TB Skin Test Reading

A TB skin test reading should be performed by a healthcare professional. During testing, a small amount of testing fluid is injected under the skin on the forearm. After 2 or 3 days, you must return for the TB skin test reading, in which the healthcare worker will inform you of the test results. A positive skin test reading is determined by the size of the swelling (induration) and the presence of risk factors.

 

An Overview of TB Skin Test Reading

For a TB skin test (also known as a PPD test), a healthcare worker will inject a small amount of testing fluid (called tuberculin or PPD) just under the skin on the underside of the forearm. After 2 or 3 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 tuberculosis 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 TB skin test reading is greater than 5 mm of induration:
 
  • HIV-infected people
  • Recent contacts to 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 TB skin test reading is greater than 10 mm of induration:
 
  • Recent arrivals (under 5 years) from high-prevalence countries
     
  • Injection drug users
     
  • Residents and employees of high-risk congregate settings (e.g., 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 disease
      
  • Children under 4 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.
 
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Written by/reviewed by: Arthur Schoenstadt, MD
Last reviewed by: Arthur Schoenstadt, MD