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Tuberculosis

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.