The QuantiFERON-TB Gold Test (QFT-G) is a modern blood test used to screen for infection with Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). This test is formally known as an Interferon-Gamma Release Assay (IGRA), offering an alternative to the traditional tuberculin skin test. It works by measuring the immune system’s cellular response to specific TB antigens, providing an objective and highly specific result. A positive result acts as a strong indicator of prior exposure, but it requires further clinical evaluation to determine the patient’s actual disease status.
What a Positive Result Indicates
A positive result from the QuantiFERON-TB Gold Test signifies that the individual has been infected with Mycobacterium tuberculosis bacteria at some point. The test does not, however, distinguish between a contained infection and an active, contagious disease.
The test works by exposing the patient’s white blood cells to synthetic peptides representing two TB proteins, ESAT-6 and CFP-10. If the person has been infected, their T-cells will recognize these antigens and release a signaling molecule called gamma interferon (IFN-γ). The test then quantifies the amount of IFN-γ released; a level above a certain cutoff, typically \(\geq 0.35\) IU/mL, is considered a positive result. A positive QFT-G result on its own does not confirm active Tuberculosis disease, but rather indicates the presence of TB infection.
Determining the Difference Between Latent and Active TB
Following a positive QFT-G result, the immediate and most important step is to determine if the infection is Latent TB Infection (LTBI) or Active TB Disease (ATBD). Active TB disease means the bacteria are multiplying and causing illness, while LTBI means the bacteria are dormant, or “sleeping,” and are not causing symptoms or being spread to others.
The first line of secondary testing is usually a chest X-ray, which is performed to look for characteristic signs of active disease. These signs can include infiltrates, cavities, or fibrotic changes in the lungs. The doctor will also conduct a thorough physical examination and medical history review, looking for clinical symptoms associated with active disease.
Active TB symptoms include:
- A persistent cough lasting three weeks or longer.
- Fever.
- Unexplained weight loss.
- Night sweats.
- Fatigue.
If the chest X-ray is clear and the patient reports no symptoms, the diagnosis is typically LTBI. If the X-ray shows abnormalities or if the patient is symptomatic, further microbiological studies, such as sputum samples for smear and culture, are necessary to confirm ATBD. The presence of culturable Mycobacterium tuberculosis in the sputum definitively confirms an active case, as ATBD is a microbiological diagnosis.
Treatment Protocols Following Diagnosis
The medical management plan is entirely dependent on the determination of whether the patient has latent infection or active disease. If the diagnosis is Active TB Disease, treatment involves a multi-drug regimen that lasts for a minimum of six months, and often longer. This aggressive treatment is necessary to fully eradicate the high bacterial load, prevent drug resistance, and stop the patient from spreading the infection. Patients with active disease must strictly adhere to the treatment schedule to ensure a complete cure.
If the diagnosis confirms Latent TB Infection, the treatment protocol shifts to preventative antibiotic therapy. The goal of treating LTBI is to prevent the “sleeping” bacteria from activating in the future, which is estimated to occur in about 5 to 10 percent of infected individuals over their lifetime. Treatment options for LTBI are often shorter than for active disease, sometimes lasting as little as three or four months, depending on the specific drug combination chosen.