TB blood tests are the preferred TB test for:

• People who have received the TB vaccine bacille Calmette–Guérin (BCG). TB blood tests (IGRAs), unlike the TB skin test, are not affected by prior BCG vaccination and are not expected to give a false-positive result in people who have received BCG. TB blood tests are the preferred method of TB testing for people who have received the BCG vaccine.
• People who have a difficult time returning for a second appointment to look for a reaction to the TST. 

TB blood tests are also called interferon-gamma release assays or IGRAs.  Two TB blood tests are approved by the U.S. Food and Drug Administration (FDA) and are available in the United States: the QuantiFERON®–TB Gold In-Tube test (QFT-GIT) and the T-SPOT®.TB test (T-Spot).

A health care provider will draw a patient’s blood and send it to a laboratory for analysis and results.

• Positive TB blood test: This means that the person has been infected with TB bacteria. Additional tests are needed to determine if the person has latent TB infection or TB disease.
• Negative TB blood test: This means that the person’s blood did not react to the test and that latent TB infection or TB disease is not likely.

 TB Blood tests

Blood tests may be used to confirm or rule out latent or active tuberculosis. These tests use sophisticated technology to measure your immune system's reaction to TB bacteria. QuantiFERON-TB Gold in-Tube test and T-Spot.TB test are two examples of TB blood tests.

These tests require only one office visit. A blood test may be useful if you're at high risk of TB infection but have a negative response to the skin test, or if you've recently received the BCG vaccine.

Imaging tests

If you've had a positive skin test, your doctor is likely to order a chest X-ray or a CT scan. This may show white spots in your lungs where your immune system has walled off TB bacteria, or it may reveal changes in your lungs caused by active tuberculosis. CT scans provide more-detailed images than do X-rays.