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Questions and Answers About Tuberculosis (TB)

2005

Latent TB Infection

How can I get tested for TB?

You should get tested for TB if

  • You have spent time with a person known to have active TB disease or suspected to have active TB disease; or
  • You have HIV infection or another condition that puts you at high risk for active TB disease; or
  • You think you might have active TB disease; or  
  • You are from a country where active TB disease is very common (most countries in Latin America and the Caribbean, Africa, Asia, Eastern Europe, and Russia); or
  • You live somewhere in the United States that active TB disease is more common such as a homeless shelter, migrant farm camp, prison or jail, and some nursing homes); or
  • You inject illegal drugs.

The TB skin test

The TB skin test may be used to find out if you have TB infection. You can get a skin test at the health department or at your doctor's office. A health care worker will inject a small amount of testing fluid (called tuberculin or PPD) just under the skin on the under side of the forearm.  After 2 or 3 days, you must return to have your skin test read by the health care worker. You may have a swelling where the tuberculin was injected. The health care worker will measure this swelling and tell you if your reaction to the test is positive or negative. A positive reaction usually means that you have been infected by someone with active TB disease.

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If you have recently spent time with and been exposed to someone with active TB disease, your TB skin test reaction may not be positive yet. You may need a second 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 TB infection.

QuantiFERON®-TB Gold

QuantiFERON®-TB Gold (QFT) is a blood test used to find out if you are infected with TB bacteria. The QFT measures the response to TB proteins when they are mixed with a small amount of blood. Currently, few health departments offer the QFT. If your health department does offer the QFT, only one visit is required, at which time your blood is drawn for the test.

What if I have a positive test for TB?

If you have a positive reaction to the TB skin test or the QFT, your doctor or nurse may do other tests to see if you have active TB disease. These tests usually include a chest x-ray and a test of the phlegm you cough up. Because the TB bacteria may be found somewhere other than your lungs, your doctor or nurse may check your blood or urine, or do other tests. If you have active TB disease, you will need to take medicine to cure the disease.

What if I have been vaccinated with BCG?

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. Your positive reaction probably means you have been infected with TB bacteria if

  • You recently spent time with a person who has active TB disease; or
  • You are from an area of the world where active TB disease is very common (such as most countries in Latin America and the Caribbean, Africa, Asia, Eastern Europe, and Russia); or
  • You spend time where TB disease is common (homeless shelters, migrant farm camps, drug-treatment centers, health care clinics, jails, prisons).

If I have latent TB infection, how can I keep from developing active TB disease?

Many people who have latent TB infection never develop active TB disease. But some people who have latent TB infection are more likely to develop active TB disease than others. These people are at high risk for active TB disease. They include

  • people with HIV infection
  • people who became infected with TB bacteria in the last 2 years
  • babies and young children
  • people who inject illegal drugs
  • people who are sick with other diseases that weaken the immune system
  • elderly people
  • people who were not treated correctly for TB in the past

If you have latent TB infection (a positive TB skin test reaction or positive QFT) and you are in one of these high-risk groups, you need to take medicine to keep from developing active TB disease. This is called treatment for latent TB infection. There are several treatment options. You and your health care provider must decide which treatment is best for you.

The medicine usually taken for the treatment of latent TB infection is called isoniazid (INH). INH kills the TB bacteria that are in the body. If you take your medicine as instructed by your doctor or nurse, it can keep you from developing active TB disease. Children and people with HIV infection may need to take INH for a longer time.

Because there are less bacteria in a person with latent TB infection, treatment is much easier. Usually, only one drug is needed to treat latent TB infection. A person with active TB disease has a large amount of TB bacteria in the body. Several drugs are needed to treat active TB disease.

Sometimes people are given treatment for latent TB infection even if their skin test reaction is not positive. This is often done with infants, children, and HIV-infected people who have recently spent time with someone with active TB disease. This is because they are at very high risk of developing active TB disease soon after they become infected with TB bacteria.

It is important that you take all the pills as prescribed. If you start taking INH, you will need to see your doctor or nurse on a regular schedule. He or she will check on how you are doing. Some people have serious side effects from INH. If you have any of the following side effects, call your doctor or nurse right away:

  • no appetite
  • nausea
  • vomiting
  • yellowish skin or eyes
  • fever for 3 or more days
  • abdominal pain
  • tingling in the fingers and toes

Warning: Drinking alcoholic beverages (wine, beer, and liquor) while taking INH can be dangerous. Check with your doctor or nurse for more information.

People who have latent TB infection need to know the symptoms of active TB disease. If they develop symptoms of active TB disease, they should see a doctor right away.

What if I have HIV infection?

A person can have latent TB infection for years.  But if that person's immune system gets weak, the infection can quickly turn into active TB disease. Also, if a person who has a weak immune system spends time with someone with active TB disease, he or she may become infected with TB bacteria and quickly develop active TB disease.

Because HIV infection weakens the immune system, people with latent TB infection and HIV infection are at very high risk of developing active TB disease. All persons with HIV infection should be tested to find out if they have latent TB infection. If they have latent TB infection, they need treatment as soon as possible to prevent them from developing active TB disease. If they have active TB disease, they must take medicine to cure the disease.


Active TB disease can be prevented and cured, even in people with HIV infection.





  
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This article was provided by U.S. Centers for Disease Control and Prevention. Visit the CDC's website to find out more about their activities, publications and services.
 

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