Questions and Answers About TB, 2007
Return to Questions and Answers
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;
- 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
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.
Gold (QFT-G) is a blood test used to find out if you are infected
with TB bacteria. The QFT-G measures the response to TB proteins when
they are mixed with a small amount of blood. Currently, few health
departments offer the QFT-G. If your health department does offer
the QFT-G, only one visit is required, at which time your blood is
drawn for the test.
Back to Top of Page
What if I have a positive test for TB?
If you have a positive reaction to the TB skin test or the QFT-G,
your doctor or nurse may do other tests to see if you have active
TB disease. These tests usually include a chest x-ray.
It may also include 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 treat 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
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);
- You spend time where TB disease is common
(homeless shelters, migrant farm camps, drug-treatment centers,
health care clinics, jails, prisons).
Back to Top of Page
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-G) 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
- 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.
Back to Top of Page
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
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 treat the disease.
Active TB disease can be treated, even in people
with HIV infection.
Last Modified: 06/26/2008
Last Reviewed: 05/18/2008
Content Source: Division of Tuberculosis Elimination
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention