The QuantiFERON-TB Gold (QFT-G) test is a serological diagnostic method specifically used to detect Mycobacterium tuberculosis infection. Unlike traditional skin contact tests (such as the PPD test), this test analyzes immune responses in the blood to accurately distinguish latent tuberculosis infection (LTBI) from active pulmonary tuberculosis. Its primary purpose is to replace more invasive skin tests and provide more objective quantitative data to assist physicians in treatment planning.
The advantages of this test include avoiding interference factors associated with skin tests, such as false positives caused by Bacillus Calmette-Guérin (BCG) vaccination or non-tuberculous mycobacteria infections. It is suitable for immunocompromised patients, groups unable to return for skin response observation, and high-precision diagnostic scenarios in healthcare settings.
QFT-G belongs to the Interferon-Gamma Release Assay (IGRA) category. Its core principle involves measuring the concentration of interferon-gamma secreted by T cells upon exposure to specific tuberculosis antigens (ESAT-6, CFP-10, and TB7.7). When blood samples are exposed to these three antigens, if the patient has been infected with tuberculosis, activated memory T cells will release corresponding cytokines, allowing determination of infection status.
This technology offers an advantage over traditional tests by directly quantifying immune responses and avoiding subjective interpretation errors associated with skin tests. The process requires drawing 5cc of whole blood, which is then dispensed into special reagent tubes for culture and analysis, all performed in the laboratory, with results typically available within 24 hours.
Primarily used for screening latent tuberculosis infection, especially among high-risk groups such as healthcare workers, HIV-infected individuals, and contacts of confirmed cases. It is also used to monitor infection status changes in immunosuppressed patients (such as organ transplant recipients and cancer chemotherapy patients).
Applicable in cases suspected of active pulmonary tuberculosis with inconclusive skin test results or when excluding BCG vaccination interference is necessary. Additionally, screening residents in long-term care facilities is a common application scenario.
The procedure requires no special preparation; patients only need to undergo venous blood collection of about 5-10cc. The blood must be immediately dispensed into special reagent tubes containing anticoagulants and stored below 4°C to maintain cell viability. Samples should be sent to the laboratory for analysis within 24 hours of collection.
The results are determined based on the relative secretion level of interferon-gamma, generally categorized as positive, negative, or indeterminate. The positive threshold varies by manufacturer calibration, but generally, levels exceeding 0.35 IU/mL are considered positive. The report usually includes response values for each antigen stimulation and control data.
Compared to traditional methods, QFT-G has a sensitivity of 80-90% and a specificity exceeding 95%, effectively reducing false positives. The results are not influenced by physiological factors such as skin itching or skin lesions, making it suitable for infection screening before surgery.
This test is non-invasive with minimal direct side effects, but may cause temporary bruising or pain at the puncture site due to blood collection. Specific risks include:
Contraindications include: 1. Severe cardiac or liver failure that may affect immune response 2. Use of immunomodulatory drugs (such as high-dose steroids) on the day of testing
Patients should inform their physicians of recent contact history, immune status, and current medications before testing. Suspected HIV-infected individuals should undergo viral load testing simultaneously, as immunosuppression may lead to false-negative results.
If the patient is on immunosuppressive therapy (such as antitumor drugs or biologics), T-cell responses may be suppressed, affecting the accuracy of results. It is recommended to wait at least 14 days after stopping such medications before testing.
There are no direct interactions with anti-tuberculosis drugs, but patients undergoing treatment should also undergo chest X-ray and sputum culture for comprehensive assessment. Concurrent interferon therapy (such as for hepatitis C) may interfere with the results and should be noted.
Clinical trials in multiple countries have shown that QFT-G has a sensitivity of 85-95% in diagnosing active tuberculosis and a specificity of up to 98% for latent infection. The World Health Organization (WHO) recommends prioritizing IGRA over skin tests in resource-available regions.
A 2018 systematic review indicated that QFT-G has a diagnostic concordance of 90% in immunocompetent individuals and can effectively reduce unnecessary treatments. During outbreaks in healthcare facilities, this test can rapidly screen high-risk contacts.
Traditional tuberculin skin testing (TST) remains an economical option but may be affected by BCG vaccination interference. Other options include:
Alternative choices should be based on the patient’s immune status and clinical presentation. For example, immunosuppressed patients are recommended to prioritize QFT-G to avoid false negatives associated with skin testing.
Do I need special preparation before taking the QuantiFERON-TB Gold test? For example, fasting or stopping medications?
Usually, no special preparation such as fasting or stopping medications is required before the QuantiFERON-TB Gold test. However, if you are taking immunosuppressants or steroids that may affect immune responses, inform your healthcare provider in advance, as these drugs could interfere with the accuracy of the test results.
If the QuantiFERON-TB Gold test is positive, what should I do next?
A positive result indicates possible tuberculosis infection, but further confirmation with clinical symptoms and imaging tests (such as chest X-ray) is necessary to determine whether it is latent infection or active pulmonary tuberculosis. Your doctor may recommend preventive treatment or follow-up observation, depending on your health status.
What is the difference between QuantiFERON-TB Gold and traditional tuberculin skin testing (TST)?
QuantiFERON-TB Gold is a blood test that avoids the false-negative results that can occur with skin tests (such as in BCG-vaccinated individuals), and results are not limited by time. The skin test requires observation of the reaction after 48-72 hours but may be affected by vaccines or individual differences. The choice between the two depends on clinical needs.
When should I retake the QuantiFERON-TB Gold test?
If the initial result is negative but there is clinical suspicion of infection (such as contact with a confirmed case) or in immunocompromised individuals (such as HIV patients), your doctor may recommend retesting after 4-8 weeks. Repeating the test may also be necessary to confirm infection after immunotherapy.
What factors can affect the accuracy of the QuantiFERON-TB Gold test?
The accuracy may be influenced by recent vaccinations (such as BCG), severe immunosuppression, or co-infection with HIV, which can lead to false-negative results. Improper handling of blood samples, such as inadequate reaction with anticoagulants, can also affect the results. Strict adherence to laboratory procedures is essential to ensure precision.
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