Learn About Reading TB Test Results

Understanding TB Test Basics and Why Testing Matters

Tuberculosis (TB) is a serious infection caused by bacteria called Mycobacterium tuberculosis. According to the World Health Organization, approximately 10 million people develop TB each year worldwide, making it one of the leading infectious diseases globally. In the United States, the Centers for Disease Control and Prevention (CDC) reported about 7,352 TB cases in 2022, though this number continues to decline due to testing and treatment programs.

TB spreads through the air when an infected person coughs, sneezes, or speaks. Not everyone exposed to TB bacteria becomes sick. Some people develop active TB disease, while others develop latent TB infection, meaning they carry the bacteria but are not sick and cannot spread the disease to others. This distinction is crucial for understanding test results.

There are two main TB tests: the tuberculin skin test (TST), also called the Mantoux test, and TB blood tests. Healthcare providers recommend testing for people with symptoms of TB, those who have been exposed to someone with active TB, individuals with weakened immune systems, healthcare workers, and people in congregate settings like prisons or shelters. Testing helps identify who needs treatment to prevent serious illness.

Understanding your TB test results requires knowing what the test measures and what different outcomes mean. A positive result does not automatically mean you have active TB disease. A negative result generally means you do not have TB infection, though in certain circumstances, additional testing may be needed. The interpretation depends on your personal health history, risk factors, and which test you received.

Practical takeaway: Before your TB test, inform your healthcare provider about any recent TB exposure, symptoms like persistent cough or night sweats, or conditions that weaken immunity, as this information helps them interpret your results correctly.

The Tuberculin Skin Test (TST) and Reading Results

The tuberculin skin test, also known as the Mantoux test, has been used for over a century to detect TB infection. A healthcare worker injects a small amount of tuberculin purified protein derivative (PPD) just under the skin, usually on the forearm. The injection itself does not hurt significantly—most people describe feeling a slight pinch. The test does not inject TB bacteria; it uses a protein that triggers an immune response in people who have been infected with TB.

The critical part of the TST is the reading, which must occur 48 to 72 hours after injection. A healthcare provider measures the raised, hardened area (called induration) that forms at the injection site. The measurement is recorded in millimeters. The person who reads the test should be trained to measure correctly, as improper technique can lead to inaccurate results. The provider looks for a firm, raised bump—not just redness, which is less significant.

Test results are interpreted based on the size of the induration and the person's risk factors. A result of 5 millimeters or larger is considered positive in people with HIV, those with recent TB exposure, or those with findings on chest X-ray consistent with TB. An induration of 10 millimeters or larger is positive for recent immigrants, people who inject drugs, residents and employees of congregate settings, and healthcare workers. For other people without risk factors, an induration of 15 millimeters or larger is considered positive. Anything smaller is generally considered negative.

False positive and false negative results can occur with the TST. A false positive may happen in people vaccinated with BCG (bacillus Calmette-Guérin) vaccine, which is used in many countries outside the United States, or in people infected with certain nontuberculous mycobacteria. A false negative can occur in people with weakened immune systems, those tested too soon after infection, or very occasionally in people with active TB disease.

Practical takeaway: If you receive a TST, mark your calendar for exactly 48 to 72 hours later and keep your appointment to have it read, as results read outside this window may not be accurate.

TB Blood Tests and Interferon-Gamma Release Assays (IGRAs)

TB blood tests, also called interferon-gamma release assays (IGRAs), represent a newer method for detecting TB infection compared to the skin test. The most commonly used IGRAs in the United States are the QuantiFERON-Gold In-Tube (QFT-Gold) test and the QuantiFERON-TB Gold Plus. These tests measure the immune system's response to TB antigens in a blood sample. A healthcare provider draws blood into special tubes that contain TB antigens, and the sample is sent to a laboratory for analysis.

IGRAs have several advantages over skin tests. They require only one visit, produce results within 24 hours, and are not affected by BCG vaccination. They are also more specific, meaning they are less likely to produce false positives in vaccinated people. Many healthcare facilities have adopted IGRAs as their preferred screening method. However, IGRAs have limitations too—they can still produce false negatives in people with weakened immune systems, and they require a blood draw, which some people find uncomfortable.

IGRA results are reported as negative, positive, or indeterminate. A negative result indicates no TB infection was detected. A positive result suggests TB infection is present, but like the skin test, it does not distinguish between latent and active TB disease. An indeterminate result means the test did not work properly and needs to be repeated. Indeterminate results occur in about 1 to 3 percent of tests and can happen if there is insufficient immune response, which sometimes occurs in people with severely weakened immune systems.

The CDC recommends IGRAs for most populations because of their advantages, though skin tests are still acceptable. In some circumstances, healthcare providers may use both tests. For example, if someone has a positive skin test but a negative IGRA, the provider may order additional testing or chest imaging to determine if TB infection is truly present. Conversely, in people with weakened immunity, a negative test of any kind does not completely rule out TB infection.

Practical takeaway: If you receive an indeterminate IGRA result, ask your healthcare provider whether repeating the test is recommended, as some causes of indeterminate results (like recent vaccination) are temporary.

Distinguishing Between Latent TB Infection and Active TB Disease

One of the most important concepts in understanding TB test results is the difference between latent TB infection and active TB disease. A positive TB test indicates infection with the TB bacteria, but it does not tell you whether the bacteria are active or dormant. This distinction determines what medical follow-up you need and whether you can spread TB to others.

Latent TB infection means you have TB bacteria in your body, but your immune system has controlled the infection. You do not have symptoms, you do not feel sick, and you cannot spread TB to other people. Latent TB usually results from exposure to someone with active TB disease, often many years ago. Approximately 5 to 10 percent of people with latent TB infection will develop active TB disease at some point in their life, often when their immune system becomes weaker due to age, illness, or medical conditions like HIV.

Active TB disease means the TB bacteria are multiplying in your body and causing illness. Symptoms include a persistent cough lasting three weeks or longer, chest pain, coughing up blood or bloody sputum, fever, night sweats, chills, and loss of appetite or weight loss. Active TB most commonly affects the lungs (pulmonary TB), but it can affect other parts of the body like the lymph nodes, kidneys, bones, or brain (extrapulmonary TB). Active TB is contagious and requires treatment with multiple antibiotics for at least six months.

If you have a positive TB test, your healthcare provider will determine whether you have latent or active TB by reviewing your symptoms, performing a physical examination, and often ordering a chest X-ray. If you have no symptoms and a normal chest X-ray, you likely have latent TB infection. If you have symptoms or abnormal chest findings, you may have active TB disease, and further testing like sputum smear microscopy or TB culture may be ordered. Some people with latent TB infection are recommended to receive preventive treatment with medications to reduce their risk of developing active disease.

Practical takeaway: After receiving a positive TB test result, attend your follow-up appointment for chest imaging and symptom evaluation, as this determines whether you have latent infection or active disease