Tuberculosis (TB) is caused by a bacterium called Mycobacterium tuberculosis. Not everyone infected with TB bacteria becomes sick. As a result, two TB-related conditions can exist: latent TB infection (LTBI) and active TB infection.
Culture of the bacteria, molecular assays, and blood tests can be performed to determine if a person has been exposed to TB. The blood test utilized at the Vermont Department of Health Laboratory is called the QuantiFERON-TB Gold Plus (QFT-Plus).
The QFT-Plus is an interferon-gamma (IFN-y) release assay, commonly known as an IGRA, and is a current alternative to the tuberculin skin test (TST or Mantoux). Unlike the TST, the QFT-Plus is a controlled laboratory test that requires only one patient visit and is unaffected by previous vaccination with Bacille-Calmette Guerin (BCG).
The QFT-Plus consists of a 4-tube collection system optimized with tuberculosis-specific antigens that elicit both CD8 and CD4 T cell responses. It has been shown that CD8 T cells have a higher frequency of detection in people with active TB disease vs. latent infection and allows for greater detection in active TB subjects with HIV co-infection and in young children.
A positive QFT-Plus is a strong indicator of exposure to TB, but it cannot differentiate between active infection with TB and latent tuberculosis infection (i.e. a person who has no signs or symptoms of disease).
Positive QFT-Plus results are used in conjunction with risk assessments, radiography, and other medical and diagnostic evaluations by the patient’s doctor.