• Reviewed on 2/13
    By Dr. Iseman

    • Michael Iseman, MDMichael Iseman, MD


Tuberculosis (TB): Diagnosis

To diagnose tuberculosis (TB), your health care provider will gather five important pieces of information during the complete history and physical examination:

You should be tested for TB if:

  • You think you might have active TB disease.
  • You have spent time with a person you know or suspect has active TB disease.
  • You are infected with HIV or have another condition that puts you at high risk for active TB disease.
  • You are from a country where active TB disease is very common (most countries in Latin America, the Caribbean, Africa, and Asia and Eastern Europe and Russia).
  • You live or work somewhere where active TB disease is more common, such as a homeless shelter, migrant farm camp, prison or jail, or some nursing homes.
  • You use illegal intravenous drugs.

History of Exposure

Getting an accurate history is important in diagnosing and treating TB. Like any disease, early intervention and treatment is very important.


Tuberculin Skin Test

The tuberculin skin test (or PPD) uses an extract of killed TB germs. The killed germs are injected into the skin. If a person has been infected with TB, a lump will form at the site of the injection. This is a positive test. This often means that TB germs have infected the body. It does not often mean the person has active disease. People with positive skin tests but without active disease cannot transmit the infection to others.

Previous vaccination for TB compromises the validity of the tuberculin skin test. Newer blood tests have helped identify those with true TB infection. Learn more.


Tuberculosis Blood Test

There are two new blood tests that have recently been developed to test for exposure to tuberculosis germs. They are still used in research settings, but some hospitals and clinics are using them in place of the tuberculin skin test. Each tests involves collection of blood that is then stimulated with a group of antigens (proteins) found in the bacteria that causes TB. If your immune system has ever seen these antigens, your cells will produce interferon-gamma, a substance produced by our immune system and, which can be measured in a laboratory. There are two tests available, the QuantiFERON-TB® Gold (QFT) Test and the T-Spot®.TB  Test. These tests appear to be more sensitive and specific than the tuberculin skin test. Importantly, unlike the tuberculin skin tests, they are not affected by previous BCG vaccination.


Chest X-ray

If a person has been infected with TB, but active disease has not developed, the chest X-ray will often be normal. Most people with a positive PPD (skin test) or TB blood test have normal chest X-rays and continue to be healthy. For such people, preventive medication may be recommended.

However, if the germ has attacked and caused inflammation in the lungs, an abnormal shadow may be visible on the chest X-rays. For these people, diagnostic tests (sputum tests) and treatment often are appropriate. Learn more.


Sputum Test

Samples of sputum coughed up from the lungs can be tested to see if TB germs are present. The sputum is examined under a microscope (a "sputum smear") to look for evidence of the TB organisms. Learn more.

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