General Information about TB Blood Tests
The FDA has approved two interferon-gamma release assay (IGRA) tests for TB infection. The two tests are:
- QuantiFERON®-TB Gold In-Tube test (GFT-GIT)
The IGRA tests identify the presence of Mycobacterium tuberculosis infection by measuring the immune response to the TB bacteria in whole blood. These tests cannot determine if a person has latent TB infection or active TB disease. Additional tests are needed to diagnose TB disease.
Blood is collected by a health care provider and sent to a laboratory for processing. The lab must begin processing the blood in 8-30 hours after collection (depending on which test is used). The test results are generally available in 24 hours.
The possible test results are:
- Positive: There has been an immune response indicating the presence of TB bacteria.
- Negative: There has not been an immune response indicating the presence of TB bacteria.
- Indeterminate: Results unclear. Possible testing error or the results are not conclusive.
- Borderline (T-SPOT®.TB only): Results in a borderline zone and cannot tell if truly positive or negative.
ALL test results should be discussed with a trained health care provider. It is important to note that TB blood tests are part of a larger toolkit used to diagnose TB infection. A negative result does not necessarily mean that a person does not have latent TB infection or TB disease.
Contact your healthcare provider or local health department for information on TB testing.
- Only one visit to health care provider to draw the blood
- Results can be available in 24 hours
- Results are not affected by prior BCG (bacille Calmette-Guérin) vaccination
- Blood samples must be processed within 8-30 hours after collection.
- Factors that decrease the accuracy of the test include errors in:
- collecting blood samples
- transporting blood samples
- running and interpreting the test
- TB blood tests may be costly
- TB tests are generally not needed for people with a low risk of infection.
- Choosing which TB test to use should be done with your health care provider. Factors in selecting which test to use include:
- reasons for testing
- test availability
- It is generally not recommended to test an individual with both a TB skin test and a TB blood test. Exceptions can be found in the Interferon-Gamma Release Assays Fact Sheet.
- TB blood tests are preferred for persons who have received the BCG vaccine.
- TB blood tests may be used for TB screening in workplace settings.
- Due to limited data on effectiveness, the TB skin test is preferred over TB blood tests for children under the age of 5.