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TB Skin Test

How Exposure to an Infected Person Affects the Test

If you have recently spent time with and been exposed to someone with active tuberculosis, your TB skin test reaction may not be positive yet. You may need a second test eight to ten weeks after the last time you spent time with the person. This is because it can take several weeks after infection for your immune system to react to the skin test. If your reaction to the second test for TB is negative, you probably do not have latent tuberculosis.
 

The BCG Vaccine and the TB Skin Test

BCG is a vaccine for TB. This vaccine is not widely used in the United States, but it is often given to infants and small children in other countries where the disease is common. BCG vaccine does not always protect people from getting the disease.
 
If you were vaccinated with BCG, you may have a positive reaction to a skin test for TB. This reaction may be due to the BCG vaccine itself or due to infection with the TB bacteria (Mycobacterium tuberculosis). Your positive reaction probably means you have been infected with the bacteria if:
 
  • You recently spent time with a person who has active TB
     
  • You are from an area of the world where active TB is very common (such as most countries in Latin America and the Caribbean, Africa, Asia, Eastern Europe, and Russia)
     
  • You spend time in places where TB is common (homeless shelters, migrant farm camps, drug treatment centers, healthcare clinics, jails, or prisons).
     

What If My Skin Test for TB Is Positive?

If you have a positive reaction to the TB skin test, your doctor or nurse may do other tests to see if you have active tuberculosis (see Tuberculosis Diagnosis for more information).
 
If you have active tuberculosis, you will need to take medicine to cure the disease. If you have latent TB infection (a positive reaction on the skin test) and you are in a high-risk group (see Causes of Tuberculosis), you need to take medications to keep from developing active tuberculosis.
 

Tuberculosis Disease

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