Not everyone infected with TB bacteria becomes sick. As a result, two TB-related conditions exist: latent TB infection (also called inactive TB) and TB disease.
CDC estimates that up to 13 million people in the United States have latent TB infection. While not everyone with latent TB infection will develop TB disease, without treatment about 5%–10% of infected people will develop TB disease over their lifetimes.
Progression from untreated latent TB infection to TB disease accounts for approximately 80% of U.S. TB cases.
Finding and treating people with latent TB infection is essential for controlling and eliminating TB disease in the United States. Treatment for latent TB infection is effective for preventing TB disease.
For Everyone: About Inactive TuberculosisTB infection occurs when a person inhales tubercle bacilli that reach the alveoli of the lungs.
While most bacilli are destroyed or inhibited, a small number may enter the bloodstream and spread throughout the body. The tubercle bacilli may reach any part of the body, including areas where TB disease is most likely to develop such as the:
Within 2 to 8 weeks, special immune cells called macrophages ingest and surround the tubercle bacilli. The cells form a barrier shell called a granuloma that keeps the bacilli contained and under control. This condition is known as latent TB infection.
CDC and the U.S. Preventive Services Task Force recommend testing people at increased risk for TB infection. Testing for TB infection should be a routine and integral part of health care for patients with increased risk for TB. Frequency of testing will depend on a person's risk for TB.
People at risk for TB fall into two broad categories:
People with latent TB infection cannot spread TB bacteria to others.
However, if these bacteria become active and multiply, latent TB infection can develop into TB disease. Once active, TB can be spread from person to person through the air.
Latent TB infection prevalence data is critical in order to track the United States’ progress in testing and treating persons with latent TB infection. CDC estimates that up to 13 million people in the United States have latent TB infection.
TB disease is a nationally notifiable disease; however, latent TB infection is not reported to CDC. Some states and localities have developed legal reporting requirements for latent TB infection as a tool to prevent TB disease.
CDC currently relies on national prevalence estimates and is exploring systems and methods to determine the best ways to measure prevalence of latent TB infection in the United States.
Keep Reading: Latent TB Infection in the United States - Published EstimatesTesting for TB infection is a routine and integral part of health care for patients with increased risk for TB. Frequency of testing depends on a person's risk factors. This could range from one-time only testing among persons at low risk for future TB exposure to annual testing among those at continued risk of exposure.
There are two types of tests that can determine if a person has been infected with TB bacteria:
TB blood tests (sometimes called IGRAs) measure the immune response to TB proteins in whole blood to determine if a person is infected with TB bacteria.
For TB blood tests, specimens are mixed with peptides that simulate antigens derived from TB bacteria and with controls. In most people infected with TB bacteria, the white blood cells recognize the simulated antigens and release interferon-gamma (IFN-γ). The tests measure the level of IFN-γ response.
The U.S. Food and Drug Administration (FDA) has approved these two TB blood tests that are commercially available in the United States:
The TB skin test is also called the Mantoux tuberculin skin test (TST) or PPD. With a TB skin test, a health care provider injects a small, premeasured amount of sterile tuberculin PPD solution into the skin on the palm side of the forearm. This is sometimes called the Mantoux TST method. PPD stands for purified protein derivative of tuberculin solution, which comes in a single standard concentration. It is the only kind of TB skin test solution that is FDA-approved for this test method.
After 48–72 hours, the skin test reaction must be examined by a trained health care worker. The health care worker measures the induration (firm swelling) where the tuberculin was injected to determine if the reaction to the test is positive or negative.
Keep Reading: Clinical Testing Guidance for Tuberculosis: Tuberculin Skin TestUsing either a TB blood test or a TB skin test is acceptable medical and public health practice.
Health care providers are encouraged to use newer TB blood tests to detect latent TB infection. TB skin tests are an acceptable alternative in situations where a TB blood test is not available, is too costly, or is too burdensome.
The bacille Calmette-Guérin (BCG) TB vaccine can cause a false positive TB skin test result. Unlike the TB skin test, TB blood tests are not affected by BCG vaccination.
Routine testing with both TB blood and skin tests is not recommended.
Consider these factors when determining which test to use.
TB blood tests or skin tests should not be performed on people who have written documentation of a previous positive TB test results (TB blood test or TB skin test) or treatment for TB disease.
A positive reaction to a TB blood test or TB skin test is interpreted as TB infection. More tests, such as a chest radiograph, are needed to rule out TB disease.
A diagnosis of latent TB infection is made if a person has a positive TB blood test or TB skin test result and a medical exam does not indicate TB disease. The diagnosis is based on information gathered from:
TB disease must be excluded before initiating treatment for latent TB infection. Failure to do so may result in treatment failure and development of drug resistance.
Keep Reading: Diagnosing TuberculosisPeople with latent TB infection should be treated to prevent the development of TB disease. Progression from untreated latent TB infection to TB disease accounts for approximately 80% of U.S. TB disease cases. Treating latent TB infection is 90% effective in preventing the development of TB disease.
There are several standard treatment regimens for the treatment of latent TB infection. Regimens use the drugs isoniazid (H), rifapentine (P), and/or rifampin (R).
CDC and the National Tuberculosis Coalition of America preferentially recommend short-course, rifamycin-based, 3- or 4-month latent TB infection treatment regimens over 6- or 9-month isoniazid monotherapy. Short-course latent TB infection treatments are effective, safe, and have higher completion rates than treatment regimens.
