From the CDC website:

Types of Testing

There are two kinds of tests that are used to detect TB bacteria in the body: the TB skin test (TST) and 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 (LTBI) 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.

Diagnosis

If a person is found to be infected with TB bacteria, other tests are needed to see if the person has latent TB infection or TB disease.

Who Should be Tested

Certain people should be tested for TB infection because they are at higher risk of being infected with TB bacteria, including:

• People who have spent time with someone who has TB disease
• 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 in high-risk settings (for example correctional facilities, long-term care facilities or nursing homes, and homeless shelters)
• Health-care workers who care for patients at increased risk for TB disease
• Infants, children, and adolescents exposed to adults who are at increased risk for latent tuberculosis infection or TB disease

Many people who have latent TB infection never develop TB disease. But some people who have latent TB infection are more likely to develop TB disease than others. Those at high risk for developing TB disease 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

TB tests are generally not needed for people with a low risk of infection with TB bacteria.

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