What is a QFT test?

A QFT (QuantiFERON-TB Gold) test is a blood test used to help detect Mycobacterium tuberculosis, the bacteria that causes tuberculosis (TB).

Here's a more detailed explanation:

• Purpose: The primary purpose of a QFT test is to determine if someone is infected with TB bacteria. It's especially useful for individuals who may have been exposed to TB or are at higher risk of infection.

• How it Works: QFT is an interferon-gamma (IFN-γ) release assay (IGRA). This means it measures how your immune system reacts to TB bacteria.

  1. A blood sample is taken.
  2. The blood is mixed with antigens (substances that can trigger an immune response) that are specific to M. tuberculosis.
  3. If you are infected with TB bacteria, your immune cells (specifically T cells) in the blood will release IFN-γ when they encounter these antigens.
  4. The amount of IFN-γ released is measured. A high level of IFN-γ indicates a TB infection.

• Advantages over Tuberculin Skin Test (TST): QFT tests offer several advantages over the traditional TST (also known as a PPD or Mantoux test):

  • Requires only one patient visit.
  • Results are available within 24-48 hours.
  • Not affected by prior BCG (Bacillus Calmette-Guérin) vaccination. The BCG vaccine is used in many countries to prevent TB, but it can cause false-positive results on the TST.
  • Less subjective interpretation compared to the TST, which requires visual interpretation of skin induration.

• Who Should Get a QFT Test? Individuals who should consider getting a QFT test include:

  • People who have been in close contact with someone who has active TB disease.
  • People who have weakened immune systems (e.g., due to HIV, certain medications, or other medical conditions).
  • People who have recently arrived in the United States from countries where TB is common.
  • People who live or work in high-risk settings, such as hospitals, homeless shelters, or correctional facilities.

• Interpreting Results: A QFT test can be positive, negative, or indeterminate.

  • Positive: Indicates TB infection. Further evaluation, including a chest X-ray and other tests, is needed to determine if the infection is active (TB disease) or latent (TB infection).
  • Negative: Suggests that TB infection is unlikely, but does not completely rule it out, especially in individuals with weakened immune systems.
  • Indeterminate: Means the test results are unclear and may need to be repeated.

• Important Note: A positive QFT test only indicates infection with TB bacteria. It does not differentiate between active TB disease (where the bacteria are actively multiplying and causing symptoms) and latent TB infection (where the bacteria are present but inactive, and the person does not have symptoms and cannot spread the disease). Additional tests are needed to make that determination.

In summary, the QFT test is a modern blood test that offers a convenient and reliable way to detect TB infection.