TB Skin Test Reading

Only a healthcare professional should read a TB skin test. During the procedure, a small amount of fluid is injected under the skin on the forearm. After a few days, you will return and the healthcare provider will inform you of the test results. A positive reading is determined by the size of the swelling (called induration) and the presence of risk factors.

Reading a TB Skin Test

For a TB skin test (also known as a PPD test), a healthcare worker will inject a small amount of fluid (called tuberculin or PPD) just under the skin on the underside of the forearm. After two or three days, you must return for the TB skin test reading.

You may have a swelling where the tuberculin was injected. The healthcare worker will measure this swelling and tell you if your reaction to the skin test is positive or negative. A positive reaction usually means that you have been infected by someone with active tuberculosis.

What Is a Positive TB Skin Test Reading?

A positive skin test reading is determined by the size of the swelling (induration) and the presence of risk factors. A positive TB skin reading, as well as your personal risk factors, will determine if tuberculosis treatment is recommended.

People in the following high-risk groups should be given treatment for latent tuberculosis if their skin test reading is greater than 5 mm of induration:

• HIV-infected people

• Recent contacts in a TB case

• People with fibrotic changes on a chest radiograph consistent with old TB

• Patients with organ transplants and other immunosuppressed patients (receiving the equivalent of more than 15 mg a day of prednisone for over a month).

In addition, people in the following high-risk groups should be considered for treatment of latent tuberculosis if their skin test reading is greater than 10 mm of induration:

• Recent arrivals (under five years) from high-prevalence countries
• Injection drug users
• Residents and employees of high-risk congregate settings, such as correctional facilities, nursing homes, homeless shelters, hospitals, and other healthcare facilities)
• Mycobacteriology laboratory personnel
• People with clinical conditions (such as diabetes) that place them at high risk for developing TB
    
• Children under four years of age, or children and adolescents exposed to adults in high-risk categories.

People with no known risk factors for TB may be considered for latent tuberculosis treatment if their TB skin test reading is greater than 15 mm of induration.