Interferon-gamma release assay

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

• There appears to be a dose-response relationship between the IGRA result and risk of progression to active TB, with the risk rising from 0 to 5 and then plateauing by about 101

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