Latent tuberculosis (latent TB) is a condition where TB bacteria live inside your body but are held in check by your immune system, causing no symptoms and no ability to spread to others. About a quarter of the world’s population, roughly 2 billion people, carry the TB bacterium in this dormant state. Most will never get sick from it, but without treatment, 5% to 10% of people with latent TB will eventually develop active TB disease at some point in their lives.
How TB Bacteria Stay Dormant
When you breathe in TB bacteria, your immune system responds by walling them off inside tiny clusters of immune cells called granulomas. Think of a granuloma as a biological containment unit. Inside it, the bacteria encounter harsh conditions: low oxygen, limited nutrients, acidic surroundings, and chemical stress from immune defenses. In response, the bacteria essentially shift into survival mode. They stop multiplying but remain alive, running a stripped-down genetic program that lets them persist without actively growing.
This standoff between the immune system and the bacteria is what defines latent TB. The bacteria are trapped, so they can’t reach your airways and be coughed out. That’s why latent TB is not contagious. Active TB, by contrast, develops when granulomas break down and bacteria spill into the lungs, where they can multiply freely and be expelled into the air.
Latent TB vs. Active TB
The most important difference is simple: latent TB produces no symptoms at all. You feel completely healthy. Active TB disease, on the other hand, typically causes persistent cough (sometimes with blood), fever, night sweats, weight loss, and fatigue. A person with latent TB has a positive result on a TB blood test or skin test, but a normal chest X-ray and no detectable bacteria in sputum samples. Someone with active TB will usually also test positive on those same screening tests, but their chest X-ray often shows lung damage, and lab cultures can identify live bacteria.
You cannot give latent TB to anyone. You don’t need to be isolated or take special precautions around other people. The infection only becomes a public health concern if it progresses to active disease.
Who Should Be Screened
Not everyone needs to be tested for latent TB. The U.S. Preventive Services Task Force recommends screening for people at increased risk, which includes:
- People born in or who previously lived in countries with high TB rates
- People who live or have lived in congregate settings like homeless shelters or correctional facilities
- People with weakened immune systems, including those living with HIV or taking immunosuppressive medications
- Healthcare workers and others with occupational exposure to TB
- Close contacts of someone with active TB disease
How Latent TB Is Diagnosed
Two types of tests detect latent TB, and both work by measuring your immune system’s response to TB proteins rather than finding the bacteria directly.
The TB skin test (sometimes called a Mantoux test) involves injecting a small amount of protein under the skin of your forearm. You return 48 to 72 hours later so a healthcare provider can check for a raised bump at the injection site, which indicates your immune system recognizes TB bacteria. The TB blood test (called an IGRA) requires a single blood draw, with results typically available within 24 hours.
The CDC encourages providers to use the blood test when possible. It has several practical advantages: it requires only one visit, results aren’t affected by reader interpretation, and it doesn’t produce false positives in people who received the BCG vaccine. The BCG vaccine is a TB vaccine given routinely in many countries outside the United States, and it can cause the skin test to react even when no real TB infection exists. If you were vaccinated against TB as a child, a blood test gives a more reliable answer.
Risk of Progressing to Active TB
For most people with latent TB, the immune system keeps bacteria contained for life. The overall lifetime risk of progression is 5% to 10%, but that risk isn’t evenly distributed. Several factors raise the odds significantly:
- HIV: The single biggest risk factor, because HIV directly weakens the immune cells responsible for containing TB
- Recent infection: People infected within the past two years are at higher risk than those who were infected long ago
- Young children: Kids under five have less mature immune defenses
- Immunosuppressive therapy: This includes medications used after organ transplants, certain cancer treatments, and drugs that block inflammatory pathways
- Chronic conditions: Diabetes, chronic kidney failure, silicosis, and certain cancers of the head, neck, or lungs all increase risk
- Low body weight: Being under 90% of ideal body weight is an independent risk factor
The risk is highest in the first two years after exposure, then decreases but never fully disappears. This is why treatment is recommended even for people who feel perfectly fine.
How Latent TB Is Treated
Treatment for latent TB is preventive. The goal is to kill dormant bacteria before they ever have a chance to reactivate. The CDC now favors shorter treatment courses over the older approach of taking a single medication for six to nine months.
The most common current options are a once-weekly two-drug combination taken for three months (12 total doses), or a single daily medication taken for four months. A three-month daily two-drug regimen is also available. These shorter courses have largely replaced the older six- or nine-month regimens because people are more likely to finish a shorter treatment, and completion is what matters. An incomplete course leaves surviving bacteria behind and wastes the effort.
The older regimen of daily medication for six or nine months is still used when shorter options aren’t suitable, such as when drug interactions are a concern or when the bacteria are suspected to be resistant to one of the medications in the shorter regimens.
Side Effects and Liver Safety
The primary safety concern during latent TB treatment is liver injury. One of the most commonly used medications can cause inflammation in the liver, which is usually mild but in rare cases becomes severe. The warning signs to watch for are unusual fatigue, nausea, vomiting, abdominal pain, or yellowing of the skin or eyes. These symptoms can start subtly, sometimes without obvious jaundice at first.
If you develop any of these symptoms during treatment, you should stop taking the medication immediately, even before you can get a clinical evaluation. Your provider may order baseline liver function tests before starting treatment, particularly if you have HIV, chronic liver disease, are pregnant or recently postpartum, or drink alcohol regularly. During treatment, periodic blood work may be done to monitor liver enzymes, especially if your baseline levels were abnormal or you have other risk factors for liver problems.
Most people tolerate treatment well and finish without significant side effects. The benefit of completing treatment, eliminating the lifelong risk of developing active TB, outweighs the temporary inconvenience for the vast majority of people with latent infection.