Self-Study Modules on Tuberculosis
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3: Diagnosis of Tuberculosis Infection and Disease
The tuberculin skin test is used to determine whether a person
has TB infection. The Mantoux tuberculin skin test is the preferred
type of skin test because it is the most accurate. This test is
done by using a needle and syringe to inject tuberculin between
the layers of the skin, usually on the forearm. After 48 to 72 hours,
the patient's arm is examined for a reaction (an induration). The
diameter of the indurated area (the swelling, not the redness) is
measured across the forearm. Most people with TB infection have
a positive reaction to the tuberculin.
Whether a reaction to the Mantoux tuberculin skin test is classified
as positive depends on the size of the induration, the person's
risk factors for TB, and for people who may be exposed to TB on
the job, the risk of exposure to TB in the person's job.
Several factors can affect how the skin test reaction is interpreted.
Some factors, such as infection with nontuberculous mycobacteria
(mycobacteria other than M. tuberculosis) and vaccination
with BCG, can cause false-positive reactions. Other factors, such
as anergy, recent infection, and very young age can cause false-negative
reactions. HIV-infected people may be tested for anergy if they
have a negative reaction to the tuberculin skin test. Also, close
contacts of someone with infectious TB disease who have a negative
reaction to the tuberculin skin test should be retested 10 weeks
after the last time they were in contact with the person who has
In many TB screening programs, two-step testing is used for skin
testing employees when they start their job. Two-step testing is
a strategy for telling the difference between boosted reactions
and reactions caused by recent infection.
There are four steps in diagnosing TB disease. The first is the
medical history. This means asking the patient whether he or she
has been exposed to a person with TB, has symptoms of TB disease,
has had TB infection or TB disease before, or has risk factors for
developing TB disease. The symptoms of pulmonary TB disease include
coughing, pain in the chest when breathing or coughing, and coughing
up sputum or blood. The general symptoms of TB disease (pulmonary
or extrapulmonary) include weight loss, fatigue, malaise, fever,
and night sweats. The symptoms of extrapulmonary TB disease depend
on the part of the body that is affected by the disease.
Second, patients with symptoms of TB disease may be given a tuberculin
skin test. However, they should always be evaluated for TB disease,
regardless of their skin test results. Furthermore, clinicians should
not wait for tuberculin skin test results when evaluating patients
who have symptoms of TB disease.
The third step is the chest x-ray. One purpose of the chest x-ray
is to help rule out the possibility of pulmonary TB disease in a
person who has a positive reaction to the tuberculin skin test.
Another purpose is to check for lung abnormalities in people who
have symptoms of TB disease. However, the results of a chest x-ray
cannot confirm that a person has TB disease.
The fourth step is to do a bacteriologic examination. First, a
specimen is obtained from the patient. A sputum specimen is obtained
from patients suspected of having pulmonary TB disease; other specimens
are obtained from patients suspected of having extrapulmonary TB
disease. Either way, this specimen is smeared onto a slide, stained,
and examined under a microscope for the presence of acid-fast bacilli.
When acid-fast bacilli are seen in a smear, they are counted, and
the smear is classified according to this number. Patients with
positive smears are considered infectious.
Next, the specimen is cultured, or grown, so that laboratory personnel
can determine whether it contains M. tuberculosis. Special
tests are used to identify the mycobacteria once they have grown
enough to be detected. A positive culture for M. tuberculosis
confirms the diagnosis of TB disease.
After the specimen has been cultured, it is tested for drug susceptibility.
The results of drug susceptibility tests can help clinicians choose
the appropriate drugs for each patient.
American Lung Association/American Thoracic Society. Control of
tuberculosis in the United States. Am Rev Respir Dis. 1992;146:1623-1633.
American Lung Association/American Thoracic Society. Diagnostic
standards and classification of tuberculosis. Am Rev Respir
Centers for Disease Control. Purified protein derivative (PPD)-tuberculin
anergy and HIV infection: guidelines for anergy testing and management
of anergic persons at risk of tuberculosis. MMWR. 1990;40(No.
Centers for Disease Control. Screening for tuberculosis and tuberculous
infection in high-risk populations. MMWR. 1990;39(No. RR-8):1-5.
Centers for Disease Control. Use of BCG vaccine in the control
of tuberculosis: a joint statement by the ACIP and the Advisory
Committee for Elimination of Tuberculosis. MMWR. 1979;28:241-244.
Core Curriculum on Tuberculosis, 3rd ed. Atlanta: Centers
for Disease Control and Prevention; 1994.