Not everyone infected with tuberculosis (TB) bacteria gets sick, so there are two conditions: latent TB infection (also called inactive TB) and TB disease. People with latent infection carry TB bacteria but don't have the disease — yet if the bacteria become active and multiply, it can progress to TB disease. Treating latent infection substantially lowers that risk.
Latent TB infection at a glance
| People with latent TB infection… | |
|---|---|
| have a small number of live but inactive TB bacteria | cannot spread TB to others |
| don't feel sick, but may become sick if the bacteria activate | usually test positive on a TB blood or skin test |
| typically have a normal chest X-ray | have negative sputum smears and cultures |
| should consider treatment to prevent TB disease | do not need respiratory isolation |
Why it matters
- CDC estimates up to 13 million people in the United States have latent TB infection.
- Without treatment, about 5%–10% will develop TB disease during their lives.
- Progression from untreated latent infection causes about 80% of U.S. TB cases, so finding and treating it is essential to eliminating TB.
- TB disease must be reported nationally, but latent infection isn't reported to CDC; some states and localities require it.
How infection happens
TB infection begins when a person inhales TB bacteria that reach the alveoli of the lungs. Most are destroyed or held in check, but a few may enter the bloodstream and spread to places where TB disease is most likely to develop — the lymph nodes, lungs, kidneys, brain and bone. Within 2 to 8 weeks, immune cells called macrophages surround the bacteria and form a walled-off granuloma that keeps them contained. That contained state is latent TB infection.
Who should be tested
CDC and the U.S. Preventive Services Task Force recommend testing people at higher risk, as a routine part of their health care; how often depends on risk, from once to every year.
| Higher risk of exposure | Higher risk of disease once infected |
|---|---|
| Contacts of people with infectious TB | People with HIV |
| People born in or who often travel to countries where TB is common | Children under 5 |
| Current or former residents of homeless shelters, correctional facilities or nursing homes | People infected within the last 2 years |
| Employees of high-risk group settings | People with untreated or poorly treated past TB |
| Health care workers who treat TB patients | People on immune-suppressing drugs — TNF blockers, corticosteroids at or above 15 mg of prednisone a day, or transplant drugs |
| Locally defined high-incidence groups, which may include underserved or low-income people or those with substance use disorders | People with silicosis, chronic kidney failure, leukemia, or head, neck or lung cancer |
| Children and teens exposed to high-risk adults | People with diabetes, a gastrectomy or jejunoileal bypass, or low body weight (under 90% of ideal) |
| People who use substances such as injected drugs |
Tests
| TB blood test (IGRA) | TB skin test (TST) | |
|---|---|---|
| How it works | Blood is mixed with TB-like peptides; infected people's white cells release interferon-gamma, which is measured | A small dose of PPD tuberculin is injected into the skin of the forearm; a trained worker measures the firm swelling after 48–72 hours |
| Products | FDA-approved QuantiFERON-TB Gold Plus and T-SPOT.TB | PPD, the only FDA-approved solution for this method |
| Preferred for | People who got the BCG vaccine (which can cause false-positive skin tests) and people unlikely to return for a skin-test reading | Children under 5, though some experts use blood tests in younger children |
Either test is acceptable; providers are encouraged to use blood tests, with the skin test as an alternative when a blood test isn't available, affordable or practical. Don't routinely use both, and don't retest people with documented past positive results or TB treatment.
A positive result means TB infection. More tests, such as a chest X-ray, are needed to rule out TB disease. Latent infection is diagnosed from the medical history, the test, the chest X-ray, a physical exam and sometimes sputum tests — and TB disease must be ruled out before treating latent infection, or treatment could fail and breed drug resistance.
Treatment
Treating latent TB infection is 90% effective at preventing TB disease. CDC and the National Tuberculosis Coalition of America preferentially recommend short, rifamycin-based regimens, which are effective, safe and more often completed:
| Regimen | What it is |
|---|---|
| 3HP | 3 months of once-weekly isoniazid plus rifapentine |
| 4R | 4 months of daily rifampin |
| 3HR | 3 months of daily isoniazid plus rifampin |
| 6H / 9H (alternatives) | 6 to 9 months of isoniazid alone — effective, but more serious toxicity and lower completion |
Choose a regimen based on the drug susceptibility of the likely source case, other medical conditions and possible drug interactions. If the source may have drug-resistant TB, or a patient needs an alternative for allergies or interactions, consult a TB expert; state and local TB programs can also help.
Sources
Based on "Clinical Overview of Latent Tuberculosis Infection," Centers for Disease Control and Prevention; a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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