Interferon-gamma release assay
From IDWiki
Background
- In vitro test that assess for a cell-mediated immune response to tuberculosis antigens
- Usually early secreted antigenic target 6 (ESAT-^) and culture filtrate protein 10 (CFP-10)
- Used for diagnosis latent tuberculosis infection
- Unclear utility in children under 5 years
- Levels often decline after treatment for LTBI or active tuberculosis, but remain positive
- In Canada, we have two tests available:
- QuantiFERON-TB Gold Plus (QFT-Plus): an in-tube ELISA whole-blood test of ESAT-6 and CFP-10, uses 2 tubes (the second designed to stimulate CD8 T-cells), and is reported in IU/mL
- T-SPOT.TB: a spot ELISA assay performed on separated peripheral blood mononuclear cells, using ESAT-6 and CFP-10, and is reported in the number of spots compared to a control
Interpretation
QuantiFERON-TB Gold Plus
- Reports the IU/mL of 4 tubes: negative control (which assesses baseline interferon activity), antigen 1, antigen 2, and mitogen control (to assess their immune response in general)
- Reported as positive if either antigen 1 or 2 is ≥0.35 above the control tube
| Tube (IU/mL) | Result | |||
|---|---|---|---|---|
| Nil | TB1 - nil | TB2 - nil | Mitogen - nil | |
| ≤8 | ≥0.35 and ≥25% of nil | any | any | positive |
| any | ≥0.35 and ≥25% of nil | |||
| <0.35, or ≥0.35 and <25% of nil | <0.35, or ≥0.35 and <25% of nil | ≥0.5 | negative | |
| <0.5 | indeterminate | |||
| >8 | any | |||
T-SPOT.TB
- Reports the number of spots in the antigen wells compared to a control well
- Positive it at least 8 more spots in the antigen wells
False Positives
- The antigens cross-react with some species of non-tuberculous mycobacteria, particularly Mycobacterium marinum, Mycobacterium kansasii, Mycobacterium szulgai, and Mycobacterium flavescens
