25498 Tuberculosis Interferon Gamma Release Assay (IGRA)

​Sodium/Lithium Heparin Whole Blood (GTT) - supplied by Oxford

Adults and children > or = 10 years of age: collect 6 mL
Children > or = 2 to <10 years of age: collect 4 mL
Children < 2 years of age: collect 2 mL

Note: Specimen MUST be sent out the same day as collection via FedEx.  Collection tubes and shippers provided by Oxford Laboratories.

Internal - COLLECTIONS ARE LIMITED TO SITES THAT HAVE BEEN SPECIFICALLY TRAINED.  See collection locations with available collection times in chart below.

T-SPOT Collection Schedule

​This test belongs to a class of diagnostic tests known as interferon gamma release assays (IGRA).  It uses enzyme-linked immunospot (ELISPOT) technology to detect a specific lymphocyte population that is reliably present when an individual has been exposed to M. tuberculosis, the causative agent of tuberculosis.  This test may be used to aid in the diagnosis of latent and active TB.

This test may be generally used in place of the tuberculin skin test (TST).  Unlike TST, IgRAs do not give positive results in BCG-vaccinated individuals.  However, three non-tuberculosis mycobacteria, Mycobacterium kansasii, Mycobacterium marinum and Mycobacterium szulgai, have been found to cause a positive reaction to this test.

LIMITATIONS
- This test is NOT a definitive test for tuberculosis; IGRA test results must be used together with other diagnostic findings, an assessment of risk factors present and clinical findings.

- This test cannot differentiate between latent and active TB.

- This test has NOT been fully validated for the following populations:

     -Individuals < or = 17 years of age;
     -Pregnant women;
     -Hemophilia patients.

- This test has NOT been validate for use in patients undergoing anti-tuberculosis chemotherapy: it thus should not be used as a test-of-cure.

- Exposure to the PPD antigen used in the TST may cause a false positive IGRA reaction for an extended period of time.  Best practice dictates completing IGRA testing before placing a TST.

Refer to the most recent CDC guidance at http://www.cdc.gov/nchstp/tb for more information.

​One of several possible interpretations will be reported for this test:

Negative
Latent or active tuberculosis is unlikely.  This result must be correlated with other diagnostic and clinical findings.

Invalid
This result is due to an elevated Nil value and/or low Mitogen value.  Consider a single repeat test in 2-4 weeks.  If the result is still Invalid, consider a tuberculin skin test or other diagnostic study.

Borderline
Latent or active tuberculosis cannot be ruled in or out.  Consider a single repeat test in 2-4 weeks, Tuberculin Skin Test, or other diagnostic study.  Consider a single repeat test in 2-4 weeks.  If the result is still Borderline, consider a tuberculin skin test or other diagnostic study.

Positive
Latent or active tuberculosis is likely.  This test result must be correlated with other diagnostic and clinical findings.

The Food and Drug Administration fundamentally considers this to be a qualitative test.  At the recommendation of the Centers for Disease Control and Prevention, the spot counts are provided in the scanned result form.  These counts are useful only as an additional guide in the interpretation of a single test result.  Spot counts should not be used in either single or serial testing to predict disease progression or severity, or to monitor therapeutic response.  Reported spot count categories, with a brief explanation of each are as follows:

Nil (Neg) control: lymphocyte production of interferon gamma with no external stimulation.  May be considered the baseline level of interferon gamma production for the patient.  A negative control.  An elevated Nil value can result from acute viral infection or other causes.  In such cases repeating the test once in 2-4 weeks may be of value.  A highly elevated Nil result will lead to an Invalid result.

Positive control: lymphocyte production of interferon gamma upon stimulation with a non-specific plant mitogen.  May be considered the maximal level of positive interferon gamma production for that patient.  A positive control.  Reduced Mitogen values will lead to an Invalid test result.  This may occur due to an absolute severe lymphopenia or to non-functional lymphocytes that may occur in various immunosuppressed states.

Panels A & B: lymphocyte production of interferon gamma upon stimulation with two panels of antigens unique to M. tuberculosis (with the known exceptions of M. kansasii, M. marinum, & M. szulgai) antigens.  A positive reaction to one or both panels indicate that lymphocyte clones recognizing these antigens are present.

Test Not Performed: This result may be reported if the quantity of blood was insufficient, the incorrect anticoagulant was used, the sample had an insufficient concentration of lymphocytes, the sample was too old, or the sample was accidentally lost or otherwise compromised.