The tuberculin skin test is used to determine whether a person has Latent TB infection. The Mantoux tuberculin skin test is the preferred type of skin test because it is the most accurate and is the only type of test that should be used.
Whether a reaction to the Mantoux tuberculin skin test is classified as positive depends on the size of the reaction, 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.
Several factors can affect how the skin test reaction is interpreted. 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.
A new serological test QuantiFERON-TB Gold ® has recently been approved by the FDA and is used in place of the skin test as it is usually more specific.
There are four steps in diagnosing TB disease: medical history, tuberculin skin test, chest x-ray, and bacteriologic examination.
A medical history includes asking the patient whether they have been exposed to a person with TB, symptoms of TB disease, if they have had latent TB infection or TB disease before, or risk factors for developing TB disease. The symptoms of pulmonary TB disease may include:
- Pain in the chest when breathing or coughing
- Coughing up sputum or blood
The general symptoms of TB disease (pulmonary or extrapulmonary) may include:
- Weight loss
- Night sweats
The symptoms of extrapulmonary TB disease depend on the part of the body that is affected by the disease.
Patients with symptoms of TB disease may be given a tuberculin skin test or QuantiFERON TB Gold test ®. However, they should be evaluated for TB disease, regardless of their skin tests or QFT Gold results.
The chest x-ray is used to help rule out the possibility of pulmonary TB disease in a person who has a positive reaction to the tuberculin skin test and check for lung abnormalities in people who have symptoms of TB disease. The results cannot confirm or rule out that a person has TB disease.
The fourth step is a bacteriologic examination. 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. The specimen is examined under a microscope for the presence of acid-fast bacilli (AFB). When AFB are seen, they are counted. Patients with positive AFB smears are considered infectious. The specimen is then cultured, or grown, to determine whether it contains M. tuberculosis. A positive culture for M. tuberculosis confirms the diagnosis of TB disease.
After the specimen has been cultured, it may be tested for drug susceptibility. The results of drug susceptibility tests can help clinicians choose the appropriate drugs for use in treatment.