QuantiFERON TB-Gold (In Tube Method) Frequently Asked Questions
Why the new test?
The new QuantiFERON-TB Gold (QFT In Tube method) allows for greater flexibility in specimen processing. This enables us to offer the test five days a week.
Who should be tested for TB with QFT?
In any situation where the TST is used, the QFT can also be used.
Which test should TB suspects receive?
SFDPH currently recommends that patients suspected of having active TB have both a TST and a QFT. The QFT, like the TST, is a diagnostic aide for the detection of TB infection. Patients with active TB can be either TST negative, QFT negative, or both. Therefore when clinical suspicion of active TB is present, the results of both tests should be interpreted in the context of the clinical, microbiologic and radiographic examinations.
Which test should persons with impaired immune function (including HIV infection) receive?
Like TB suspects, maximum sensitivity for detection of M. tuberculosis infection is preferred in patients with the highest risk of reactivation TB. Because limited data is available to document the sensitivity of the QFT in patients with impaired immune function, SFDPH recommends that both tests be routinely performed. While this is operationally challenging, in this patient population the added sensitivity of both tests is cost effective. Patients with discordant results should be managed as if they are infected with M. tuberculosis, unless some overriding concern exists for false positive TST results (e.g. active non-tuberculous mycobacteria infection).
When can a patient receive both a QFT and TST?
As above, unless they are a TB suspect or have impaired immune function, patients should not routinely receive both QFT and TST. If you already have a test result from either TST or QFT, you should act based on that information.
What if a patient recently received a TST?
In general, patients who recently received a TST should not be getting a TB test again, unless there is some reason to be suspicious of the results. Consult with Tuberculosis Clinic, (415) 206-8524, before requesting the test.
My patient has a negative QFT can they still be a TB suspect, or are they cleared?
Anyone with TB symptoms or TB risk factors and a new abnormal chest radiograph may be a TB suspect, regardless of the QFT or TST result. Like the TST, the QFT is a useful but imperfect diagnostic aide. It should never replace clinical judgment. Patients with a negative TST, as well as patients with a negative QFT, can have active TB. Remember: contacts to active TB cases who are newly infected with TB can take up to 8 weeks to convert their QFT or TST to a positive test.
Can I confirm a positive TST with a QFT?
You can, however there is no gold standard for the diagnosis of LTBI. False negative tests can occur with the QFT. A confirmatory QFT, i.e., QFT after a positive TST is most useful in BCG vaccinated individuals or patients refusing treatment for LTBI. Please note, however, that a TST may cause boosting that may result in a subsequent positive QFT.
The QFT result is indeterminate. What do we tell the patient and what do we do?
Indeterminate results can be caused by high background level of interferon-gamma (failure of the negative control), or lack of response to the mitogen (failure of the positive control). Repeat QFT or TST placement should depend on patient and provider preference. In San Francisco, repeating the QFT will result in a valid result 65-75% of the time.
My patient was QFT-positive OR indeterminate, but he/she never came back for their results. What do I do?
Provide routine follow up per your clinic protocol. For high risk patients, e.g. (HIV positive and QFT positive), assistance can be obtained from Sheila Davis-Jackson at (415) 206-8524.
My patient was QFT-negative, but he/she never came back for their results. What do I do?
If the patient needs to have proof of their result, they can get it anytime. In the future, we may mail out negatives to reduce clinic visits.
If you have any additional questions regarding the QuantiFERON-TB Gold (In Tube) blood test, please contact:
TB Control: (415) 206-8524 or the Curry International TB Center warmline (for general, non-SF related questions): (415) 502-4700