Frequently Asked Questions
- How Tuberculosis spread?
- What is TB infection?
- What is TB active disease?
- What are the symptoms?
- What is a TB skin test?
- What does a positive TB test mean?
- What does a negative skin test mean?
- What does a TB blood test mean?
- Could BCG vaccine affect TB skin test results?
1. How Tuberculosis spread?
Tuberculosis (TB) is an infectious disease that spreads from person to person through the air when a person with active TB disease of the lungs or throat (include) coughs, or sneezes. TB primarily affects the lungs but can also affect other organs.Top of Section
2. What is TB infection?
TB bacteria can live in the body without making you sick. This is called latent TB infection. In most people who breathe in TB bacteria and become infected, the body is able to fight the bacteria to stop them from growing. People with latent TB infection do not feel sick and do not have any symptoms. People with latent TB infection are not infectious and cannot spread TB bacteria to others. However, if TB bacteria become active in the body and multiply, the person will go from having latent TB infection to being sick with TB disease.Top of Section
3. What is TB active disease?
TB bacteria become active if the immune system can't stop them from growing. When TB bacteria are active (multiplying in your body), this is called TB disease. People with TB disease are sick. They may also be able to spread the bacteria to people they spend time with every day.
If TB disease is not treated, it can cause serious illness or even death. There is effective medical treatment for both latent TB infection and active TB disease.Top of Section
4. What are the symptoms?
Symptoms of TB disease include fever, coughing, fatigue, weight loss, diminished appetite, or coughing up blood. TB can take six to nine months to cure, longer if the strain is resistant to drugs.Top of Section
5. What is a TB skin test?
The TB skin test is a safe method to detect if you have been infected with TB. It can be given to anyone who needs it but must be read by a trained health care worker within 48 to 72 hours of administering the test. It has no adverse effects and can be given during pregnancy.Top of Section
6. What does a positive TB test mean?
A positive TB test means that someone has been exposed to the TB germ and follow up evaluation is needed. Once a positive TB test has been confirmed to be truly positive, there is no point in having repeat TB tests in the future. They will not hurt you, or cause disease, but will always continue to be positive.Top of Section
7. What does a negative skin test mean?
A negative skin test means that the person’s body did not react to the test, and that latent TB infection or TB disease is not likely.Top of Section
8. What does a TB blood test mean?
TB blood tests (also called interferon-gamma release assays or IGRAs) measure how the immune system reacts to the bacteria that cause TB. An IGRA measures how strong a person’s immune system reacts to TB bacteria by testing the person’s blood in a laboratory.
Two IGRAs are approved by the U.S. Food and Drug Administration (FDA) and are available in the United States:
- QuantiFERON®–TB Gold In-Tube test (QFT-GIT)
- T-SPOT®.TB test (T-Spot)
A positive IGRA means that the person has been infected with TB bacteria. Additional tests are needed to determine if the person has latent TB infection or TB disease. A health care worker will then provide treatment as needed.
A Negative IGRA means that the person’s blood did not react to the test and that latent TB infection or TB disease is not likely.
IGRAs are the preferred method of TB infection testing for the following:
- People who have received bacille Calmette–Guérin (BCG). BCG is a vaccine for TB disease.
- People who have a difficult time returning for a second appointment to look for a reaction to the TST.
9. Could BCG vaccine affect TB skin test results?
Bacille Calmette-Guérin (BCG) is a vaccine for tuberculosis (TB) disease. 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 does not always protect people from getting TB.
People who were previously vaccinated with BCG may receive a TB Skin Test to test for TB infection. Vaccination with BCG may cause a positive reaction to a TB skin test. A positive reaction to a TB skin test may be due to the BCG vaccine itself or due to infection with TB bacteria.
A positive reaction to a TB skin test probably means you have been infected with TB bacteria if:
You recently spent time with a person who has TB disease.
You are from an area of the world where TB disease is very common (such as most countries in Latin America and the Caribbean, Africa, Asia, Eastern Europe, and Russia.)
You spend time where TB disease is common (such as homeless shelters, migrant farm camps, drug-treatment centers, health care clinics, jails, or prisons).
TB Blood tests (IGRAs) unlike the TB skin test, are not affected by prior BCG vaccination and are not expected to give a false-positive result in people who have received BCG.
For children under the age of five, the TB skin test is preferred over TB blood tests.
A positive TB skin test or TB blood test only tells that a person has been infected with TB bacteria. It does not tell whether the person has latent TB infection or has progressed to TB disease. Other tests, such as a chest x-ray and a sample of sputum, are needed to see whether the person has TB disease.Top of Section