Quantiferon-TB Gold Test – A Modern Tool for Diagnosing Tuberculosis

The Quantiferon-TB Gold Test is an advanced blood test used to detect latent tuberculosis (TB) infection by measuring the immune system’s response to Mycobacterium tuberculosis, the bacteria responsible for TB. It is widely recommended for diagnosing latent TB infection and differentiating it from active TB, especially in individuals at high risk or those with previous BCG vaccinations.

Indications for Quantiferon-TB Gold Test:

● Screening for latent TB: The test is primarily used in individuals at high risk of TB, such as healthcare workers, individuals living in crowded conditions, and those exposed to people with active TB.

● People with HIV: Those living with HIV are at higher risk of progressing from latent TB to active TB, making routine screening necessary.

● Prevention in immunocompromised individuals: Patients on immunosuppressive therapy, such as biologic drugs, need to be screened for latent TB before starting treatment to avoid the risk of reactivation.

● Close contacts of TB patients: Individuals who have had close contact with someone diagnosed with active TBare also recommended for testing.

How Quantiferon-TB Gold Test Works:

The test measures the interferon-gamma (IFN-γ) release from T-cells in response to specific TB antigens. Unlike the traditional Tuberculin Skin Test (TST), it does not require multiple visits and is not affected by previous BCG vaccinations. A blood sample is drawn, and the immune response is measured in the lab.

Normal Range:

● Negative result: No detectable immune response, indicating no infection or latent TB.

● Positive result: Indicates exposure to Mycobacterium tuberculosis, which could be a sign of latent TB infection. Further testing (like chest X-ray and sputum tests) is necessary to rule out active TB.

Abnormal Test Significance:

● Positive result: In case of a positive result it suggests that the individual has been infected with latent TB. However, it does not differentiate between latent TB and active TB, and therefore, additional tests are needed for a definitive diagnosis.

● False positives: The test can sometimes return a false-positive result in people infected with non-tuberculous mycobacteria (NTM) or those with a history of BCG vaccination.

● False negatives: Immunocompromised individuals (e.g., those with HIV or on immunosuppressive drugs) may receive false-negative results due to weakened immune responses.

Limitations of the Quantiferon-TB Gold Test:

● False negatives: Although more accurate than the TST, the Quantiferon-TB Gold test can still yield false-negative results, particularly in people with weakened immune systems or those who are in the early stages of infection.

● No distinction between latent and active TB: The test does not differentiate between latent TB (which is not contagious) and active TB (which can spread to others). A chest X-ray or sputum culture is necessary to confirm active disease.

● Cost and accessibility: The Quantiferon-TB Gold test is generally more expensive and less widely available than the TST, particularly in low-resource settings.

Guidelines on the Utility of Quantiferon-TB Gold Test:

Recent WHO and CDC guidelines suggest that the Quantiferon-TB Gold Test is a reliable diagnostic tool for screening latent TB, especially in high-risk populations. It is preferred over the TST for individuals who have received the BCG vaccine, as it avoids the false positives that can occur with the skin test. However, these guidelines emphasize the need for clinical evaluation in conjunction with the test results, as it cannot definitively confirm active TB.

The CDC specifically recommends the Quantiferon-TB Gold Test for screening in populations at risk of latent TBinfection, such as those with HIV, exposure to TB patients, and those undergoing immunosuppressive therapies. It is also increasingly used as part of routine TB screening programs in high-burden countries to monitor and control the spread of TB.

Management Based on Results:

● For positive results: Further diagnostic tests, such as a chest X-ray, sputum culture, or acid-fast bacillus (AFB)smear, are necessary to rule out active TB. If latent TB is confirmed, the standard treatment is a 6-month course of isoniazid (INH), sometimes in combination with rifampicin or rifapentine, depending on the individual’s health status and the presence of any drug resistance.

● For negative results: While a negative test typically rules out latent TB, follow-up testing may be required if the individual develops symptoms later or is exposed to active TB.

Price in India:

Approx. ₹3,000 – ₹7,000 for the Quantiferon-TB Gold test, depending on the clinic or laboratory.