QUANTIFERON - TB GOLD (INTERFERON-GAMMA RELEASE ASSAY)

QuantiFERON-TB Gold Plus — Interferon-Gamma Release Assay

The QuantiFERON-TB Gold Plus test, commonly called QuantiFERON-TB Gold or QFT-Plus, is a laboratory blood test used to detect immune sensitization to Mycobacterium tuberculosis, the bacterium responsible for tuberculosis.

It belongs to a group of tests known as interferon-gamma release assays (IGRAs). The test measures how the patient’s immune cells respond when exposed to specific tuberculosis antigens in a blood sample. Sensitized T lymphocytes release interferon-gamma, which is then measured by an immunoassay.

The test is primarily used to identify TB infection, including latent tuberculosis infection. In latent TB infection, the bacteria remain in the body without causing symptoms and cannot normally be spread to other people. However, latent infection may later progress to active TB disease, particularly in people with weakened immune systems.

A positive QuantiFERON result indicates that infection with organisms in the Mycobacterium tuberculosis complex is likely. However, the test cannot determine whether the patient has latent TB infection or active tuberculosis disease. A positive result therefore requires clinical assessment and additional investigations, which may include a chest X-ray, sputum smear microscopy, molecular testing and mycobacterial culture.

A negative result suggests TB infection is less likely, but it does not completely exclude infection. False-negative results may occur when testing is performed too soon after exposure or in people with weakened immune responses, severe illness or certain medical conditions.

One important advantage of QuantiFERON testing is that it is not expected to produce a false-positive result because of previous Bacille Calmette-Guérin vaccination, commonly known as BCG vaccination. It also requires only one patient visit, unlike the tuberculin skin test, which requires a second visit for interpretation.

Test Purpose

The test may help:

• Detect infection with Mycobacterium tuberculosis.
• Support the diagnosis of latent TB infection.
• Screen people who have been in contact with someone with infectious TB.
• Evaluate healthcare workers and other occupationally exposed individuals.
• Screen patients before immunosuppressive or biological therapy.
• Assess people with previous BCG vaccination.
• Support immigration, employment or institutional TB screening when required.

Benefits

QuantiFERON-TB Gold Plus requires a single blood collection and does not require the patient to return for skin-test reading. It uses TB-specific antigens and is therefore more specific than the traditional tuberculin skin test in people who have received BCG vaccination.

The test contains Nil, TB1, TB2 and Mitogen controls. These controls help assess background interferon-gamma activity, immune responsiveness and the validity of the test result.

Why Doctors Recommend It

Doctors may recommend this test after exposure to a person with active TB, before starting medicines that suppress the immune system, during occupational screening or when latent TB infection needs to be evaluated.

WHO recognizes IGRAs as one of the recommended approaches for testing for TB infection.

Preparation

No fasting or special dietary preparation is normally required.

The patient should inform the healthcare provider about:

• Recent exposure to tuberculosis
• Symptoms such as persistent cough, fever or weight loss
• HIV infection or other immune disorders
• Immunosuppressive treatment
• Recent live-virus vaccination
• Previous TB treatment
• Recent tuberculin skin testing

Correct blood collection, tube filling, mixing, transport and incubation are essential for reliable results.

Reporting Time

Typically: 2–5 working days

Reporting time may vary depending on specimen transport, incubation schedules and laboratory workflow.

Who Should Take the Test?

The test may be recommended for:

• Close contacts of patients with infectious tuberculosis
• Healthcare and laboratory workers
• People with HIV or weakened immunity
• Patients preparing to start biological therapy
• Organ transplant candidates or recipients
• People undergoing long-term steroid treatment
• Individuals from settings with a high prevalence of TB
• Patients with abnormal chest imaging suggestive of previous TB
• People requiring occupational or institutional TB screening
• Anyone advised by a qualified healthcare professional