The tuberculin skin test (TST), also known as the Mantoux test, is a diagnostic tool used to detect
tuberculosis (TB) infection. As a nursing professional, understanding the administration, interpretation, and implications of the TST is crucial for effective patient care and management of TB.
What is the Tuberculin Skin Test?
The TST is a simple and widely used method to determine if an individual has been exposed to Mycobacterium tuberculosis. This test involves the intradermal injection of purified protein derivative (
PPD) into the forearm. The immune response to the PPD is measured in millimeters of induration (swelling) at the site of injection, typically 48-72 hours after administration.
How is the Tuberculin Skin Test Administered?
The TST is administered by injecting 0.1 mL of PPD into the inner surface of the forearm using a
syringe with a fine gauge needle. It is important to ensure the injection is intradermal, forming a small, pale elevation of the skin known as a wheal. Nursing professionals must ensure the patient returns for a reading within the prescribed time frame.
How is the Test Read?
The test is read by measuring the diameter of the induration, not the redness, at the injection site. Measurements are taken transverse to the forearm, and results are recorded in millimeters. A ruler or caliper can be used for accuracy. Proper training is essential for nurses to accurately interpret and document the results.
What Do the Results Mean?
Interpretation of the TST results depends on the size of the induration and the individual's risk factors for TB. Common thresholds for a positive test include:
5 mm or more: Considered positive in individuals with HIV infection, recent contacts of TB cases, or those with fibrotic changes on chest X-ray consistent with prior TB.
10 mm or more: Positive in recent immigrants from high-prevalence countries, injection drug users, residents and employees of high-risk settings, and children under 4 years old.
15 mm or more: Considered positive in individuals with no known risk factors for TB.
Positive test results indicate TB exposure but do not confirm active TB disease. Further evaluation, including a chest X-ray and sputum test, is necessary to confirm active TB.
What are the Limitations of the TST?
The TST has limitations that nursing professionals should be aware of. False positive results can occur in individuals who have received the
BCG vaccination or have been exposed to non-tuberculous mycobacteria. False negative results may occur in individuals with weakened immune systems, recent TB infection, or very young children. Nursing professionals must consider these factors when interpreting test results.
Ensuring proper
intradermal injection technique to avoid false results.
Educating patients about the need to return for reading and the importance of follow-up testing if necessary.
Documenting the test site, size of induration, and any patient reactions comprehensively.
Recognizing and managing adverse reactions, such as allergic responses.
What are the Alternatives to the TST?
In certain situations, alternative tests like the
Interferon Gamma Release Assays (IGRAs) may be used. IGRAs, such as the QuantiFERON-TB Gold test, measure the immune response to TB antigens in blood samples and are not affected by BCG vaccination status. Nursing professionals should understand when these alternatives are appropriate, especially in patients with a history of BCG vaccination or in populations where follow-up for TST reading is challenging.
Conclusion
The tuberculin skin test remains a crucial tool in the identification of TB infection. As nursing professionals, understanding the administration, interpretation, and limitations of the TST enables effective patient education and management. By staying informed about the latest guidelines and developments, nurses can ensure the highest standard of care in TB prevention and control.
