QFT accurately diagnoses Latent TB Infections (LTBI) which is an asymptomatic infection and is believed to affect over one-quarter of the global population. Approximately 5-15% of infected individuals will progress to active TBI if not diagnosed and treated. This progression is called the reactivation of LTBI.

QFT measures the amount of interferon gamma that is released when infected white blood cells in a blood sample come into contact with specific antigens of M. tuberculosis. The use of whole blood in QFT makes T cell incubation simple and fast, and reduces turn-around time (TAT). It is simple as there is no lymphocyte isolation, washing, counting, diluting, or culturing.

The QFT Test can be used be used for individuals who’ve had contact with individuals diagnosed with active pulmonary TB, are at increased risk of progression to active TB disease due to impaired immunity, have radiographic evidence of old, healed TB without history of treatment and have an increased risk for LTBI due to injection-drug use. Additionally, the following individuals should be screened for tuberculosis:

• Foreign-born persons and visitors from TB-endemic countries, especially immigrants who have arrived in the last two years
• Residents and employees of prisons and jails
• Individuals eligible for LTBI surveillance programs such as health-care workers, aid workers and military personnel

QFT works on patients who have previously been vaccinated against TB with the Bacille Calmette-Guerin (BCG) vaccination. QFT requires only one patient visit, has a low (1%) false-positive rate and high specificity (>99%) which reduces unnecessary evaluations (labs, X-rays).

The blood sample is collected in three specialized tubes (NIL, TB Antigen and Mitogen) that enable immediate exposure of the white blood cells to highly specific TB antigens. The QFT results are reported qualitatively as negative or positive for the bacteria, as well as quantitatively with a numerical value (reported in IU/mL). The numerical value indicates the amount of interferon gamma that is released when infected blood cells are exposed to specific antigens of M.tuberculosis.

For a test to be considered positive, the TB Antigen tube value minus the NIL tube value must be greater than or equal to 0.35 IU/mL.

QFT is a blood test that is collected in three sample tubes that allow for control-based testing in a laboratory environment. An immune response is observed when antigen from M. tuberculosis comes into contact with infected white blood cells. The test is simple, fast and considered the most clinically tested and proven. QFT can accurately diagnose LTBI, which is a state of persistent immune response which evidence of clinically active TB.

QFT is usually ordered to detect LTBI in the following conditions:  

• Individuals who’ve had contact with persons recently diagnosed with active TB
• Foreign-born persons and visitors from TB prone countries, especially immigrants who have arrived in the last two years
• People who are at increased risk of progression to active TB disease due to impaired immunity
• People with evidence of old, healed TB without history of treatment
• People with an increased risk for LTBI due to injection-drug use, and residents and employees of prisons and jails
• Persons who are, by history, at low risk for LTBI but whose future activity might place them at increased risk for exposure, and others eligible for LTBI surveillance programs (e.g., health-care workers, aid workers and military personnel)

The current fee for this test is C$95.00. It is not covered by the provincial health insurance plans, but it may be covered by extended health insurance plans.

The QFT test is available in select LifeLabs locations in Ontario; please refer to the table below for more details.

Your doctor will receive the results between 5 to 10 business days of the blood test.

1. Qiagen, QuantiFERON®–TB Gold product information
2. Pai et. al. Canadian Tuberculosis Standards 7th Edition: 2014 – Diagnosis of latent Tuberculosis infection
3. Centers for Disease Control and Prevention, Guidelines for Using the QuantiFERON®-TB Test for Diagnosing Latent Mycobacterium tuberculosis Infection, 2012
4. CDC. Updated guidelines for using gamma release assays to detect m. tuberculosis infection. MMWR. 2010;59(RR05):1-25
5. City of Toronto, Tuberculosis Information for Healthcare Providers: Management of Latent Tuberculosis Infection and Active Disease – Information for Health Professionals
6. Latent tuberculosis infection: An overview – CCDR: Volume 43-3/4, March 2, 2017: Tuberculosis (TB)
7. World Health Organization’s Fact Sheet on Tuberculosis