TB Gold

The TB Gold test is an Interferon-Gamma Release Assay (IGRA), a type of blood test used to diagnose Mycobacterium tuberculosis infection. It works by measuring the immune system's response to TB bacteria. This test plays a significant role in public health initiatives to control tuberculosis and is essential for individual patient management, helping doctors make informed decisions about further testing and potential treatment.

One of the key questions patients often have is about TB Gold vs skin test (also known as the Mantoux tuberculin skin test or TST). The TB Gold test offers several advantages over the traditional skin test:

• Single Visit: It usually requires only one visit to have blood drawn, unlike the TST which requires a second visit for the result to be read.
• BCG Vaccine: The TB Gold test is not affected by prior BCG (Bacille Calmette-Guérin) vaccination, which can cause a false-positive reaction in the TST. This leads to greater accuracy in individuals who have received the BCG vaccine.
• Objectivity: Results are based on a laboratory measurement, reducing the subjectivity that can sometimes occur with reading a skin test reaction.

The importance of the TB Gold test lies in its ability to accurately detect TB infection, allowing for early intervention, particularly for latent TB infection, to prevent progression to active TB disease.

The TB Gold test measures the cell-mediated immune response to specific antigens of Mycobacterium tuberculosis. Here's a more detailed explanation:

1. Blood Sample Collection: A small sample of your blood is collected in specialized tubes.
2. Exposure to TB Antigens: In the laboratory, your blood cells (specifically T lymphocytes) are mixed with synthetic peptides representing TB antigens, such as ESAT-6 and CFP-10.
3. Interferon-Gamma (IFN-γ) Release: If your T lymphocytes have been previously sensitised to TB bacteria, they will recognise these antigens and release IFN-γ.
4. Measurement: The amount of IFN-γ released is then measured. A significant amount of IFN-γ indicates a positive result.

The TB Gold test is recommended in various situations, primarily for identifying individuals with TB infection who might benefit from treatment or further evaluation. Common scenarios include:

• Screening for Latent TB Infection (LTBI)
• Contact tracing
• Healthcare worker screening
• Pre-biologic therapy screening
• Immigration medical exams
• Evaluation for active TB disease
• Screening in high-risk congregate settings

Risks:

• Slight pain or discomfort at the needle insertion site
• Bruising or minor bleeding
• Fainting or feeling lightheaded (rare)
• Infection at the puncture site (very rare)

Limitations:

• Cannot distinguish between latent and active TB
• While accuracy is generally high, false positives and negatives can occur:
• False positives: Rare, but possible
• False negatives: More likely in recent exposures, immunocompromised individuals, or very young children
• Indeterminate results: Can happen due to lab issues or immune suppression
• Window period: 2–8 weeks post-exposure for immune response to show
• Does not predict progression from latent to active TB

• Positive: Indicates likely TB infection (latent or active)
• Negative: TB infection unlikely
• Indeterminate: Inconclusive result; may need retesting
• Borderline: Near cutoff levels; further testing may be required

Internal controls:

• Nil Control: Background IFN-γ levels
• Mitogen Control: Ensures T-cell functionality

• Individuals with close contact with TB-positive persons
• Immigrants or travellers from TB-endemic areas
• Healthcare workers and residents in high-risk environments
• Immunocompromised individuals (e.g., HIV, cancer, autoimmune disorders)
• People starting biologic medications
• Symptomatic individuals
• Children requiring immune-impacting therapies

• Higher accuracy and specificity
• Not affected by BCG vaccination
• Only one visit needed
• Lab-based objectivity
• Early detection of latent TB
• Aids clinical decision-making
• Suitable for a wide range of patients

• Mycobacterium tuberculosis infection
• Latent TB infection (LTBI)
• Supportive in diagnosing active TB (with other tests)

Instructions:

• No fasting required
• Disclose all medications and supplements
• Inform if taking blood thinners or if you have bleeding disorders

Prerequisites:

• Doctor’s referral
• ID and insurance documents

Eligibility:

• Suitable for adults, children, pregnant women, and immunocompromised individuals

Procedure:

1. Arrival and check-in
2. Site preparation and blood draw
3. Sample collection into QFT-Plus tubes
4. Post-draw care (bandage, pressure)
5. Duration: a few minutes

Caution:

• Mention allergies to latex or adhesives
• Inform if you faint during blood draws
• Notify about IVs, scarring, or preferred arm