cell culture techniques (where live bacteria are grown
on a plate in the laboratory) are still seen as the
gold standard for active TB as they are extremely sensitive
if live mycobacteria can be obtained in the sample.
M. tuberculosis can be cultured (grown) from
a variety of specimens and can be used to detect pulmonary
as well as extra-pulmonary disease. By assessing the
effect of antibiotics on the cultured bacteria, this
technique can also provide data on likely effectiveness
of certain antibiotics. However, it is not always possible
to obtain bacteria in the sample, especially in non-pulmonary
TB and the test is therefore not always reliable. A
drawback of this test is the time to result, which can
be anything from two to six weeks.
Blood
Test for TB detection
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