A physician may suspect that a patient is suffering from tuberculosis (TB) based on reported symptoms and the patient’s risk of contracting TB. The physician may also compile a medical history and perform a physical examination. If TB is suspected, a number of additional tests may also be conducted.

The most common test for TB is the Mantoux tuberculin skin test (or TST). This test is safe for use in infants, children, pregnant women and people with compromised immune systems (including patients with HIV). However, it is not recommended for people who have had an allergic reaction to previous TSTs. A TST is sometimes carried out as a routine screening test for children and adults at risk of contracting TB.

During the TST, a small quantity of fluid called tuberculin is injected just under the skin on the inside of the forearm. Within 48 to 72 hours, the patient must return to the physician, who will inspect the site of the injection for a raised lump or welt. For people who have no known risk factors, the test is considered positive for TB if the patient displays a lump of over 15 millimeters (mm) in diameter. For those in high-risk groups, a 5-mm lump could indicate infection. If there is no lump, the test is considered negative.

The TST is only capable of indicating the presence of TB bacteria. It does not differentiate whether the infection is latent (when bacteria are dormant in the body) or active (bacteria are actively causing disease).

Also, the TST may sometimes produce results that are false-positive (results incorrectly indicate TB although TB bacteria are not present) or false-negative (results incorrectly indicate absence of TB even though TB bacteria are present).

A false-positive reaction could be due to several factors, including:

• Latent TB. A false-positive result may indicate the presence of dormant TB bacteria in the body. These bacteria are not active and not currently causing symptoms of disease.
  
  
• Infection with bacteria that are similar to the bacterium that causes TB (Myobacterium tuberculosis).
  
  
• Previous vaccination to TB with the live vaccine, Bacille Calmette-Guerin (BCG).
  
  
• Incorrect administration of the TST (e.g. fluid injected too deep under the skin).
  
  
• Inaccurate reading of the TST.

A false-negative reaction is more common and can be due to the following factors:

• Recent TB infection. It can take up to eight or 10 weeks after exposure to the TB bacteria for the body to react to a skin test such as a TST. Generally, symptoms of TB appear (typically within two to eight weeks after exposure to TB bacteria) before the bacteria can begin to be identified by the TST.
  
  
• Impaired immune function. In some cases, the immune system may be too weak to mount a defense against the TST to produce a reaction. This may occur if the patient is suffering from an illness such as HIV or is taking medication that weakens the immune system. This can also happen if the body is overwhelmed with TB bacteria. Some people with impaired immune systems may be unable to react to skin tests such as the TST (a condition called cutaneous anergy).
  
  
• Young age. Children under 6 months old often do not react to skin tests.
  
  
• Recent vaccination with a live virus. Live-virus vaccinations such as those administered to prevent measles or smallpox may produce a false-negative TST reaction. People are advised to wait four to six weeks after a live-virus vaccination before they undergo a TST.
  
  
• Incorrect administration or reading of the TST.

If the TST results are positive, or if a physician suspects that results are a false-negative, additional tests may be performed to confirm the presence of active TB. In these cases, a physician may recommend a chest x-ray to look for signs of TB in the lungs. In people with latent TB, a chest x-ray may show the small, scab-like areas (macrophages) where white blood cells have walled off TB bacteria. In people with active TB, a chest x-ray may show cavities that have formed after the infection broke out of the macrophages.

A physician may also perform a culture, which involves sending samples of sputum (mucus coughed up from the lungs) to a laboratory, where they are tested for signs of TB bacteria. Cultures are also used to assess what TB medications may most effectively kill the bacteria. This can help a physician develop a specific antibiotic treatment plan for a patient.

People with HIV are more likely to develop TB in additional organs of the body and thus may be tested for evidence of TB bacteria in organs such as the brain, spine, kidneys or lymph nodes.

Other tests that may be used to diagnose TB include the tine test, where a small amount of the tuberculosis antigen is injected under the skin in the arm. It is administered with a multi-pronged instrument that contains the antigen on each of the tines (spokes) that penetrate the skin. Like the TST, this test produces a small, raised bump if the body has been exposed to TB bacteria. In addition, other blood tests can also be used to diagnose the presence of TB bacteria in the body. However, the TST is considered the most accurate and is the preferred method of diagnosing TB in the United States. Researchers are investigating new methods to diagnose TB that may be more accurate than the TST. 

It is advised that anyone at risk of TB, including children, be periodically tested for the disease.

Because of their increased risk, people with HIV should be tested for TB on a yearly basis. They should also get tested under the following circumstances:

• When first diagnosed with HIV
  
  
• When starting HIV therapy
  
  
• After coming into contact with someone with active TB
  
  
• When experiencing any symptoms which could indicate TB

Physicians are required by law to report all confirmed cases of TB to state or local health agencies within 24 hours in order to prevent an outbreak of the disease.