Tuberculosis Skin Test: What You Need to Know
Doctors use the skin test to detect TB bacteria, called Mycobacterium tuberculosis (M. tuberculosis). Testing allows people to receive treatment quickly and take precautions to avoid passing the bacteria to others.
With latent TB infection (LTBI), M. tuberculosis does not cause symptoms. However, the activated infection causes symptoms and is known as TB. Both conditions can be detected with a skin test and are usually curable with antibiotic treatment.
Learn more about the symptoms of TB.
Healthcare professionals also use the TB skin test to screen people at high risk, including:
- healthcare workers and volunteers in outpatient and inpatient facilities
- lab workers
- healthcare workers in correctional facilities
- clinicians in homeless shelters
- healthcare workers providing home-based care or working in long-term health facilities
Learn more about TB prevention.
You’ll need to schedule two visits when your clinician or workplace orders a TB skin test. During the first visit, the initial test is performed. A healthcare professional reads and documents the TB skin test results during the next appointment.
For the initial test, a healthcare professional injects a small amount of tuberculin liquid just under the inner skin of your forearm. The liquid is harmless and does not cause TB. The injection takes only a few minutes and may cause mild, short-lasting discomfort.
During the next visit, usually within 48 or 72 hours, your clinician will read the result of the TB skin test by measuring the size of the skin reaction, if one exists. The result will show whether M. tuberculosis is present in your system. Other tests will determine whether you have LTBI or TB.
The healthcare professional will assess your skin test result during your second visit. If you miss the second appointment, you will have to start the test again.
To determine whether your TB skin test is positive, meaning the test detects the bacteria that causes TB, a clinician will measure the size of the reaction at the injection site. They also consider your risk factors, such as reporting close contact with someone with TB.
According to the Centers for Disease Control and Prevention (CDC), the guidelines for evaluating the size of the skin reaction are as follows:
- For people at high risk for developing TB, a skin reaction that measures at least 5 millimeters (mm) is considered positive.
- For people with some risk factors, a skin reaction measuring 10 mm or more is considered positive.
- For people with no known TB risk factors, a skin reaction of 15 mm or more is considered positive.
If you receive a positive TB skin test, starting treatment as soon as possible is essential. Medical professionals can cure TB if you follow their instructions and treatment plan.
If your TB skin test result is positive, your doctor may recommend more diagnostic testing to determine whether you have LTBI or TB. They will also discuss potential treatment options for both.
Learn more about the treatments for TB.
In some cases, a false-positive TB skin test result can occur if you have received the Bacille Calmette-Guérin (BCG) vaccine. While the vaccine is not widely administered in the United States, other countries have vaccination programs for infants and children.
Aside from medical professionals whose employers require regular testing for TB, people with certain risk factors may need to undergo TB testing:
- people in close contact and casual contact with others who have TB
- people emigrating from countries where TB is common, such as Southeast Asia and Africa
- some people whose immune systems are weakened by certain factors, such as:
- organ transplants
- medications that suppress the immune system, such as steroids
- people at increased risk who:
- smoke cigarettes
- consume excessive amounts of alcohol
- have a body mass index (BMI) lower than 20
- have suspicious findings on a chest X-ray
Your local city, county, or state health department may offer TB skin testing. If you have a primary care provider and health insurance, they can help you schedule a test.
Veterans can schedule a TB skin test at their nearest Veterans Administration hospital or clinic. Also, many clinics in drug stores offer TB skin tests.
The TB skin test identifies LTBI and TB cases. A healthcare professional injects a small amount of tuberculin into your forearm. When you return at your next appointment, the clinician assesses the result by measuring the size of the skin reaction.
Doctors can cure TB with extensive medication regimens. It is critical to follow their treatment plan.
Talk with a doctor if you need a TB skin test.