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Testing for Tuberculosis (TB)

National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention 

Division of Tuberculosis Elimination

Tuberculosis (TB) is a disease that is spread through the air from one person to another. When someone who is sick 

with TB coughs, speaks, laughs, sings, or sneezes, people nearby may breathe TB bacteria into their lungs. TB usually 

attacks the lungs, but can also attack other parts of the body, such as the brain, spine, or kidneys. 

There are two types of TB: 

TB disease. People with TB disease can spread the 

bacteria to others, feel sick, and can have symptoms 

including fever, night sweats, cough, and weight loss. 

There are two kinds of tests that are used to determine 

if a person has been infected with TB bacteria: the 

tuberculin skin test and TB blood tests.

TB bacteria can live in 

the body without 

making a person sick. 

This is called latent 

TB infection. People 

with latent TB infection do not feel sick, do not have TB 

symptoms, and cannot spread TB bacteria to others. 

Some people with latent TB infection go on to develop

1. Latent TB infection

2. TB disease

Tuberculin Skin Test (TST)

What is a TST?

The Mantoux tuberculin skin test is a test to check if a 

person has been infected with TB bacteria.

How does the TST work?

Using a small needle, a health care provider injects a 

liquid (called tuberculin) into the skin of the lower part of 

the arm. When injected, a small, pale bump will appear. 

This is different from a Bacille Calmette-Guerin (BCG) 

shot (a TB vaccine that many people living outside of the 

United States receive). 

The person given the TST must return within 2 or 3 days 

to have a trained health care worker look for a reaction 

on the arm where the liquid was injected. The health 

care worker will look for a raised, hard area or swelling, 

and if present, measure its size using a ruler. Redness by 

itself is not considered part of the reaction.

What does a positive TST result mean?

The TST result depends on the size of the raised, hard 

area or swelling. It also depends on the person’s risk of 

being infected with TB bacteria and the progression to 

TB disease if infected. 

• Positive TST: This means the person’s body was 

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.

• Negative TST: This means the person’s body did not 

react to the test, and that latent TB infection or TB 

disease is not likely.

Who can receive a TST?

Almost everyone can receive a TST, including infants, 

children, pregnant women, people living with HIV, and 

people who have had a BCG shot. People who had a 

severe reaction to a previous TST should not receive 

another TST. 

How often can a TST be given?

Usually, there is no problem with repeated TSTs unless a 

person has had a severe reaction to a previous TST.

Testing for TB in People with 

a BCG

People who have had a previous BCG shot may receive a 

TST. In some people, the BCG shot may cause a positive 

TST when they are not infected with TB bacteria. If a TST 

is positive, additional tests are needed.
TB Blood Tests

What is an Interferon Gamma Release 

Assay (IGRA)? 

An IGRA is a blood test that can determine if a person 

has been infected with TB bacteria. 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:

1) QuantiFERON®-TB Gold In-Tube test 

 (QFT-GIT)

2) T-SPOT®.TB test (T-Spot)

How does the IGRA work?

Blood is collected into special tubes using a needle. The 

blood is delivered to a laboratory as directed by the 

IGRA test instructions. The laboratory runs the test and 

reports the results to the health care provider. 

What does a positive IGRA result mean?

• Positive IGRA: This 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. 

• Negative IGRA: This means that the person’s blood did 

not react to the test and that latent TB infection or TB 

disease is not likely.

Who can receive an IGRA?

Anyone can have an IGRA in place of a TST. This can 

be for any situation where a TST is recommended. In 

general, a person should have either a TST or an IGRA, 

but not both. There are rare exceptions when results 

from both tests may be useful in deciding whether a 

person has been infected with TB.

IGRAs are the preferred method of TB infection testing 

for the following:

• People who have received the BCG shot

• People who have a difficult time returning for a 

second appointment to look at the TST after the test 

was given

How often can an IGRA be given?

There is no problem with repeated IGRAs.

Who Should Get Tested for TB?

TB tests are generally not needed for people 

with a low risk of infection with TB bacteria. 

Certain people should be tested for TB bacteria because 

they are more likely to get TB disease, including: 

• People who have spent time with someone who has 

TB disease

• People with HIV infection or another medical problem 

that weakens the immune system 

• People who have symptoms of TB disease (fever, night 

sweats, cough, and weight loss)

• People from a country where TB disease is common 

(most countries in Latin America, the Caribbean, 

Africa, Asia, Eastern Europe, and Russia)

• People who live or work somewhere in the United 

States where TB disease is more common (homeless 

shelters, prison or jails, or some nursing homes)

• People who use illegal drugs

Choosing a TB Test

Choosing which TB test to use should be done by the 

person’s health care provider. Factors in selecting which 

test to use include the reason for testing, test availability, 

and cost. Generally, it is not recommended to test a 

person with both a TST and an IGRA.

Diagnosis of Latent TB Infection or TB 

Disease

If a person is found to be infected with TB bacteria, other 

tests are needed to see if the person has TB disease.

TB disease can be diagnosed by medical history, 

physical examination, chest x-ray, and other laboratory 

tests. TB disease is treated by taking several drugs as 

recommended by a health care provider. 

If a person does not have TB disease, but has TB bacteria 

in the body, then latent TB infection is diagnosed. The 

decision about taking treatment for latent TB infection 

will be based on a person’s chances of developing TB 

disease.

Related Links

CDC. Tuberculosis (TB): http://www.cdc.gov/tb 

Basic TB Information: http://www.cdc.gov/tb/

publications/factsheets/general/tb.htm 

November 2011