Causes and Diagnoses

Causes and Diagnoses of Tuberculosis

Tuberculosis is spread in the saliva, sputum and nasal secretions of people who have active TB disease. Some people can be exposed to the bacterium that causes TB and not get infected. Others will develop a latent TB infection. Those with weakened immune systems and other risk factors will likely develop active TB disease when they breathe in the TB bacteria. This means they are contagious and can spread the disease to others.

Diagnosing tuberculosis

If you have been exposed to someone who has active TB disease, or have symptoms of TB, you should be tested for the disease. There are two common tests:

  • TB skin test: A small amount of liquid called tuberculin is injected under the skin. Two to three days later, a health care provider must inspect the injection site to determine if it is positive or negative. If it is positive, further testing is required. If it is negative, you do not have latent or active TB. Skin tests sometimes produce false negatives and false positives.
  • TB blood test: A sample of blood is tested in a lab to measure your immune system’s reaction to TB bacteria.

Other tests may include:

  • Chest X-ray: This may show white spots on your lungs where your immune system is fighting TB bacteria.
  • Sputum test: A sample of the mucus you cough up may be tested in a lab for the presence of TB bacteria.

Diagnostic Tests