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Education Materials > Publications > Self-Study Modules on TB > Module 3 > Summary

Self-Study Modules on Tuberculosis

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Module 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 TB.

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.

Additional Reading

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 Dis. 1990;142:725-735.

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. RR-5).

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.


Released October 2008
Centers for Disease Control and Prevention
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
Division of Tuberculosis Elimination -

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