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The diagnosis of TB is made on the basis of
laboratory test results. The standard test for tuberculosis--which
is the so-called tuberculin skin test--detects the presence of
infection, not of active TB. Tuberculin is an extract prepared from
cultures of M. tuberculosis. It contains substances belonging
to the bacillus (antigens) to which an infected person has been
sensitized. When tuberculin is injected into the skin of an infected
person, the area around the injection becomes hard, swollen, and red
within one to three days. Today skin tests utilize a substance
called purified protein derivative (PPD) that has a standard
chemical composition and is therefore is a good measure of the
presence of tubercular infection. The PPD test is also called the
Mantoux test. The Mantoux PPD skin test is not, however, 100%
accurate; it can produce false positive as well as false negative
results. What these terms mean is that some people who have a skin
reaction are not infected (false positive) and that some who do not
react are in fact infected (false negative). The PPD test is,
however, useful as a screener. Anyone who has suspicious findings on
a chest x ray, or any condition that makes TB more likely should
have a PPD test. In addition, those in close contact with a TB
patient and persons who come from a country where TB is common also
should be tested, as should all healthcare personnel and those
living in crowded conditions or institutions.
Because the symptoms of TB cover a wide range of
severity and affected body parts, diagnosis on the basis of external
symptoms is not always possible. Often, the first indication of TB
is an abnormal chest x-ray or other test result rather than physical
discomfort. On a chest x ray, evidence of the disease appears as
numerous white, irregular areas against a dark background, or as
enlarged lymph nodes. The upper parts of the lungs are most often
affected. A PPD test is always done to show whether the patient has
been infected by the tubercle bacillus. To verify the test results,
the physician obtains a sample of sputum or a tissue sample (biopsy)
for culture. Three to five sputum samples should be taken early in
the morning. If necessary, sputum for culture can be produced by
spraying salt solution into the windpipe. Culturing M.
tuberculosis is useful for diagnosis because the bacillus has
certain distinctive characteristics. Unlike many other types of
bacteria, mycobacteria can retain certain dyes even when exposed to
acid. This so-called acid-fast property is characteristic of the
tubercle bacillus.
Body fluids other than sputum can be used for
culture. If TB has invaded the brain or spinal cord, culturing a
sample of spinal fluid will make the diagnosis. If TB of the kidneys
is suspected because of pus or blood in the urine, culture of the
urine may reveal tubercular infection. Infection of the ovaries in
women can be detected by placing a tube having a light on its end (a
laparoscope) into the area. Samples also may be taken from the liver
or bone marrow to detect the tubercle bacillus. |
