TB Skin Test

Tuberculosis (TB) is a disease caused by the bacteria Mycobacterium tuberculosis. This bacteria can be spread easily through the air, transmitting the infection from person to person. TB primarily affects the lungs but can also impact other organs and bodily systems such as the brain, kidney, and spine.

A TB skin test is used to screen for tuberculosis infection when someone has potentially been exposed to tuberculosis. It is also used as a diagnostic tool when someone is showing symptoms of tuberculosis disease. This test is conducted by injecting a small amount of testing fluid, also called tuberculin, into the inside of the forearm and measuring the resulting swelling several days later.

Purpose of the test

A TB skin test is used to detect exposure to TB bacteria by measuring a patient’s immune response to an inactivated, or killed, version of Mycobacterium tuberculosis.

There are two TB-related conditions. The first is called TB infection or latent TB infection. The second is referred to as TB disease, active TB, or reactivation TB.

TB infection describes a stage of tuberculosis in which a person’s immune system is able to control the infection. Patients with TB infection don’t become ill or spread tuberculosis to others, but may develop TB disease if left untreated.

TB disease occurs in 5 to 10% of people with TB infection. Patients with TB disease usually develop symptoms of tuberculosis and can spread the disease to others.

A TB skin test may be performed to screen for TB infections or to assist in diagnosing TB disease:

• Screening for TB infection: Screening for TB infection means testing for TB in a patient without symptoms. Screening is often conducted when someone has a high risk of having tuberculosis and would benefit from treatment if a TB infection is diagnosed.
• Diagnosing TB disease: Diagnostic tests are used when a patient has symptoms of tuberculosis. A positive TB test supports a diagnosis of TB disease. In addition to the results of a TB test, doctors consider a patient’s medical history and the results of a physical exam, imaging, and other lab tests to diagnose TB disease.

A TB skin test detects if a patient has ever been infected with TB but does not determine if a patient currently has TB infection or TB disease. Further testing is required to confirm or rule-out a diagnosis of TB disease.

What does the test measure?

A TB skin test measures a person’s immune response to a testing solution that is made from Mycobacterium tuberculosis antigens. Antigens are protein markers that exist on the surface of the bacteria and trigger an immune response.

During a TB skin test, the testing solution is injected under the skin of the forearm, which creates an elevated, swollen spot on the surface of the skin. After 48 to 72 hours, a health professional reads the results of a TB skin test by measuring the size of the elevated spot. To interpret this test, a doctor considers a person’s risk of TB infection and the diameter of the swelling, measured in millimeters.

The TB skin test is one of two types of tests used to detect TB. The other type of test is a TB blood test called an IGRA TB Test. The decision of which type of TB test to use for an individual patient depends on several factors including where the test is conducted, availability, and the cost of each test.

When should I get a TB skin test?

A TB skin test may be recommended to screen a person who is at an increased risk of TB infection. People whose job or living condition puts them at an increased risk of TB infection include those who live or work in group settings where tuberculosis is more common, such as:

• Health care settings
• Correctional facilities
• Homeless shelters
• Nursing homes
• Countries where TB infection is common, including Mexico, India, and China

If a patient is showing symptoms of TB disease, a TB skin test may be ordered to assist in making a diagnosis. Symptoms of TB disease include:

• A bad cough that lasts longer than 3 weeks
• Coughing up blood and mucus
• Chest pain
• Fatigue
• Lack of appetite or weight loss
• Fever, chills, or night sweats

How to get tested

A TB skin test is typically ordered by a doctor and administered in a health care setting such as a doctor’s office, clinic, or hospital. A TB skin test may be a requirement for employment, especially in healthcare settings.

Can I take the test at home?

A TB skin test cannot be conducted at home. This test must be performed and interpreted by a trained health care professional.

How much does the test cost?

The cost of a TB skin test depends on several factors. If a TB skin test is an employment, the cost of this test may be covered by the employer. In other cases, it may be helpful for the patient to consult with their doctor or insurance provider to determine the specific cost of testing including any copays or deductibles.

A TB skin test is performed by injecting a test fluid under the skin on the inside of the forearm, between the wrist and the elbow.

The test takes two separate visits to complete. The first visit is to administer the test, which typically takes about 5 minutes. The second visit is to interpret the test results. Test results must be read within a 48 to 72 hour window to be considered valid. If the test is not read within that time frame, another TB skin test can be administered as soon as possible.

Before the test

No pre-test preparations are needed for a TB skin test. If a patient has experienced a severe reaction to a previous TB skin test, this should be shared with the health care professional administering the test. Another type of TB test may be more suitable.

During the test

During a TB skin test, a health care professional will wipe the inner forearm with alcohol and let the skin dry. Using a syringe and needle, the health care professional will then inject a small amount of test solution just under the skin. When performed correctly, the injection forms a small elevated spot on the skin. The test site should be left uncovered and undisturbed until the test result can be interpreted after several days.

After the test

After a TB skin test, the site must be examined by a health care provider between 48 and 72 hours later to see if a local skin reaction has occurred.

Receiving test results

TB skin test results are available as soon as 48 hours and up to 72 hours after the test is administered. The health care professional reading the test may report test results immediately and can recommend further testing if needed.

Interpreting test results

A TB skin test is read and interpreted by a trained health care provider between two and three days after the test is administered. To read this test, the provider determines if a skin reaction has occurred at the site where the test fluid was injected.

If a reaction has occurred, the diameter of the induration, which describes a firm area of tissue, is measured across the forearm. The test is then interpreted using two criteria:

• The measurement of the diameter of the swelling in millimeters
• The person’s risk of TB infection or the risk of progression to TB disease if infected

These criteria mean that the size of an induration required for a positive test result depends on other aspects of a patient’s risk profile. For example, an induration of 5 or more millimeters is considered positive in people living with HIV or who have imaging test results suggestive of TB disease. An induration of 15 or more millimeters is required for a positive diagnosis in patients with no known risk factors for TB.

Are test results accurate?

The TB skin test is a widely used test. There are known circumstances that can lead to false negative and false positive test results.

There are several factors that can contribute to false positive test results, in which a person has a positive test result despite not having an infection. Known causes for false positive test results include:

• Previous vaccination with the bacille Calmette-Guérin (BCG) TB vaccine
• An infection from another type of bacteria from the same family as Mycobacterium tuberculosis

False negative results, in which a person has a negative test result despite having a TB infection, can occur for several reasons, including:

• Recent vaccination using the live-virus measles or smallpox vaccine
• A recent TB infection acquired within 8 to 10 weeks before a TB skin test
• Testing on infants
• A phenomenon known as anergy, which describes a lack of normal immune response to the test fluid

Do I need follow-up tests?

In the event of a positive TB skin test result, follow-up tests are used to rule out TB disease. Tests used to rule out TB disease include a physical exam, chest x-rays and sputum culture. In some cases, doctors may suggest an IGRA TB test to confirm a positive TB skin test result.

If the result of a TB skin test is negative, follow-up testing depends on the patient’s circumstances. For example, if a patient with a compromised immune system is exposed to someone with TB disease, they may be treated for TB even after a negative TB skin test. TB testing is then repeated eight weeks after the patient begins treatment. In older adults who were previously infected with TB bacteria, a second TB skin test may be administered after an initial negative test result, known as two-step testing.

If TB skin test results are not read by a trained provider within the 48 to 72 hour window, another test may be ordered.

Questions for your doctor about test results

You may find it helpful to ask your doctor the following question about your TB skin test:

• What are the measurements of the induration?
• What are my risk factors for TB infection?
• What is the interpretation of my test?
• Do any follow-up tests need to be completed?
• Are there any precautions I need to take after receiving this test result?

How is a TB skin test test different from an IGRA TB test?

Another test used to detect TB infections, the interferon gamma release assay (IGRA) is a blood test that requires a blood sample. An IGRA TB test may be used instead of a TB skin test for a number of reasons, including the test setting, the cost of testing, and test availability. Providers may also recommend an IGRA TB test because this test only requires one visit. However, both tests are acceptable for detecting TB infections.