The Tuberculin Skin Test (TST), also known as the Mantoux test, is a diagnostic tool used to assess if a person has been infected with Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). The test involves injecting a small amount of tuberculin into the skin and measuring the reaction after 48 to 72 hours. Understanding how to read a TB skin test result is crucial for diagnosing latent TB infection and guiding further management.
Interpreting TB Skin Test Results
The interpretation of TST results is based on the size of the induration (the hardened area) and not the redness or swelling. The Centers for Disease Control and Prevention (CDC) provide guidelines for interpreting TST results, which vary depending on the individual's risk factors for TB.
Measurement and Interpretation
The TST result is measured in millimeters (mm) of induration. The measurement is taken 48 to 72 hours after the injection. The CDC guidelines recommend the following interpretations based on the induration size:
| Induration Size (mm) | Interpretation |
|---|---|
| 0-4 | Negative: No significant induration. |
| 5 or greater | Positive: Indicates infection with M. tuberculosis, though further evaluation is needed to determine if the infection is latent or active. |
Factors Influencing Interpretation
Several factors can influence the interpretation of TST results, including:
Vaccination with BCG
The Bacille Calmette-Guérin (BCG) vaccine, which is used in many countries to prevent TB, can cause a false-positive TST result. However, the BCG vaccine's effect on TST results wanes over time.
Previous TB Infection or Vaccination
Individuals who have previously been infected with TB or vaccinated with BCG may have a positive TST result, even if they do not have active TB.
Immune Status
People with weakened immune systems, such as those with HIV/AIDS, may have false-negative TST results due to their inability to mount an adequate immune response.
Key Points
- The TST is a critical tool for diagnosing latent TB infection.
- Interpretation of TST results depends on the size of the induration.
- CDC guidelines provide a framework for interpreting TST results based on induration size.
- Factors such as BCG vaccination, previous TB infection, and immune status can influence TST result interpretation.
- Further evaluation is needed for individuals with positive TST results to determine if the infection is latent or active.
What to Expect After the Test
After the TST, individuals should return to the testing site 48 to 72 hours later to have the test read. The healthcare provider will measure the induration and interpret the result based on the guidelines.
Follow-Up Care
For individuals with positive TST results, further evaluation, including chest X-rays and possibly sputum tests, may be necessary to determine if the TB infection is latent or active.
Preventive Therapy
Individuals with latent TB infection may be offered preventive therapy to reduce the risk of progressing to active TB disease.
What does a positive TB skin test result mean?
+A positive result indicates that you have been infected with M. tuberculosis. However, it does not differentiate between latent TB infection and active TB disease. Further evaluation is needed to determine the status of the infection.
Can a TB skin test result be false-positive?
+Yes, a TB skin test can result in a false-positive in individuals who have been vaccinated with BCG or who have previously been infected with TB.
What if my TB skin test result is negative?
+A negative result generally indicates that you are not infected with M. tuberculosis. However, in certain cases, such as in individuals with weakened immune systems, a false-negative result may occur. If you have been exposed to TB, your healthcare provider may recommend repeat testing or other diagnostic methods.
In conclusion, understanding how to read a TB skin test result is vital for the early detection and management of TB infection. By recognizing the implications of TST results and considering individual risk factors, healthcare providers can make informed decisions regarding further evaluation and treatment.
