Even when you don't have TB symptoms, this century-old test can still detect the disease.

USA Today

11-05-2025

Even when you don't have TB symptoms, this century-old test can still detect the disease.
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Tuberculosis history and treatment
Tuberculosis is spread through the air from one person to another, says the Center for Disease Control and Prevention.
Tuberculosis, or TB, is a disease that infects nearly 11 million people worldwide, killing some 1.25 million of them every year, per data from the World Health Organization.
A person infected with the disease may experience a range of symptoms such as a low-grade fever, coughing or fatigue on the mild end to a loss of appetite, difficulty breathing, night sweats, back and chest pain and a persistent cough (often with accompanying blood) on the more severe end.
TB is spread through airborne particles that are released when an infected person talks, sneezes or coughs and another person breathes in those particles. "It's a disease that is highly contagious," explains Amira Roess, a professor of global health and epidemiology at George Mason University in Virginia.
Detecting TB early, she adds, goes a long way toward avoiding some of the disease's worst outcomes and preventing its spread to others.
What is a TB test? What is a PPD test?
Forms of testing for TB vary depending on what stage of the disease a person is in. The three main stages of the disease are primary (exposure), latent (inactive) and active TB, each reflecting whether TB germs are still contained in the lungs or are actively wreaking havoc. Only someone with active TB has significant symptoms and can spread the disease to others, but it's still important to know if you have the less serious stages of the disease as primary and latent TB can develop into active TB at any time.
Symptoms aren't usually present in the primary or latent stage of TB, but there are still two ways to test whether TB germs are present in the body. The first of these is a blood test "which measures the body's immune response to tuberculosis proteins," explains Dr. Richard Doyle, a physician and clinical assistant professor of emergency medicine at Northwestern University.
The second diagnostic test option is known as a purified protein derivative (PPD) test, but it's commonly referred to as a TB skin test. "TB skin tests have been used in medicine for over a century," says Doyle. The skin test is administered by a healthcare professional using a small needle to inject testing materials under the skin, "typically in the forearm," says Doyle.
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How long do PPD test results take?
Results of a PPD test are not immediate because the test has to determine "if the immune system has been previously (affected) by TB exposure," says Doyle, which can take a couple of days. "If your immune system reacts to the injected protein," he explains, "it will cause a firm, raised area at the site of injection." This thickening and hardening of tissue is called an induration and the size of it is measured by your doctor in a return visit 48 to 72 hours after the initial injection took place.
If you're unable to return to the doctor within two or three days after the test is first administered, "you may be given instructions for how to read the test results yourself," says Roess.
When checked by a healthcare professional, "an induration on the skin of 15 millimeters or more is usually considered a positive TB result in healthy individuals," says Doyle. "But for people at higher risk, such as those with HIV, organ transplant recipients or people on immunosuppressive medications, an induration of just 5 millimeters may be considered positive due to their increased vulnerability to developing active TB."
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Who are PPD tests recommended for?
PPD tests are suggested for a number of reasons but "are most commonly recommended when a person has been in contact with a person infected with tuberculosis," says Dr. Cathy Hewison, a physician and tuberculosis advisor at the Paris-based humanitarian organization, Médecins Sans Frontières.
Education enrollment, new employment, traveling to or from certain countries or an immigration application can all be additional reasons a PPD test may be recommended. The U.S. Centers for Disease Control and Prevention (CDC) also suggests some groups of people be periodically tested under other circumstances. Such groups may include people who have a weakened immune system or conditions such as cancer, diabetes or HIV; if a person has been infected with TB in the past; if one is elderly; has done illegal intravenous drugs; or if one has not received proper treatment for a past TB infection.
PPD tests are also commonly recommend for people who work in homeless shelters, hospitals, prisons or nursing facilities. "Healthcare workers are routinely screened as part of occupational health protocols," says Doyle. "Patients who are about to start immunosuppressive therapies such as certain biologic agents or chemotherapy should also be screened for latent TB before treatment begins."
If exposure is detected or TB germs are found through PPD testing, Hewison says, "then preventive treatment can be offered to reduce your likelihood of developing active TB or spreading it to others."