Learn About TB Test Results and What They Mean

Understanding TB Tests: The Basics

Tuberculosis (TB) is an infection caused by bacteria called Mycobacterium tuberculosis. It spreads through the air when an infected person coughs, sneezes, or speaks. According to the World Health Organization, approximately 10 million people develop TB disease each year worldwide, and about 1.5 million die from it. In the United States, the Centers for Disease Control and Prevention (CDC) reports around 7,000 to 8,000 TB cases annually, though this number has been declining over recent decades.

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TB tests are medical tools used to determine whether someone has been infected with the TB bacteria. It's important to understand that having TB infection is different from having TB disease. TB infection means the bacteria is in your body but not making you sick. TB disease means the infection has progressed and you have symptoms and can spread the disease to others. Two main types of TB tests exist: the tuberculin skin test (TST), also called the Mantoux test, and TB blood tests, also called interferon-gamma release assays (IGRAs).

These tests don't directly show whether you have active TB disease. Instead, they indicate whether your immune system has been exposed to TB bacteria. Additional tests, like chest X-rays and sputum samples, are needed to confirm active TB disease. Understanding how these tests work and what results mean helps you and your healthcare provider take appropriate next steps.

Practical takeaway: Recognize that a positive TB test doesn't automatically mean you have TB disease. Further medical evaluation is always required to reach a diagnosis.

The Tuberculin Skin Test (TST) Explained

The tuberculin skin test, also known as the Mantoux test or purified protein derivative (PPD) test, has been used for over 100 years to screen for TB infection. A healthcare provider injects a small amount of purified protein derivative solution into the skin of your forearm, just below the surface. The injection creates a small, pale bump called a wheal. This procedure is quick and takes only a few seconds.

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The critical part happens 48 to 72 hours after the injection. You must return to the healthcare provider's office to have the injection site checked and measured. The provider looks for a raised, hardened bump called an induration and measures its width in millimeters using a ruler. Only the hardness and swelling count—redness alone doesn't determine the result. The size of the induration determines whether the test is negative, positive, or possibly positive depending on your risk factors.

The TST works by measuring your immune system's reaction to TB protein. If you have been infected with TB bacteria, your immune system recognizes the protein and causes swelling at the injection site. If you have never been exposed to TB, typically no swelling occurs. However, several factors can affect TST results. Some vaccines, particularly the BCG vaccine (Bacillus Calmette-Guérin) given in many countries outside the United States, can cause false-positive results. Previous TB infection, recent TB exposure, or weakened immunity can all influence how your body reacts to the test.

Practical takeaway: Mark your calendar for your TST follow-up appointment 48 to 72 hours after the injection. Missing this appointment means you'll need to get the test repeated, as results cannot be read after 72 hours.

Understanding TB Blood Test Results (IGRAs)

TB blood tests, known as interferon-gamma release assays, represent a newer approach to TB screening that was approved by the FDA in the early 2000s. These tests measure how a person's blood cells react when exposed to TB antigens in a laboratory setting. Common IGRA tests include the QuantiFERON-Gold In-Tube test, QuantiFERON-TB Gold Plus test, and the T-SPOT.TB test. Unlike the skin test, blood tests require only one office visit and provide results within 24 hours to a few days.

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IGRAs work differently from skin tests. A healthcare provider draws a small blood sample, which is then sent to a laboratory. The lab exposes the blood sample to TB antigens and measures whether immune cells called T-lymphocytes release a chemical called interferon-gamma (IFN-gamma). If your immune system has encountered TB bacteria before, these T-cells recognize the antigens and release IFN-gamma, resulting in a positive test. If your immune system hasn't encountered TB, little or no IFN-gamma is released, resulting in a negative test.

Blood tests have certain advantages over skin tests. They are not affected by BCG vaccination history, making them more specific for true TB infection in people who received BCG vaccines. They require only one visit instead of two. They are less likely to be affected by improper administration or delayed reading. However, blood tests may be less sensitive in people with weakened immune systems, particularly those with advanced HIV disease, and they typically cost more than skin tests. Some healthcare settings use both tests together for confirmation, or they may use blood tests as follow-up when skin tests show uncertain results.

Practical takeaway: Ask your healthcare provider which TB test they're using and why. Blood tests and skin tests measure slightly different things and may be recommended for different situations.

What Different TB Test Results Mean

For the tuberculin skin test, results are interpreted based on the size of the induration (raised bump) measured in millimeters and your risk factors for TB. The CDC provides specific guidelines that healthcare providers use. A result of less than 5 mm is typically considered negative. However, the interpretation of 5 mm or larger depends on your personal circumstances and TB risk factors. For people with HIV infection, those in close contact with TB patients, those with chest X-ray findings suggesting TB, or those who use injection drugs, an induration of 5 mm or larger may be considered positive. For people with diabetes, kidney disease, or those who work in healthcare settings, an induration of 10 mm or larger may be considered positive. For people with no known TB risk factors, an induration of 15 mm or larger is considered positive.

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For TB blood tests, results are reported as positive, negative, or indeterminate. A positive result means TB infection is likely. A negative result means TB infection is unlikely. An indeterminate result means the test couldn't be properly evaluated, usually because the immune system didn't respond adequately to the test controls. Indeterminate results can occur in people with weakened immune systems, those taking certain medications, or those with certain medical conditions. When an indeterminate result occurs, the test is usually repeated or the healthcare provider may recommend a skin test instead.

A positive TB test result means you have been infected with TB bacteria at some point. However, this alone does not mean you have active TB disease. People with TB infection can remain well for years without developing symptoms. Only about 5 to 10 percent of people with TB infection develop active TB disease during their lifetime. When TB infection is detected, your healthcare provider will recommend additional testing, such as a chest X-ray and possibly other tests, to determine whether you have active TB disease or latent TB infection.

Practical takeaway: A positive TB test requires follow-up testing but doesn't automatically mean you have TB disease. Work with your healthcare provider to determine what additional testing you need.

False Results and Factors That Can Affect Accuracy

TB test results can sometimes be false positive or false negative, meaning the result doesn't accurately reflect your true TB status. Understanding these possibilities helps you work with your healthcare provider to reach an accurate diagnosis. False positives occur when a test shows positive results despite no true TB infection. For the skin test, false positives can result from previous BCG vaccination, which is given in many countries to prevent severe TB in children. BCG vaccination can cause skin test induration for years or even decades after vaccination. Other infections caused by organisms related to TB, called nontuberculous mycobacteria, can also cause false-positive skin tests. These include bacteria found in soil and water that people can encounter in certain occupations or environments.

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False negatives occur when a test shows negative results despite actual TB infection. In people with weakened immune systems—such as those with advanced HIV disease, those taking immunosuppressive medications, or those with certain medical conditions—TB tests may fail to show infection because their immune systems cannot mount an adequate response. Recent TB infection within the past 8 to 10 weeks may not yet show up on tests, as it takes time for immune sensitization to develop. Very young children under 5 years old may have false-negative skin tests because their