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Interferon Gamma/TB FERON/TB Gold/ QuantiFERON/ IGRAs : Blood Based TB Test

Electron Micrograph of a Human T Cell

 Introduction:

Interferon-Gamma Release Assays (IGRAs) are whole-blood tests that can aid in diagnosing Mycobacterium tuberculosis infection.
This test is used to diagnose TB when site specific sample is not available. This is blood based TB test. They do not help differentiate latent tuberculosis infection (LTBI) from tuberculosis disease. This could help in prompt intervention resulting in the preventionof an active TB disease.

Latent Tuberculosis: 

Latent tuberculosis (TB) infection is a non-communicable form of tubercular infection in which, the patient is not symptomatic. In latent tuberculosis, the TB infection persists for a long period of time (may last for years). When an individual’s immune system decreases due to a variety of reasons, the infection may become active and cause an active tuberculosis disease

Other Brand Name: TB Feron, TB Gold, TB Platinum, Quantiferon

Features of IGRAs :

Interferon Gamma Receptor
1. IGRA assay used for quantitative detection of interferon gamma(IFN-y) released by T -lymphocytes when exposed to MTB
2. More accurately identifies those patients who have TB infection and are at greater risk of developing active TB
3. Designed for specificity and unaffected by BCG vaccination status

Meaning of Positive Results:

  1. Prior exposure to tuberculosis infection, either as a latent infection, or as an active TB infection
  2. This Test may be false positive. False positive test results can occur with individuals who have infection from other types of mycobacterium, such as mycobacterium szulgai, mycobacterium kansasii, and mycobacterium marinum

Meaning of Negative Results:

  1. Non-exposure to tuberculosis infection in the past, or rarely,the result may be false negative
  2.  A negative result also does not rule out an active tuberculosis infection, because certain patients, who have a very severe, active tuberculosis infection, may have a negative test result
    3. T-Lymphocytes
  3.  False negative result can result from a specimen that is older than twelve hours; causing the white cells to become less active or dysfunctional. These inactive white cells may end-up giving a false negative report

Meaning of Intermediate Results:

  • Test needs to be repeated after some time, as determined by your Doctor

Advantages of Test:

  1. Easy blood sampling, only one visit required
  2.  Result available within 24-hours, unlike a tuberculin skin test which may require up to 72-hours to be performed
  3. If we compare with tuberculin skin test, which is affected in cases, where individuals have received prior BCG vaccination, a QuantiFERON test is not affected by a prior BCG vaccination

Disadvantages of Test: 

  1.  The specimen has to be processed within 12-hours after collection. A specimen older than 12-hours, results in the white cells becoming less active, or dysfunctional, which may lead to a false negative report
  2. Testing in children, younger than 17 years age, is being debated
  3. Certain medications that you may be currently taking may influence the outcome of the test
  4.  Limited in individuals, who have been recently exposed to mycobacterium tuberculosis bacteria. It may give rise to false negative results
  5. Limited diagnostic value in immune-compromised individuals, in case of: 1) genetic immune-compromised disorders. 2) individuals who have active HIV infection, 3) those who have acquired immunodeficiency syndrome. The test may result in false negative results
  6. Result may be false negative in case of leukemias, lymphomas, and patients on chemotherapy, individuals with severe diabetes, those who have had recent transplants, and also individuals suffering from chronic renal failure.

Interpretation: 

Test results are NOT to be interpreted as results of a Alone this test only. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental information or tests.

Refrences:
1. Dis 2008; 67:84-90, 10. Ponce de Leon D et al. J Rheumatol 2008; 35:776-781
2. 3-Cobanoglu N et al. Int J Tuberc Lung Dis 2007; 11;1177-1182
3. http://www.dovemed.com/common-procedures/procedures-laboratory/tb-screening-test-interferon-gamma-release-assays/
 4. http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5202a2.htm
5. http://www.nyc.gov/html/doh/downloads/pdf/tb/tb-qtf-factsheet.pdf
6. http://www.lehigh.edu/health/PDFs/QuantiFERONGoldFactSheet.pdf
7. http://doh.sd.gov/tb/Quantiferon.aspx
8. http://www.ncbi.nlm.nih.gov/pubmed/18713053


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