The Tuberculin Skin Test (TST), also known as the Mantoux test, is a widely used diagnostic tool for detecting tuberculosis (TB) infection. A positive result indicates that a person has been infected with Mycobacterium tuberculosis, the bacteria that causes TB. However, a positive result does not necessarily mean that the person has active TB disease. In this article, we will explain the significance of a positive TST result, the factors that influence the test outcome, and the next steps for individuals with a positive result.
The TST involves injecting a small amount of tuberculin, a substance derived from TB bacteria, into the skin. The skin's reaction to the tuberculin is measured after 48 to 72 hours. A positive result is indicated by a certain level of skin induration, or swelling, which is measured in millimeters. The interpretation of the result depends on various factors, including the individual's immune status, age, and history of TB exposure.
Interpreting a Positive TST Result
A positive TST result can be classified into different categories based on the level of skin induration. The Centers for Disease Control and Prevention (CDC) provide guidelines for interpreting TST results, which are as follows:
- For individuals with no risk factors, a result of 15 mm or more is considered positive.
- For individuals with certain risk factors, such as recent exposure to TB, a result of 10 mm or more is considered positive.
- For individuals with compromised immune systems, such as those with HIV/AIDS, a result of 5 mm or more is considered positive.
Factors Influencing TST Results
Several factors can influence the outcome of a TST, including:
| Factor | Description |
|---|---|
| Immune Status | Individuals with weakened immune systems, such as those with HIV/AIDS or taking immunosuppressive medications, may have a false-negative result. |
| Age | Children under 5 years old may have a false-negative result due to immature immune systems. |
| TB Exposure | Individuals with recent exposure to TB may have a positive result even if they do not have active disease. |
| Vaccination | BCG vaccination, which is commonly used in countries with high TB prevalence, can cause a false-positive result. |
Next Steps for Individuals with a Positive TST Result
Individuals with a positive TST result should undergo further evaluation to determine if they have active TB disease or latent TB infection (LTBI). The evaluation may include:
Chest Radiograph
A chest X-ray is used to assess for signs of active TB disease, such as lung lesions or cavities.
Sputum Smear Microscopy
A sputum sample is examined for the presence of TB bacteria to diagnose active disease.
IGRA (Interferon-Gamma Release Assay)
An IGRA is a blood test that measures the immune response to TB bacteria and can help confirm LTBI.
Treatment
Individuals with active TB disease require prompt treatment with antibiotics. Those with LTBI may be offered preventive therapy to reduce the risk of progressing to active disease.
Key Points
- A positive TST result indicates TB infection but does not confirm active disease.
- The interpretation of TST results depends on individual risk factors and immune status.
- Further evaluation, including chest radiograph and sputum smear microscopy, is necessary to diagnose active TB disease.
- Individuals with LTBI may benefit from preventive therapy to reduce the risk of disease progression.
- Healthcare providers should consider the individual's overall clinical picture when interpreting TST results and developing a treatment plan.
Conclusion
In conclusion, a positive TST result is an important indicator of TB infection, but it requires careful interpretation and further evaluation to determine the individual's disease status. By understanding the factors that influence TST results and the next steps for individuals with a positive result, healthcare providers can provide effective care and prevent the progression of TB disease.
What does a positive TST result mean?
+A positive TST result indicates that a person has been infected with Mycobacterium tuberculosis, but it does not confirm active TB disease.
Can a positive TST result be false?
+Yes, a positive TST result can be false due to various factors, such as BCG vaccination or cross-reactivity with other mycobacteria.
What are the next steps after a positive TST result?
+Further evaluation, including chest radiograph, sputum smear microscopy, and IGRA, is necessary to determine if the individual has active TB disease or latent TB infection.