Latent tuberculosis (latent TB) is a condition where TB bacteria live inside your body but are held in check by your immune system. You don’t feel sick, you have no symptoms, and you can’t spread TB to anyone else. About a quarter of the world’s population carries TB bacteria this way. The critical fact: without treatment, 5% to 10% of people with latent TB will eventually develop active TB disease at some point in their lives.
What Happens Inside Your Body
When you breathe in TB bacteria, your immune system doesn’t always kill them outright. Instead, immune cells surround the bacteria and wall them off in tiny structures called granulomas. Think of a granuloma as a microscopic containment zone: layers of immune cells form an organized barrier, sometimes reinforced with scar-like tissue, that keeps the bacteria trapped.
Inside these granulomas, conditions are harsh for the bacteria. The environment is low in oxygen, acidic, and flooded with immune signals. In response, the bacteria shift into a dormant state, dramatically slowing their metabolism. They’re alive but barely active, essentially hibernating. This standoff between immune system and bacteria can last decades. As long as the granulomas hold, you remain healthy and non-infectious.
How Latent TB Differs From Active TB
The distinction is straightforward. With latent TB, you feel completely normal. There are no coughs, no fevers, no weight loss, and no way to pass the infection to others. A chest X-ray typically looks normal. The bacteria are contained.
Active TB disease is the opposite. The bacteria have broken free from immune containment and are multiplying. This usually causes persistent cough (often lasting three weeks or more), fever, night sweats, fatigue, and unintentional weight loss. People with active TB disease in the lungs can spread bacteria to others through the air when they cough or speak. Active TB requires a longer, more intensive course of treatment than latent TB.
What Makes Latent TB Reactivate
Reactivation happens when the immune system can no longer maintain those granuloma walls. The biggest driver is anything that weakens your immune defenses. HIV infection is the most significant risk factor because it directly attacks the immune cells responsible for keeping TB bacteria contained.
Certain medications also raise the risk substantially. Anti-TNF drugs, used to treat conditions like rheumatoid arthritis and inflammatory bowel disease, are considered high-risk because they interfere with the very immune signaling that holds granulomas together. High-dose corticosteroids (roughly equivalent to 15 mg or more of prednisone daily for at least a month) also carry significant risk. Calcineurin inhibitors, commonly used after organ transplants, further increase susceptibility.
Beyond medications, several health conditions raise reactivation risk: diabetes, severe kidney disease, certain cancers (particularly of the head, neck, or lung), silicosis, and low body weight. A recent TB infection (within the past two years) also carries higher reactivation risk compared to an infection acquired long ago. For people with none of these risk factors, the lifetime risk stays in that 5% to 10% range, with most reactivation occurring in the first two years after initial infection.
How Latent TB Is Detected
Since latent TB causes no symptoms, it’s found only through testing. Two types of tests are available: the tuberculin skin test and blood tests.
Tuberculin Skin Test
A small amount of protein derived from TB bacteria is injected just under the skin of your forearm. You return to a healthcare provider 48 to 72 hours later so they can measure any raised bump (not redness, but the firm swelling) at the injection site. What counts as a “positive” result depends on your risk level:
- 5 millimeters or more: positive for people with HIV, organ transplant recipients, those on immune-suppressing medications, recent close contacts of someone with active TB, or people with chest X-ray findings suggesting prior TB
- 10 millimeters or more: positive for people born in countries where TB is common, people who live or work in high-risk settings (nursing homes, shelters, correctional facilities), people with diabetes or kidney disease, children under 5, and those who use injection drugs
- 15 millimeters or more: positive for people with no known risk factors
The thresholds are lower for higher-risk groups because missing a diagnosis in someone likely to reactivate carries greater consequences.
Blood Tests
Blood-based tests, known as interferon-gamma release assays, measure how your immune cells react to TB proteins in a lab setting. These require only a single visit and aren’t affected by prior TB vaccination (BCG), which can cause false positives on the skin test. The two main versions have sensitivity ranging from about 81% to 90%, with specificity above 95%. That means they’re good at correctly identifying people who do and don’t have latent TB, though no test is perfect.
Who Should Be Tested
Testing is recommended for people at increased risk of either getting infected or progressing to active disease. This includes people born in or who frequently travel to countries with high TB rates, such as Mexico, the Philippines, Vietnam, India, China, Haiti, and Guatemala. People who live or have lived in group settings like homeless shelters, prisons, or nursing homes should also be tested, along with healthcare workers in those environments.
Anyone who has spent time with a person who has infectious TB disease needs testing. So do people with immune-suppressing conditions or medications, particularly before starting anti-TNF therapy or after an organ transplant. Children under 5 who fall into any risk group are a priority because young children progress from latent to active disease more quickly than adults.
Treatment Options and What to Expect
Treating latent TB eliminates the bacteria while they’re dormant, removing the risk of future reactivation. The CDC recommends shorter treatment courses over the older approach of taking isoniazid alone for six to nine months. The preferred regimens now are rifamycin-based combinations lasting three to four months.
One common regimen combines two medications taken daily for three months, totaling about 90 doses. Shorter courses have a practical advantage: people are far more likely to finish them. Completing the full course matters because stopping early leaves surviving bacteria that could still reactivate.
Side Effects of Treatment
The most important side effect to know about is liver irritation. As many as 10% to 20% of people taking isoniazid develop mildly elevated liver enzymes during treatment, though levels usually return to normal on their own. In rare cases, more serious liver inflammation (hepatitis) can develop. Signs to watch for include yellowing of the skin or eyes, brown-colored urine, persistent nausea, stomach pain, and unexplained fatigue lasting more than a few days.
Some people also experience peripheral neuropathy, a tingling or numbness in the hands and feet caused by the medication affecting sensory nerves. Other possible reactions include dizziness, loss of appetite, joint pain, rash, and flu-like symptoms. If you notice any of these, stopping the medication and contacting your provider promptly is important, as most side effects resolve once the drug is discontinued.
Your provider will typically monitor liver function with blood tests during treatment, especially if you’re over 35, drink alcohol regularly, or have pre-existing liver conditions. For most people, treatment is straightforward and completed without significant problems.