Research
HAPS Executive
Locked Bag 1
Hunter Region Mail Centre
NSW 2310, Australia
Phone: (02) 4921 4000
Fax: (02) 4921 4400
International Enquiries
Phone: +61 2 4921 4000
Fax: +61 2 4921 4400
QuantiFERON® - TB Gold (QFT) Screening
Tuberculosis is a communicable disease caused by infection with M.tuberculosis complex organisms (M.tuberculosis, M.bovis, M.africanum) which typically spreads to new hosts via airborne droplet nuclei from patients with respiratory tuberculosis disease.
A newly infected individual can become ill from TB within weeks to months, but most infected individuals remain well. Latent tuberculosis infection (LTBI) is a non-communicable asymptomatic condition, persists in some, who might develop tuberculosis disease months or years later. The QuantiFERON®-TB Gold test is both for LTBI and as a helpful aid in diagnosing M.tuberculosis complex infection in sick patients.
How does it work?
QFT uses specially designed (1mL) blood collection tubes that are coated with M. tuberculosis specific antigens along with a negative and a positive control tube. Stimulation of T cells in whole blood with these highly specific antigens results in the production of the cytokine gamma interferon (IFN-γ) only in individuals infected with M. tuberculosis which is then measured in the laboratory. The magnitude of the measured IFN- γ level cannot be correlated to stage or degree of infection, level of immune responsiveness, or likelihood for progression to active disease.
A negative result
A negative QuantiFERON®-TB Gold result does not preclude the possibility of M.tuberculosis infection or tuberculosis disease:
- False-negative results can be due to the stage of infection (e.g. specimen collected prior to the development of the immune response),
- Co-morbid conditions which affect immune functions,
- Incorrect performance of the assay in the laboratory
- Or other immunological variables.
Positive results
While the specific antigens in the QuantiFERON®-TB Gold tubes are absent from all BCG strains and from most known non-tuberculous mycobacteria, it is possible that a positive QuantiFERON®-TB Gold result may be due to infection by M.kansasii, M.szulgai or M.marinum. If such infections are suspected, alternative tests should be investigated.
A positive QuantiFERON®-TB Gold result should be followed by further medical evaluation and diagnostic evaluation for active tuberculosis disease *(e.g. AFB smear and culture, chest x-ray).
Indeterminate results
This result may occur with insufficient lymphocytes in the patient sample (ie. Low CD4 count), reduced lymphocyte activity due to improper specimen handling, incorrect filling/mixing of the positive control tube, or inability of the patient’s lymphocytes to generate IFN-γ.
The test should be repeated to confirm the results and/or other clinical and medical examinations should be performed.
How to order the test
This test is currently rebated by Medicare ONLY if the patient is immuno-compromised or immuno-suppressed. This information needs to be recorded on the request form or the patient may be charged a fee of $60 for the test (the cost of this test for patients in hospital or medical outpatient clinics is usually met by the department).
Most HAPS pathology collection centres have the special collection tubes required (refer www.haps.gov.nsw.au for a list of collection centres).
The test cannot be collected on Fridays or weekends due to the processing required.
Further information
Please contact the Immunology laboratory on 49214018 if further information is required.
Cellestis Limited is the Australian company that developed and markets the QuantiFERON®-TB Gold kit, their website has other useful information that may be of interest (www.cellestis.com).