Tuberculosis (TB)

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:

• 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 medicines 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 if you have ever had a TB infection. See a picture of a tuberculin skin test.

Rapid blood tests have been approved by the U.S. Food and Drug Administration (FDA) to 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.

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

During treatment, sputum tests are done once a month—or more often—to make sure 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

All cases of TB are reported to the local or state health department because the disease can spread to others and cause outbreaks. Major health authorities keep track of TB outbreaks and encourage early testing for people who are at risk for developing the disease.

The Centers for Disease Control and Prevention (CDC) recommend TB testing for people who:⁵

• Have a human immunodeficiency virus (HIV) infection or another condition that puts them at risk for TB.
• Spend a lot of time with a person who has active TB disease, which can be spread to others.
• Inject illegal drugs.
• Were born in parts of the world where tuberculosis is common, such as Latin America and the Caribbean, Africa, Asia, Eastern Europe, and Russia.
• Live or work in nursing homes, homeless shelters, migrant farm camps, prisons, or jails.

People who have a high risk for developing TB usually have a skin test (tuberculin test) on a regular basis. Health professionals often are given a tuberculin skin test when they begin work in a hospital or nursing home, with retesting every 6 to 12 months.