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General Information about TB Blood Tests

The Test:

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)
  • T-SPOT®.TB.

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.

The IGRA process:

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.

Where to get tested:

Contact your healthcare provider or local health department for information on TB testing.

Advantages of IGRAs

  • 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

Limitations or Disadvantages of IGRAs

  • 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

Recommendations:

  • 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
    • cost
  • 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.

Additional Information:

Interferon-Gamma Release Assays (fact sheet)
BCG Vaccine (fact sheet)
Tuberculosis and Pregnancy (fact sheet)
Diagnosis of Tuberculosis Disease (fact sheet)
 

 
Contact Us:
  • Centers for Disease Control and Prevention
    Division of Tuberculosis Elimination (DTBE)
    1600 Clifton Rd., NE
    MS E10
    Atlanta, GA 30329
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    TTY: (888) 232-6348
  • Contact CDC–INFO
  • Page last reviewed: May 25, 2011
  • Page last updated: May 25, 2011
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