From the Guidelines
The T-SPOT TB test is a specialized blood test used to detect tuberculosis (TB) infection, which works by measuring the immune response to TB bacteria in a blood sample, and is preferred over the tuberculin skin test (TST) due to its higher specificity and lack of cross-reactivity with the BCG vaccine, as suggested by the most recent study 1.
How the Test Works
The T-SPOT test identifies T-cells that have been sensitized to TB antigens, indicating exposure to the tuberculosis bacterium. It uses an ELISpot assay to determine the number of cells producing interferon-γ, which is a key component of the immune response to TB. This test is particularly useful for individuals who have received BCG vaccines, have compromised immune systems, or cannot return for skin test readings.
Test Results and Interpretation
The test results are typically available within 24-48 hours and are reported as positive (TB infection likely), negative (no TB infection detected), or indeterminate (inconclusive). It is essential to note that this test cannot distinguish between latent TB infection and active TB disease, so additional testing such as chest X-rays or sputum cultures may be needed if the result is positive, as recommended by recent guidelines 1.
Advantages and Recommendations
The T-SPOT test has several advantages over traditional TB testing methods, including its high specificity, lack of cross-reactivity with the BCG vaccine, and ability to detect TB infection in individuals with compromised immune systems. Based on the most recent evidence, it is recommended to use the T-SPOT test or other interferon-gamma release assays (IGRAs) for TB screening, especially in individuals with autoimmune inflammatory rheumatic diseases or those who have received BCG vaccines, as suggested by the 2022 EULAR recommendations 1.
From the Research
T-Spot TB Blood Work Overview
- T-Spot TB, also known as T-SPOT.TB, is a blood test used to diagnose tuberculosis (TB) infection by measuring the release of interferon-gamma from T-cells in response to specific TB antigens 2, 3, 4.
- The test is designed to detect latent TB infection (LTBI) and active TB, and is considered more accurate than the traditional tuberculin skin test (TST) 2, 5.
How T-Spot TB Works
- The T-SPOT.TB test uses two specific antigens, ESAT-6 and CFP 10, to stimulate T-cells and measure the release of interferon-gamma 2, 4.
- The test is performed on whole blood samples and can provide results within a few days 2.
- The test has been shown to have high sensitivity and specificity, and is not affected by BCG vaccination or cross-reaction with non-tuberculous mycobacteria 3, 4, 5.
Clinical Applications of T-Spot TB
- T-SPOT.TB has been licensed for in-vitro diagnosis in Europe and the United States, and has been shown to be useful in distinguishing active TB from LTBI 4.
- The test has also been used to predict active TB among individuals with LTBI, and has been shown to have better performance than TST in diagnosing active TB 5.
- However, large prospective longitudinal studies are needed to fully evaluate the performance of T-SPOT.TB in diagnosing LTBI 5.
Limitations of T-Spot TB
- The test may not be reliable in high-risk populations, such as those with close contact with active TB patients, due to interference from LTBI 6.
- The sensitivity of T-SPOT.TB may be lower in sputum smear-negative populations, and the test results should be interpreted with caution in combination with subject characteristics 6.