how to know if tb skin test is positive

3 min read 17-12-2024

Tuberculosis (TB) is a serious bacterial infection that primarily affects the lungs. A Tuberculin Skin Test (TST), also known as a Mantoux test or PPD test, is a common way to screen for TB infection. But how do you know if your TB skin test is positive? Understanding the results is crucial for appropriate medical follow-up. This article explains how to interpret your TB skin test results and what steps to take next.

Understanding the TB Skin Test

The TST involves injecting a small amount of tuberculin (a protein derived from the TB bacteria) just under the skin of your forearm. You'll return 48-72 hours later for a reading. It's not a test for active TB disease itself, but rather for evidence of exposure to the bacteria and the body's immune response.

How to Read a TB Skin Test: Determining a Positive Result

A nurse or doctor will measure the area of induration (hardness and swelling) at the injection site, not the redness (erythema). This measurement is crucial in determining if the test is positive. The size of the induration that indicates a positive result depends on several factors:

Your risk factors: People with higher risk factors for TB (e.g., recent close contact with someone with active TB, weakened immune system, recent immigration from a high-TB-prevalence country) may be considered positive with a smaller induration size.
Your age: The interpretation may differ slightly based on age.

General Guidelines (Consult your doctor for definitive interpretation):

Positive: An induration of 15 millimeters (mm) or greater is generally considered positive in most individuals with no known risk factors.
Positive (Higher Risk): Individuals with known risk factors might be considered positive with an induration size of 10 mm or greater.
Positive (Very High Risk): Individuals with very high risk factors (e.g., HIV infection, recent organ transplant) may be considered positive with an induration of 5 mm or greater.

Important Note: Redness alone is not a sign of a positive test. The measurement is based solely on the raised, hardened area (induration).

What to Do if Your TB Skin Test is Positive

A positive TB skin test does not mean you have active TB disease. It indicates you've been exposed to the bacteria and your immune system has reacted. You'll need further testing to determine if you have latent TB infection (LTBI) or active TB disease.

This usually involves a chest X-ray and possibly other tests, such as a sputum culture (to check for the presence of TB bacteria in your cough). Your doctor will discuss the appropriate next steps and treatment options based on your individual circumstances.

Latent TB Infection (LTBI) vs. Active TB Disease

Latent TB infection (LTBI): The bacteria are present in your body, but they are not actively causing illness. It's usually treatable with medication to prevent it from progressing to active TB.
Active TB disease: The bacteria are actively multiplying and causing illness. It requires more extensive treatment with multiple antibiotics.

What if My TB Skin Test is Negative?

A negative TB skin test generally means you haven't been infected with TB bacteria, or if you were infected, your immune system didn't mount a detectable response. However, it's important to remember that a negative test doesn't completely rule out infection. It’s always best to discuss the results with your physician.

Frequently Asked Questions (FAQs)

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

A: You'll need to return 48-72 hours after the injection for the test to be read.

Q: Can I get a false positive or negative TB skin test?

A: Yes, false positives and negatives are possible, although relatively rare. Factors such as prior BCG vaccination (a TB vaccine) can affect the results.

Q: What is the treatment for latent TB infection?

A: Treatment typically involves taking antibiotics for several months. Your doctor will determine the best course of treatment for you.

Q: How long does the induration from a TB skin test last?

A: The induration will usually subside within a few weeks.

Conclusion:

A TB skin test is an important screening tool, but it's only one part of the diagnostic process. If your test is positive, don't panic. Consult your doctor immediately for further evaluation and appropriate treatment. Understanding your test results and taking the necessary steps is critical in preventing the spread of tuberculosis and protecting your health. Remember, early diagnosis and treatment are key.