Interferon-gamma release assay

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Interferon-gamma release assays (IGRAs) offer higher sensitivity (and specifity) in the diagnosis of active and latent tuberculosis over the traditional mantoux test ( sensitivity 77% (95% CI 71% to 82%)[1], particularly in immunosuppressed patients. However they do not predict viable Tb infection and distinguish between latent Tb and active Tb. T cells that have previously encountered Tb infection produce interferon-gamma when stimulated with Tb antigens. Usefully the commercial assays detect Mycobacteria tuberculosis, Mycobacteria kansasii, Mycobacteria marinum and Mycobacteria szulgai, but not BCG.

  • QuantiFERON-TB Gold In-Tube assay. Has replaced QuantiFERON®-TB test
    • Estimated pooled sensitivity 70% (95% CI, 63% - 78%)[1]
  • T-SPOT.TB assay. (formally known as the Elispot Test)
    • Estimated pooled sensitivity 90% (95% CI 86% to 93%)[1]

Currently national guidelines differ as to the place of IGRAs in tuberculosis diagnosis but the general trend is for increased use where they can be afforded. This is because a progression rate in asymptomatic contacts who test positive of 15% is typical for IGRAs compared to 2.3% for the mantoux test. Please refer to the appropriate information as the literature is expected to answer over the next few years unresolved questions which include:

  • Role in the diagnosis of:
    • Latent Tb
      • Before treatment with TNF inhibitors although increasingly being used
    • Predicting progression from latent TB to active TB
    • Tb in immunosuppressed
  • Mantoux interfering with subsequent IGRAs by boosting


References

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