What is a Tuberculin Skin Test?
A tuberculin skin test (TST), also known as the Mantoux test, is a diagnostic tool used to detect tuberculosis (TB) infection. It involves the intradermal injection of a purified protein derivative (PPD) into the skin, typically on the forearm. The test measures the immune response to the PPD, which includes swelling and redness at the injection site.
Why is the Tuberculin Skin Test Important?
The TST is crucial in identifying individuals who have been exposed to Mycobacterium tuberculosis, the bacterium that causes TB. Early identification allows for timely intervention, reducing the risk of transmission and progression from latent TB infection to active TB disease.
Who Should Be Tested?
Healthcare professionals recommend TST for individuals at high risk of TB, including:
1. Those with close contact with TB patients.
2. Individuals with weakened immune systems.
3. Persons from regions with high TB prevalence.
4. Residents and employees of high-risk settings, such as correctional facilities and nursing homes.
5. Healthcare workers who might be exposed to TB through their work.
How is the Test Administered?
A nurse will administer the TST by injecting a small amount of PPD into the skin of the forearm. The injection should create a small, raised area called a wheal. The patient is then instructed to return within 48 to 72 hours for the test to be read.
How is the Test Read?
The nurse will assess the injection site for induration, which is a raised, hardened area. Redness alone is not considered. The induration is measured in millimeters (mm) using a ruler. Based on the size of the induration and the patient’s risk factors, the test is interpreted as positive or negative.
What Do the Results Mean?
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Negative Result: Typically, an induration of less than 5 mm is considered negative. This indicates that the person likely does not have TB infection.
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Positive Result: The size of induration that signifies a positive result varies based on risk factors:
- ≥5 mm: Positive for high-risk individuals (e.g., HIV-infected persons).
- ≥10 mm: Positive for moderate-risk individuals (e.g., recent immigrants from high-prevalence countries).
- ≥15 mm: Positive for individuals with no known risk factors.
What are the Limitations?
The TST has several limitations:
1.
False Positives: These can occur in individuals who have received the Bacillus Calmette-Guérin (BCG) vaccine.
2.
False Negatives: These can result from recent TB infection, very young or old age, or immunosuppression.
3.
Need for Two Visits: Patients must return for reading, which can be inconvenient.
What are the Alternatives?
An alternative to the TST is the
Interferon-Gamma Release Assay (IGRA), a blood test that measures the immune response to TB proteins. This test does not require a return visit and is not affected by prior BCG vaccination.
1. Patient Education: Inform patients about the purpose of the test, the need to return for reading, and possible side effects.
2. Proper Technique: Ensure correct injection technique to produce an accurate wheal.
3. Accurate Measurement: Use a ruler to measure induration accurately and document results.
4. Follow-Up: Provide guidance based on test results, including further evaluation for positive tests and preventive measures for negative tests.
Conclusion
Tuberculin skin tests are a vital tool in the detection and management of TB. Nurses must be proficient in administering and interpreting these tests, educating patients, and understanding the implications of test results. By doing so, they contribute significantly to TB control and prevention efforts.
