what is a positive tb test look like

3 min read 17-12-2024

Tuberculosis (TB) is a serious bacterial infection that primarily affects the lungs. Early detection is crucial for successful treatment. This article explains what a positive TB test looks like, covering both the traditional skin test (TST) and the newer blood test (IGRA). Remember, a positive test result doesn't automatically mean you have active TB; further testing is always necessary.

Understanding the Tuberculin Skin Test (TST)

The TST, also known as the Mantoux test, is the older method for screening for TB infection. It involves injecting a small amount of tuberculin (a protein from the TB bacteria) just under the skin of your forearm.

What a Positive TST Looks Like:

You'll need to return 48-72 hours after the injection for a reading. A healthcare professional will measure the area of induration (hardened, raised area) at the injection site, not the redness (erythema).

Positive Result: A positive TST is generally defined as an area of induration of 15 mm or more in diameter in most people. However, the threshold can be lower (≥5 mm or ≥10 mm) depending on your risk factors (see below). The area will feel firm and raised to the touch. It might be slightly red, but the size of the raised area is the key factor.
Indeterminate Result: Sometimes, the reaction is neither clearly positive nor negative. This requires further testing and evaluation.
Negative Result: A negative TST shows minimal or no induration.

Factors Affecting TST Interpretation:

Risk Factors: Individuals with certain risk factors (recent contact with someone with TB, weakened immune system, certain medical conditions) may be considered positive with a smaller induration size.
Previous BCG Vaccination: Bacille Calmette-Guérin (BCG) vaccination, common in many countries, can cause a positive TST, even in the absence of TB infection. This makes interpretation more complex.

Understanding Interferon-Gamma Release Assays (IGRAs)

IGRAs, like QuantiFERON-TB Gold and T-SPOT.TB, are blood tests that detect your immune system's response to TB bacteria. They are considered a more specific test than the TST, especially in people who have received the BCG vaccine.

What a Positive IGRA Looks Like:

The results are usually available within 24 hours. A positive result means your blood sample shows a significant immune response to TB bacteria. The exact interpretation depends on the specific test used, but the lab will provide a clear positive or negative result.

Positive Result: Indicates that you have been exposed to TB bacteria at some point in your life. It doesn't differentiate between latent TB infection and active TB disease.
Negative Result: Suggests you haven't been exposed to TB bacteria.
Indeterminate Result: Similar to the TST, an indeterminate IGRA requires additional testing.

What Happens After a Positive TB Test?

A positive TST or IGRA does not mean you have active TB disease. It indicates you've been exposed to the bacteria and may have latent TB infection (LTBI). LTBI means the bacteria are present in your body but are not actively causing illness. However, it can progress to active TB if left untreated.

Your healthcare provider will likely order further tests, such as a chest X-ray and possibly sputum cultures, to determine if you have active TB disease. If you have LTBI, they may recommend treatment to prevent the infection from becoming active.

Conclusion: Seek Professional Medical Advice

This information provides a general overview of what a positive TB test looks like. It is essential to consult with a healthcare professional for accurate interpretation of your test results and appropriate medical advice. Early detection and treatment of TB are critical for preventing serious health consequences. Don't attempt self-diagnosis based solely on the visual appearance of a skin test.