What is the Tuberculin Skin Test (TST)?
The Tuberculin Skin Test (TST), also known as the Mantoux test, is a diagnostic tool used to determine if a person has been exposed to the bacterium
Mycobacterium tuberculosis, which causes
Tuberculosis (TB). The test involves injecting a small amount of tuberculin purified protein derivative (PPD) into the skin of the forearm and examining the injection site after 48-72 hours for a reaction.
How is the TST Administered?
The test is administered by injecting 0.1 mL of PPD intradermally into the forearm. A small, raised wheal should form at the injection site. Healthcare providers should avoid using the same site for repeated tests and ensure the injection is intradermal rather than subcutaneous for accurate results.
How is the TST Interpreted?
The interpretation of the TST is based on the size of induration (swelling) at the site of injection, measured in millimeters. The threshold for a positive test varies depending on the individual’s risk factors:
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≥5 mm: Considered positive in HIV-infected persons, recent contacts of TB cases, persons with fibrotic changes on chest radiograph consistent with prior TB, organ transplant recipients, and other immunosuppressed patients.
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≥10 mm: Considered positive in recent immigrants from high-prevalence countries, injection drug users, residents and employees of high-risk congregate settings, mycobacteriology laboratory personnel, persons with clinical conditions that place them at high risk, children under 4 years of age, or children and adolescents exposed to adults in high-risk categories.
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≥15 mm: Considered positive in any person, including those with no known risk factors for TB.
What are the Limitations of TST?
The TST has several limitations:
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False Positives: Can occur in individuals who have been vaccinated with the
BCG vaccine or exposed to non-tuberculous mycobacteria.
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False Negatives: Can occur in individuals with weakened immune systems, very young children, or those recently infected where the immune response has not yet developed.
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Booster Effect: Repeat testing can sometimes cause a boosted reaction in individuals previously sensitized to TB antigens.
What is the Role of TST in Epidemiology?
In epidemiology, the TST is used for:
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Screening: Identifying individuals with latent TB infection (LTBI) who may benefit from preventive therapy.
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Surveillance: Monitoring TB infection rates in populations, especially in high-risk settings such as healthcare facilities, prisons, and shelters.
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Contact Investigation: Identifying and evaluating people who have been in close contact with infectious TB cases to prevent further transmission.
Alternatives to TST
Alternatives to the TST include Interferon-Gamma Release Assays (
IGRAs), such as the QuantiFERON-TB Gold and T-SPOT.TB tests. These blood tests measure the immune response to TB antigens and have advantages like not being affected by BCG vaccination and requiring only a single patient visit.
Conclusion
The Tuberculin Skin Test remains a valuable tool in the fight against tuberculosis, despite its limitations. Understanding its administration, interpretation, and role in public health can significantly aid in TB control and prevention strategies. Regular updates and training for healthcare providers on TST and its alternatives are essential for accurate diagnosis and effective epidemiological surveillance.
