TB Skin Test and Exposure to an Infected Person
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 TB skin test 8 to 10 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 TB skin test. If your reaction to the second test is negative, you probably do not have
latent tuberculosis.
TB Skin Test and the BCG Vaccine
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 TB is common. BCG vaccine does not always protect people from getting TB.
If you were vaccinated with BCG, you may have a positive reaction to a TB skin test. 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 TB 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).
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.
If you have active tuberculosis, you will need to take medicine to cure the disease. If you have latent TB infection (a positive TB skin test reaction), and you are in a high-risk group
(see Causes of Tuberculosis), you need to take
tuberculosis medications to keep from developing active tuberculosis.
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