Diagnosing active TB in the lungs

Doctors diagnose active tuberculosis (TB) in the lungs (pulmonary TB) by using a medical history and physical examination, and by checking your symptoms (such as an ongoing cough, fatigue, fever, or night sweats). Doctors will also look at the results of:

• Sputum cultures. Testing mucus from the lungs (sputum culture) is the best way to diagnose active TB. If TB bacteria grows from your samples, sensitivity testing is done on the bacteria. These tests will show which medications will kill the bacteria. Results of sensitivity tests can take between 1 and 6 weeks because TB-causing bacteria grow very slowly. Your doctor may start treatment before results are returned if it's likely that you have TB. Researchers are working on new tests that may give quicker results.
• Chest X-rays. A chest X-ray cannot diagnose active TB. A chest X-ray usually is done if you have:
  • A positive tuberculin skin test (also called a TB skin test, PPD test, or Mantoux test).
  • Symptoms of active TB, such as a persistent cough, fatigue, fever, or night sweats.
  • An uncertain reaction to the tuberculin skin test because of a weakened immune system, or to a previous bacille Calmette-Guerin (BCG) vaccination.

Diagnosing latent TB in the lungs

A tuberculin skin test will show whether you have latent TB. The test also will show if you have ever had a TB infection. See an illustration of a tuberculin skin test.

QuantiFERON-TB Gold is a blood test that has been approved by the U.S. Food and Drug Administration (FDA) to help detect latent TB.³ It can help diagnose TB when results from a tuberculin skin test are uncertain. The test can also tell if a person who has had a BCG vaccination has a TB infection. It requires only one visit to the doctor or clinic, instead of two visits as required for the tuberculin skin test.

Diagnosing TB outside the lungs

Diagnosing TB in other parts of the body (extrapulmonary TB) requires more testing. You may have:

• A sample of the affected area taken out (biopsy). The sample is sent to a lab to look for TB-causing bacteria.
• A urine culture to look for TB infection in the kidneys (renal TB).
• A sample of fluid around the spine (cerebrospinal fluid) taken to look for a TB infection in the brain (TB meningitis).
• A CT scan to diagnose TB that has spread throughout the body (miliary TB) and to detect lung cavities caused by TB.
• An MRI scan to look for TB in the brain or the spine.

Testing for HIV infection is often done at the time of TB diagnosis. You may also have a blood test for hepatitis.⁴

Tests during TB treatment