Interferon-gamma release assay (IGRA)

What it measures

Measures interferon-gamma released by a patient's T cells when stimulated with antigens specific to M. tuberculosis. It detects TB infection and is not affected by BCG vaccination.

Specimen and preparation

• Specimen: Whole blood in kit tubes or lithium-heparin blood
• Tube: Green top
• Preparation: Fill kit tubes to the mark, shake well and incubate or deliver within the kit time limit.

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Latent TB infection
• Active TB (cannot distinguish from latent)
• Recent TB contact (conversion)

Causes of low values

• Indeterminate: low mitogen response in immunosuppression, lymphopenia or poor handling

Method

ELISA-based (QuantiFERON) or ELISpot (T-SPOT.TB)

Good to know

Preferred over Mantoux in BCG-vaccinated people; a negative result does not rule out active TB.

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