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what is a tb test supposed to look like

what is a tb test supposed to look like

3 min read 30-12-2024
what is a tb test supposed to look like

A tuberculin skin test (TST), also known as a Mantoux test or PPD test, is used to screen for tuberculosis (TB) infection. Understanding what a positive and negative reaction looks like is crucial for interpreting the results. This article will guide you through the expected appearance of a TB test, both positive and negative, and discuss what to do next.

Understanding the TB Test Procedure

Before we dive into the appearance of the test, let's briefly recap the procedure. A small amount of tuberculin solution is injected just under the skin of your forearm. The injection site should be marked with a pen. You will return 48-72 hours later for a reading.

Close-up image of a TB skin test injection site

Image caption: Example image of a TB skin test injection site. (Replace with actual image and ensure it is compressed).

What a Negative TB Test Looks Like

A negative TB test typically shows very little to no reaction at the injection site. You might see a very small, slightly raised bump, but it should be less than 5mm in diameter. There should be no redness or induration (hardening of the skin). This indicates you haven't been infected with TB bacteria.

  • Less than 5mm in diameter: This is considered a negative result regardless of any redness.

What a Positive TB Test Looks Like

A positive TB test result shows a significant reaction at the injection site. This is measured as induration (hardening of the skin), not just redness. The induration is measured across the area of hardened skin, perpendicular to the long axis of the forearm. The size of the induration determines the interpretation of the test. The following are guidelines, but interpretation might vary based on individual risk factors and physician judgment.

  • Greater than or equal to 15mm: This is typically considered positive for any individual.
  • 10-14mm: This may be positive in individuals with certain risk factors, such as those with weakened immune systems, recent contact with someone with TB, or those who live in areas with a high prevalence of TB.
  • 5-9mm: This may be considered positive for those at very high risk, including individuals with HIV, organ transplant recipients, or those who have undergone recent immunosuppressive therapy.
Image showing a positive TB skin test reaction

Image caption: Example image of a positive TB skin test reaction, clearly showing induration. (Replace with actual image and ensure it is compressed).

Important Considerations

  • Only a healthcare professional can interpret the results. Do not attempt to self-diagnose based on the appearance of your test.
  • Redness alone is not sufficient for a positive result. It's the induration (hardness) that's measured.
  • False positives can occur. Factors like previous BCG vaccinations (a TB vaccine common in other parts of the world) can influence the results.
  • A positive test does not mean you have active TB. It means you have been infected with the bacteria. Further testing, such as a chest X-ray and sputum culture, is needed to determine if the infection is active.

What to Do After Your TB Test

Regardless of the result, schedule a follow-up appointment with your doctor to discuss the results. They will interpret your test and advise you on the next steps. If the test is positive, you will likely need further testing to determine if you have active TB. If active TB is present, appropriate treatment will be recommended.

Frequently Asked Questions

How long does it take for a TB test to show results?

You need to return to your healthcare provider 48-72 hours after the injection for a reading.

What if I have a reaction but it’s not quite 15mm?

The size of the reaction isn't the only factor considered. Your risk factors will also be taken into account by your healthcare provider when interpreting your results.

Is a TB test painful?

The injection itself is usually only slightly uncomfortable, similar to a small needle prick.

Can I get a TB test at my doctor's office?

Yes, most doctors' offices and clinics offer TB skin testing. You may also be able to receive the test at local health departments.

This information is for educational purposes only and should not be considered medical advice. Always consult with a healthcare professional for any health concerns or before making any decisions related to your health or treatment.

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