Latent vs. Active TB: What Are the Differences?

The typical symptoms of active tuberculosis (TB) affect the lungs and throat and include chronic cough, sputum (thick, colored phlegm), and blood while coughing, among others. However, most people exposed—approximately nine in 10—have latent TB, meaning they aren’t symptomatic or contagious.

Though closely related, there are key differences between active and latent TB. This article discusses these distinctions and provides an overview of their symptoms, causes, and treatment, as well as what you can do to prevent exposure.  

Symptoms

Though both latent and active TB arise from exposure to bacteria, there are key differences in how these conditions present. Active TB causes symptoms and can spread, while latent TB doesn’t cause any physical signs or symptoms and is not contagious.

Causes

Like other airborne diseases, such as the common cold or influenza (flu), TB is transmitted via infected droplets exhaled by a symptomatic person. Expelled when they cough, talk, sneeze, or sing, the bacteria access the lungs when others inhale these drops. The infecting bacteria is Mycobacterium tuberculosis.

Given this means of transmission, your chances of exposure to TB increase when you spend significant periods of time close to a symptomatic person. For example, cases often spread among family members, classmates, co-workers, and friends.

Latent TB

Most people exposed to TB can successfully contain and control the infection. The key difference between active and latent TB hinges on the immune system and its ability to prevent the infection from spreading. Put simply, those with stronger immunity are better able to stop the disease from becoming serious; however, they will still have small amounts of the bacteria in their systems.  

Active TB

Roughly 5–10% of those exposed to TB bacteria develop active forms of the disease. Several risk factors that make some more susceptible to developing symptoms include:

• Young age, especially infants and children
• Older age (over 65 years old)
• Human immunodeficiency virus (HIV/AIDS)
• Organ transplants
• Late-stage kidney disease
• Blood cancers (leukemia, lymphoma, and multiple myeloma, among others)
• Use of immunosuppressive drugs or corticosteroids

Diagnosis

During diagnosis, healthcare providers want to ensure symptoms are the result of TB and not another condition and assess how much the disease has spread and developed. Several tests can help with diagnosis.

Latent TB Diagnosis

Latent TB is more difficult to diagnose because many people carry it without knowing it. Only two tests can detect both latent and active cases. These include:

• TB skin test: The TB skin test measures your skin’s reaction to an injection of tuberculin, a protein the infecting bacteria emit. Around 48–72 hours after the injection, your healthcare provider will measure the raised, hardened bump around the injection site. A more robust reaction indicates the presence of TB.
• TB blood test: The immune system releases antigens into the blood when fighting infection. Two blood tests, the QuantiFERON-TB Gold Plus and the T-SPOT.TB, can detect these. Blood samples are collected during an office visit and sent to a clinical laboratory for testing.  

Active TB Diagnosis

Additional tests and evaluations that can detect active TB include:

• Medical evaluation: Healthcare providers will assess your medical history, medications, and symptoms and look for signs of other conditions.
• Chest X-ray: Through imaging, healthcare providers can detect the presence of lung lesions, which may indicate infection. However, a chest X-ray alone will not confirm the diagnosis, and additional testing is necessary.
• Sputum smear: Clinical laboratory assessments of sputum (a viscous buildup in the lungs) samples detect the presence of infectious bacteria.
• Bacteria culture test: To ensure symptoms result from TB and not another bacterial infection, the sputum sample will also undergo culture evaluation. This testing determines the specific type of bacteria causing symptoms.

Treatment

Three- to nine-month antibiotic drug regimens treat latent and active TB.

Latent TB Treatments

Though people with latent TB don’t have symptoms and aren’t contagious, treatment is still necessary to prevent it from becoming active. Weight, age, and other health factors determine specific dosages, with typical approaches including:

• Rifampin (RIF) is taken daily for four months, with specific dosages varying based on age and weight.
• Isoniazid (INH) is taken for six to nine months, with doses taken either daily or twice a week.
• INH and rifapentine (RPT) therapies are taken once a week for three months.  
• INH and RIF are taken daily over a three-month course.

Short-course three- to four-month rifamycin-based treatment is preferable to six- to nine-month isoniazid therapy.

Active TB treatments

As with latent TB, prolonged courses of antibiotic therapy treat active cases. Primarily, there are two combination therapies, with specific dosages shifting throughout treatment. These include:

• RPT-moxifloxacin (MOX) therapy: This therapy involves daily doses of RPT, supplemented with several other antibiotics, MOX, INH, and pyrazinamide (PZA), for an intensive phase of eight weeks. A continuation phase of daily RPT, MOX, and INH follows this for an additional nine weeks.
• Six- to nine-month RIPE treatment: A combination therapy of RIF, INH, PZA, and ethambutol (EMB) taken daily, five days a week, or in three-day-a-week doses for eight weeks. A continuation phase of four to seven months follows this, often with reductions in frequency.

Prevention

Because TB is an airborne disease transmitted by close contact with an infected person, prevention involves limiting such contact. Strategies to avoid catching or spreading TB include:

• Ensuring good ventilation in your home or workplace
• Isolating if you have symptoms or ensuring that a symptomatic person is isolated
• Promptly screening for TB when exposed to an infected person
• Testing for TB following travel in or immigration from regions with higher rates, such as countries in Southeast Asia, Eastern Europe, South America, and others
• Regularly testing if you’re a healthcare worker or an employee at a nursing home, correctional facility, or similar environment
• Taking medications to treat latent TB if detected

Summary

Latent and active tuberculosis (TB) are two forms of bacterial infection, from Mycobacterium tuberculous. Most exposed people have latent TB, meaning they are without symptoms and not contagious.

Active TB occurs when bacteria are multiplying in the body, leading to chronic cough, blood or sputum while coughing, chills, and other symptoms. These conditions are treated with prolonged courses of antibiotic therapy. Timely diagnosis is essential in their management.