What Tests Do Doctors Use to Diagnose Tuberculosis?
The doctor will complete the following tests to diagnose tuberculosis. You may not be hospitalized for either the initial tests or the beginning of treatment.
- Chest X-ray: The most common diagnostic test that leads to the suspicion of infection is a chest X-ray.
- In primary TB, an X-ray will show an abnormality in the mid and lower lung fields, and lymph nodes may be enlarged.
- Reactivated TB bacteria usually infiltrate the upper lobes of the lungs.
- Miliary tuberculosis exhibits diffuse nodules at different locations in the body.
- The Mantoux skin test also known as a tuberculin skin test (TST or PPD test): This test helps identify people infected with M. tuberculosis but who have no symptoms. A doctor must read the test.
- The doctor will inject 5 units of purified protein derivative (PPD) into your skin. If a raised bump of more than 5 mm (0.2 in) appears at the site 48 hours later, the test may be positive.
- This test can often indicate disease when there is none (false positive). Also, it can show no disease when you may in fact have TB (false negative).
- QuantiFERON-TB Gold test: This is a blood test that is an aid in the diagnosis of TB. This test can help detect active and latent tuberculosis. The body responds to the presence of the tuberculosis bacteria. By special techniques, the patient's blood is incubated with proteins from TB bacteria. If the bacteria is in the patient, the immune cells in the blood sample respond to these proteins with the production of a substance called interferon-gamma (IFN-gamma). This substance is detected by the test. If someone had a prior BCG vaccination (a vaccine against TB given in some countries but not the U.S.) and a positive skin test due to this, the QuantiFERON-TB Gold test will not detect any IFN-gamma.
- Sputum testing: Sputum testing for acid-fast bacilli is the only test that confirms a TB diagnosis. If sputum (the mucus you cough up) is available, or can be induced, a lab test may give a positive result in up to 30% of people with active disease.
- Sputum or other bodily secretions such as from your stomach or lung fluid can be cultured for growth of mycobacteria to confirm the diagnosis.
- It may take one to three weeks to detect growth in a culture, but eight to 12 weeks to be certain of the diagnosis.
When Should Someone Seek Medical Care for Tuberculosis?
If someone among your family or close associates is found to be sick with active TB, you should see your doctor and be tested for tuberculosis.
- The dangerous contact time is before treatment. However, once treatment with drugs starts, the sick person is noncontagious within a few weeks.
- If you develop any side effects from medications prescribed to treat tuberculosis -- such as itching, change in color of skin, tiredness, visual changes, or excessive fatigue -- call your doctor immediately.
Medically Reviewed by a Doctor on 6/24/2016
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