TB Quantiferon Gold Blood Test (Interferon-gamma release assay [IGRA]) in London
MAKE SURE you bring your PHOTO ID with you (passport/ drivers licence)
|Blood Test by Doctor||One-off £39 admin fee (per visit)|
– plus labaratory cost (select your blood test for prices).
– £5 surcharge on weekends
To See Our Main Blood testing section for this, please visit our Microbiology & Infections Blood Testing page.
This is a blood test which is used to aid in the diagnosis of Latent Tuberculosis infections and active Tuberculosis (TB) disease. You should wait for 6 weeks after exposure to a person with active TB to do this blood test (as it takes about 6 weeks for the test to become positive after exposure).
TB is a very infectious disease that can affect many body areas, but primarily affects the lungs. It spreads from person to person by coughing, sneezing and breathing. Most of those who become infected manage to confine it to a few cells in their body, where it stays alive in an inactive form– this is called Latent TB (it does not make you feel sick and you are not infectious and in many, does not progress to active TB– but in some, it does).
However in a few, especially those who are immunocompromised, it may proceed directly to active progressive TB, which can be very dangerous. In classical active lung TB, you may have a chronic cough (sometimes with bloody sputum), a fever, weight loss and weakness. If TB affects other organs, you may have back pain, joint pains, anaemia, urinary tract pain, abdominal pain, heart chest pain or a headache/ confusion. Sputum for TB culture may also be requested.
The disadvantage with this test is that it is not suitable to check for in children under 17 years of age and those who are immunocompromised. It may also give a false positive if the patient has another type of mycobacterial infection.
After this, an Xray of the chest may be requested by your doctor to look for active TB or latent TB.
If you are worried that you may have been in contact with someone with ‘open chest TB’ (that has TB and was coughing around you), if you are over 35 years of age, you can ask for an Xray of your chest to check if you have contracted it. If you are under 35 years of age, your doctor will refer you to the TB clinic to have a mantoux skin prick test.
FREQUENTLY ASKED QUESTIONS
(From Department of Health and Human Services, City of Long Beach, 1997)
Is QuantiFERON®-TB Gold as good as the skin test?
Studies show that QuantiFERON®-TB Gold is as good as, if not better than, the TB skin test in helping to diagnose TB infection. Also, since the QuantiFERON®-TB Gold test uses proteins that are found only in the TB bacteria, it is much more accurate than the TB skin test, which can react to other bacteria in the same group as TB, giving a false positive test.
I’ve had TB skin tests in the past – will that make my QuantiFERON®-TB Gold test positive?
No. the QuantiFERON®-TB Gold will only be positive if you have a TB infection. The solution injected under your skin during the TB skin test does not contain the same parts of TB bacteria as the QuantiFERON®-TB Gold test uses.
I’ve had the Bacille Calmette-Guérin (BCG) vaccine and I’ve heard that can cause a false positive skin test. Will that make my QuantiFERON®-TB Gold test positive, too?
No. BCG does not affect the results of the QuantiFERON®-TB Gold test. While it’s true that BCG can give you a false positive skin test, the proteins used in the QuantiFERON®-TB Gold test are not found in BCG, so the BCG will not give you a false positive QuantiFERON®-TB Gold test.
What kind of results do you get with the QuantiFERON®-TB Gold test?
Most times, the QuantiFERON®-TB Gold test gives a positive or negative result. The positive result means you are infected with TB, and the negative result usually means you are not infected with TB. Sometimes, the result will not be either positive or negative. In this case, the result is called “indeterminate.” An indeterminate result may mean that your white blood cells are not working well due to a weakened immune system. If your result comes out as indeterminate, the test should be repeated, and if the second result is also indeterminate, you will need to talk to your doctor about what to do next.
What are the QuantiFERON®-TB Gold test advantages over a TB skin test?
QuantiFERON®-TB Gold has a number of important advantages compared to the TB skin test:
• Since it is a blood test, the QuantiFERON®-TB Gold:
o Is easier to do than the TB skin test;
o Avoids the risk of having a severe reaction to the TB skin test;
o Is done in a single visit. Does not require a return visit for TB skin test results;
o Reduces the chances of misreading the TB skin test result;
o Is a more accurate test (fewer false positives) than the TB skin test, and this means it is less likely you will be asked to take an unnecessary chest X-ray or treatment for possible TB infection.
What happens if my TB skin test or QuantiFERON®-TB Gold test is positive?
If either test is positive, it means you are infected with TB. A positive result cannot say if the TB infection is active or not, so you will need to have a chest X-ray and a review of any symptoms related to TB. You may need other tests as well, and treatment for the infection will be recommended.
What happens if my TB skin test or QuantiFERON®-TB Gold test is indeterminate?
Unfortunately around 5% of results can come back as being ‘indeterminate’. This means that your result is neither positive or negative. This usually does not mean you are positive and often it is a sign of being negative. In some, this may mean that you have an immunodeficiency of some sort that does not allow a result to be produced. You may wish to repeat the test if you desire, but this may be a needless expense for you as invariably the result on the repeat is often the same.