QuantiFERON Gold Test: Purpose and Management
Primary Purpose
The QuantiFERON-TB Gold (QFT) test is a blood-based interferon-gamma release assay used to detect latent tuberculosis infection (LTBI) by measuring immune response to TB-specific antigens, offering superior specificity compared to tuberculin skin testing, particularly in BCG-vaccinated populations. 1, 2
When to Use QuantiFERON Gold
Screening populations at increased risk for LTBI, including recent immigrants from high TB burden countries, injection drug users, prisoners, and healthcare workers 1, 2
Serial testing of persons at low baseline risk but with future exposure potential, such as healthcare workers and military personnel 1
Preferred over TST in BCG-vaccinated individuals because QFT is not affected by prior BCG vaccination, unlike TST which produces false positives 1, 3, 4
Can be used in all circumstances where TST is traditionally used, including contact investigations and TB screening programs 1
Critical Management Steps for Positive Results
Immediate Evaluation (Before Treatment)
A positive QFT result requires the same evaluation as a positive TST and should never be followed by confirmatory TST. 1, 2
Obtain chest radiograph immediately to exclude active tuberculosis disease before diagnosing LTBI 1, 2, 5
Screen for TB symptoms including cough, fever, night sweats, and weight loss 2
Perform sputum examination if any respiratory symptoms are present 2
Offer HIV counseling, testing, and referral because HIV infection dramatically increases both the risk of active TB and the urgency of treating LTBI 1, 2
Never initiate single-drug LTBI treatment until active TB is definitively excluded 2
Treatment Regimens for LTBI
Once active TB is excluded, treatment should be offered based on risk stratification, with high-priority groups including HIV/AIDS patients, those on TNF-α antagonists, silicosis patients, and recent immigrants from high TB burden countries 2, 5
Recommended evidence-based regimens include:
Isoniazid 5 mg/kg (maximum 300 mg) daily for 9 months (preferred regimen) 2
Isoniazid 5 mg/kg (maximum 300 mg) daily for 6 months (alternative) 2
Rifampin 10 mg/kg (maximum 600 mg) daily for 4 months 2
Isoniazid plus rifampin daily for 3-4 months 2
Rifapentine plus isoniazid once weekly for 12 weeks (requires directly observed therapy) 2
Monitoring During Treatment
Monthly clinical assessment for all patients with education about hepatitis symptoms 2
Baseline and periodic liver function tests for patients with risk factors including pregnancy, HIV infection, and heavy alcohol use 2
Important Limitations and Contraindications
When NOT to Use QFT
Not recommended for diagnosing active tuberculosis because active TB suppresses interferon-gamma responses, resulting in lower sensitivity than TST 1
Insufficient data for children aged <17 years at the time of the 2005 guidelines 1
Blood must be incubated within 12 hours of collection, requiring coordination with qualified laboratories 1
Special Populations Requiring Caution
Immunocompromised patients (HIV, organ transplant recipients, those on immunosuppressive therapy) may have decreased interferon-gamma production, potentially causing false-negative results 1
Negative QFT results do not exclude LTBI in severely immunocompromised persons or those with recent exposure 1
Contact Investigation Protocol
For recent TB contacts with negative QFT results, repeat testing should be performed 8-10 weeks after the end of exposure, similar to TST protocols 1
Window period prophylaxis is indicated for contacts aged <5 years or severely immunocompromised persons exposed to highly contagious TB, even with negative initial testing 1
Test Interpretation Nuances
QFT is not affected by prior TST administration, so QFT can be performed after TST if needed 1
However, TST should NOT be used to confirm QFT results because PPD injection may affect subsequent QFT results 1
The probability of LTBI is greatest when both QFT and TST are positive, but in low-risk populations with positive QFT, TST confirmation may be considered before initiating treatment 1
QFT typically remains positive after LTBI treatment (87.5% at 3 months, 84.6% at 15 months), so the test should not be used to monitor treatment response 6