The Tuberculin Skin Test (TST) is a diagnostic method used for screening Mycobacterium tuberculosis infection. This test involves subcutaneous injection of purified protein derivative (PPD) of tuberculin to evaluate the body's allergic reaction to tuberculosis antigens. Its primary purpose is to determine whether the individual has been exposed to tuberculosis bacteria or has immunological memory against tuberculosis, commonly used in health screenings for high-risk groups.
The principle of this test is based on the immune response mechanism: if the individual has previously been infected with tuberculosis or vaccinated with Bacillus Calmette-Guérin (BCG), a red, hard, swollen area will appear at the injection site within 48 to 72 hours. The reaction's intensity helps clinicians assess infection status. Due to its low cost and simplicity, it has become a standard procedure in tuberculosis control programs worldwide.
The Tuberculin Skin Test is classified as an immune response test, with the core component being purified protein derivative (PPD). After intradermal injection of PPD, if the individual has been exposed to tuberculosis bacteria, T lymphocytes in the body release cytokines, causing a localized inflammatory response at the injection site. The size of the induration correlates with the strength of immune memory, with the measurement at 72 hours being the primary basis for interpretation.
This test's mechanism is closely related to Type IV hypersensitivity (delayed-type hypersensitivity), allowing differentiation between natural infection and vaccination. Compared to blood tests, it does not require complex equipment but necessitates the patient to return for observation within 24 to 72 hours.
This test is mainly suitable for high-risk groups with a history of tuberculosis exposure, such as healthcare workers, contacts of infected individuals, or immunocompromised patients (e.g., HIV-infected). Additionally, it is commonly used in routine screenings for school entry, military enlistment, or occupational health examinations.
Specific populations, such as contacts of tuberculosis patients, residents in high-incidence areas, or patients receiving biological therapies, may require periodic testing to monitor infection status. However, it is important to note that this test cannot distinguish latent infection from active tuberculosis disease and should be combined with other diagnostic methods for confirmation.
The procedure involves three steps: first, injecting 0.1 ml of PPD solution containing 5 tuberculin units intradermally on the forearm to form a small blister; second, an assessment by healthcare professionals 48 to 72 hours later; third, measuring the induration diameter in millimeters and interpreting the result based on risk factors.
The injection site should be free from irritants, and other vaccines or skin tests should be avoided during the testing period. If redness or swelling exceeds 10 mm after injection, further imaging or molecular biological tests may be necessary to confirm the diagnosis.
This test has a sensitivity of approximately 85-90% for detecting tuberculosis contact history and can effectively identify individuals with an immune response. Compared to blood tests, its interpretation is less affected by certain immunomodulatory treatments, making it suitable for patients on specific medications.
The vast majority of individuals will only experience local redness and swelling at the injection site, but rarely, allergic shock may occur. Therefore, observation for 15 minutes after the first injection is recommended. If blisters, ulceration, or severe swelling develop at the site, immediate medical attention is required.
Potential risks include:
Contraindications include severe immunodeficiency, acute infections, or allergy to tuberculin components. Pregnant or breastfeeding women should consult a physician before proceeding. Those vaccinated with BCG within the past three months should delay testing for 4 to 6 weeks to avoid interference with results.
Before testing, patients should inform their healthcare provider of recent exposure history, medication use, and allergies. If the patient is on steroids or other immunosuppressants, the response may be weakened, and this should be noted in the report.
Concurrent use with biological agents (such as TNF-α inhibitors) may suppress the test response, requiring adjusted interpretation. For patients undergoing immunotherapy, it is recommended to use interferon-gamma release assays (IGRAs) for improved accuracy.
There are no direct interactions with tuberculosis treatment drugs, but results should be interpreted in conjunction with clinical symptoms in patients on anti-tuberculosis therapy, as treatment may affect immune response strength.
Clinical studies show that PPD testing has a sensitivity of about 70-90% for screening contacts, with a specificity exceeding 90%. In resource-limited areas, results can serve as a key basis for initiating preventive therapy.
Interferon-gamma release assays (T-SPOT.TB or QuantiFERON-TB Gold) are the main alternatives, with advantages in distinguishing BCG vaccination from natural infection and not being affected by subjective skin reaction interpretation.
Blood tests, although highly accurate, require specialized equipment and laboratory facilities, are more costly, and do not provide immediate results. The choice between the two depends on patient characteristics and available healthcare resources.
It is recommended to avoid other vaccinations or non-steroidal anti-inflammatory drugs (NSAIDs) within 48 to 72 hours before the skin test to prevent interference with the results. Additionally, avoid scratching or rubbing the injection site and keep it clean to prevent infection.
Is redness or itching after the skin test normal? How can I relieve it?Mild redness and itching are common allergic reactions and are considered normal. If symptoms are severe (such as blisters or intense swelling), seek medical attention immediately. To alleviate discomfort, avoid scratching the area, apply cold compresses, or use antihistamines as advised by a healthcare provider.
Can I resume normal activities or bathing after the skin test?Generally, normal bathing and exercise are permitted after the test, but avoid vigorous rubbing of the injection site. Use lukewarm water for gentle cleaning and keep the area dry for 24 hours to reduce infection risk.
If the test is positive, how often should follow-up examinations be conducted?Positive individuals usually require chest X-rays and sputum tests to confirm active tuberculosis. Follow-up frequency depends on individual risk; high-risk groups (such as immunocompromised) may need monitoring every 3 to 6 months, while contacts are typically checked annually.
Does a negative result completely exclude tuberculosis infection?A negative result may be due to the immune system not mounting a sufficient response (e.g., 1-3 months after initial infection) or absence of infection. If clinical suspicion remains high, repeat testing or use interferon-gamma release assays (IGRAs) for increased accuracy may be recommended.
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