INTERFERON GAMMA RELEASE ASSAYS (IGRA) FOR LATENT TB TESTING.

Written by Stephen Rimmer Created on Updated on .

The QuantiFERON-TB Gold In Tube and the T-SPOT are two in-vitro tests for measuring cell-mediated immune responses to peptide antigens from mycobacteria. These antigens, ESAT-6, CFP-10 and TB7.7 (p4) (which is used only in QFT-G) are absent from all BCG strains and from most non tubercular mycobacterial strains (NTMs) with the exception of M. kansasii, M. szulgai and M. marinum. Individuals infected with M. tuberculosis complex organisms (M. tuberculosis, M. bovis, M. africanum, M. microti, M. canetti) have mononuclear cells in their blood that recognise these mycobacterial antigens. This recognition process leads to the stimulation and secretion of IFN-γfrom sensitized T-cells. The detection and quantification of IFN-γ, measured by enzyme-linked immunoassay (Quantiferon) or enzyme-linked immunospot (T-SPOT), forms the basis of these tests – detectable as early as two weeks after infection with M. tuberculosis.

Both tests are intended for use in conjunction with risk assessment, radiography, and other medical and diagnostic evaluations.  QuantiFERON Gold-IT is the laboratory recommended method for routine screening of patients with a normal lymphocyte count.   T spots are recommended in specific circumstances (see below). 

 

 

 

Sensitivity

 

Specificity

 

Suggested use

 

 

QFT

 

 

84

 

 

99

·         Routine TB contact screening

·         Healthcare worker screening

·         Patients with normal lymphocyte count

·         Immigrants from high incidence areas (WHO >40/100,000/yr)

 

 

T spot

 

 

89

 

 

86

·         T lymphopenia patients (malignancy, infection)

·         Immunosuppressed (iatrogenic/immunodeficiency)

·         Neonates /paediatrics and small volume  samples (<3mls).

From: Diel et al. Chest 2010; 137(4):952-968.

IGRA results should be considered in conjunction with other epidemiologic, historic, physical, and diagnostic findings. Results cannot be correlated to stage or degree of infection, level of immune responsiveness, or likelihood for progression to active disease.  

The Hospital Trust TB service will be notified of all positive and borderline results.  

For test specific information click on the links below

Tspot test Information:

http://www.heftpathology.com/index.php?option=com_zoo&task=item&item_id=594&Itemid=627

Quantiferon plus IGRA test Information; 

http://www.heftpathology.com/index.php?option=com_zoo&task=item&item_id=1115&Itemid=627