From the Guidelines
The most appropriate option for TB testing is the Interferon-Gamma Release Assay (IGRA), which is preferred over the tuberculin skin test (TST) for individuals who have received the BCG vaccine or are unlikely to return for TST reading, as recommended by the American Thoracic Society, Centers for Disease Control and Prevention, and Infectious Diseases Society of America 1.
Options for TB Testing
There are several options for tuberculosis (TB) testing, with the two main categories being skin tests and blood tests. The tuberculin skin test (TST), also known as the Mantoux test, involves injecting a small amount of purified protein derivative (PPD) under the skin of the forearm and checking for a reaction 48-72 hours later. A positive reaction, indicated by induration (hardened area) of 5-15mm depending on risk factors, suggests TB infection.
Blood Tests
Blood tests, known as Interferon-Gamma Release Assays (IGRAs), include QuantiFERON-TB Gold Plus and T-SPOT.TB, which measure the immune response to TB proteins in a blood sample. IGRAs are generally preferred for people who have received the BCG vaccine since they don't cross-react with the vaccine like the skin test can.
Factors Influencing Test Choice
The choice between tests depends on factors like:
- Previous BCG vaccination
- Age
- Immune status
- Local availability Testing is particularly important for high-risk groups including:
- Close contacts of TB patients
- Immunocompromised individuals
- Healthcare workers
- Those from countries with high TB prevalence
Additional Tests for Active TB
For active TB disease diagnosis, additional tests include:
- Chest X-rays
- Sputum microscopy
- Culture tests
- Molecular tests like GeneXpert MTB/RIF, which can detect TB bacteria and rifampin resistance within hours As noted in the 2019 study, guidelines by WHO, USPSTF, ATS, CDC, and IDSA support testing with IGRA over TST for diagnosis of LTBI in individuals with low-to-intermediate risk of progression to active disease, and either IGRA or TST or dual testing (if first one is negative) at highest risk of developing active MTB 1.
From the Research
Options for TB Testing
There are several options available for TB testing, including:
- Tuberculin skin test (TST)
- Interferon-gamma release assays (IGRAs) such as T-SPOT.TB and QuantiFERON-TB Gold In-Tube (QFT-GIT) 2, 3, 4
- C-Tb skin test, a novel specific skin test based on ESAT-6 and CFP10 antigens 5
Characteristics of TB Tests
The characteristics of these tests are:
- TST: less sensitive and specific than IGRAs, especially in BCG-vaccinated populations 2, 3
- IGRAs: more sensitive and specific than TST, especially in BCG-vaccinated populations 2, 3, 4, 6
- C-Tb skin test: delivers IGRA-like results in a field-friendly format, unaffected by BCG vaccination status 5
Predictive Value of TB Tests
The predictive value of these tests for the progression of latent infection to active tuberculosis disease is:
- IGRAs: better predictive ability than TST, with a pooled risk ratio of 9.35 (95% CI 6.48-13.49) compared to 4.24 (3.30-5.46) for TST 4
- TST: lower predictive ability, with a pooled positive predictive value of 2.3% (1.5-3.1) compared to 4.5% (3.3-5.8) for IGRAs 4
Recommendations for TB Testing
Based on the available evidence, the following recommendations can be made: