Exams and Tests

Diagnosing active TB in the lungs

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

• Sputum culture. Testing mucus from the lungs (sputum culture) is the best way to diagnose active TB. But a sputum culture can take 1 to 8 weeks to provide results.
• Sputum cytology.
• Chest X-ray. 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.
• Rapid sputum test. This test can provide results within 24 hours. This test is done only when a person is strongly suspected of having TB.

Diagnosing latent TB in the lungs

• A tuberculin skin test will show if you have ever had a TB infection. See a picture of a tuberculin skin test.
• Rapid blood tests help detect latent TB.³ They can help diagnose TB when results from a tuberculin skin test are uncertain. These tests also can tell if a person who has had a BCG vaccination has a TB infection. A rapid test requires only one visit to the doctor or clinic, instead of two visits as required for the tuberculin skin test. Rapid blood tests are also called interferon-gamma release assays (IGRAs).

Diagnosing TB outside the lungs

Diagnosing TB in other parts of the body (extrapulmonary TB) requires more testing. Tests include:

• Biopsy. A sample of the affected area is taken out and sent to a lab to look for TB-causing bacteria.
• Urine culture. This test looks for TB infection in the kidneys (renal TB).
• Cerebrospinal fluid test. A sample of fluid around the spine is taken to look for a TB infection in the brain (TB meningitis).
• CT scan. This test is used to diagnose TB that has spread throughout the body (miliary TB) and to detect lung cavities caused by TB.
• MRI. This test looks 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.

Tests during TB treatment

During treatment, a sputum culture is done once a month—or more often—to make sure that the antibiotics are working. You may have a chest X-ray at the end of treatment to use as a comparison in the future.

You may have tests to see if TB medicines are harming other parts of your body. These tests may include:

• Liver function tests.
• Eye tests, especially if you are taking ethambutol for TB treatment.
• Hearing tests, especially if you are taking streptomycin for TB treatment.

Early detection

Public health officials encourage early testing for people who are at risk for getting TB.