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how does positive tb test look like

how does positive tb test look like

2 min read 31-12-2024
how does positive tb test look like

How Does a Positive TB Test Look Like? Understanding Tuberculin Skin Tests and Interferon-Gamma Release Assays

A positive tuberculosis (TB) test doesn't look like one specific thing; it depends on the type of test administered. There are two main types of TB tests: the Tuberculin Skin Test (TST) and the Interferon-Gamma Release Assay (IGRA). Let's examine how a positive result manifests for each.

Understanding the Tuberculin Skin Test (TST) – The Mantoux Test

The TST, also known as the Mantoux test, is the more traditional method. A healthcare professional injects a small amount of tuberculin (a protein from the TB bacteria) just under the skin of your forearm. You return 48-72 hours later for a reading.

What a Positive TST Looks Like:

A positive TST isn't simply the presence of any reaction. The size of the raised, hard area (induration) is what matters. The interpretation depends on several factors, including:

  • Your Risk Factors: People with higher risk factors (e.g., close contact with someone with TB, weakened immune system, recent immigration from a high-TB-prevalence country) will have a lower threshold for a positive result.
  • Your Age: Children and older adults may have different interpretation guidelines.

Generally, a positive TST is defined as an induration of:

  • ≥5 mm: In individuals with high-risk factors (as listed above).
  • ≥10 mm: In individuals with moderate risk factors (e.g., recent immigrants from TB endemic areas, injection drug users, residents or employees of high-risk congregate settings).
  • ≥15 mm: In individuals with low risk factors (meaning they have no known risk factors).

Important Note: Only the induration (the hardened area, not redness) is measured. Redness alone does not indicate a positive result. A healthcare professional will measure the induration in millimeters using a clear ruler.

Interferon-Gamma Release Assays (IGRAs) – Blood Tests for TB

IGRAs, such as QuantiFERON-TB Gold and T-SPOT.TB, are blood tests that detect your immune system's response to TB bacteria. These tests are becoming increasingly common as they avoid some of the limitations of the TST.

What a Positive IGRA Looks Like:

A positive IGRA result is reported numerically, indicating the level of interferon-gamma (a protein produced by your immune system in response to TB). The laboratory will provide a clear positive or negative result. There's no visible reaction to interpret as with the TST. The threshold for positivity varies between tests and labs but is typically defined by the manufacturer.

What to Do if You Have a Positive TB Test Result

A positive TB test does not automatically mean you have active TB disease. It indicates that you have been exposed to the TB bacteria and have developed an immune response. You will need further testing, such as a chest X-ray and possibly sputum cultures, to determine if you have latent TB infection (LTBI) or active TB disease. Your healthcare provider will explain the next steps and treatment options.

False Positives and False Negatives

It's crucial to understand that both TST and IGRA tests can have false positives (positive result despite no infection) and false negatives (negative result despite infection). Factors such as recent vaccination with the BCG vaccine (used in many countries to prevent TB) can affect the results. Your doctor will consider these factors when interpreting your test results.

In Conclusion

Determining whether a TB test is positive depends on the type of test and individual risk factors. A healthcare professional will interpret the results and recommend further testing and treatment as necessary. Don't hesitate to ask your doctor any questions you have about your TB test results and their implications. Early diagnosis and treatment are vital in preventing the spread of TB.

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