The Tuberculosis Skin Test

How It Works, How It Is Done, and How It Is Interpreted

A wheal created at the injection site of a Mantoux tuberculin skin test.. Photo Credit: U.S. Centers for Disease Control and Prevention

Tuberculosis (TB) is a disease that is spread through the air from a person with an active infection and is particularly dangerous for those co-infected with HIV. In fact, TB today is the leading cause of death worldwide among people living with HIV, accounting for more than 400,000 death annually.

Detection of latent TB is done using a simple test called the Mantoux tuberculin skin test. A latent TB infection is one in which a person has been infected with the bacterium but does not have an active infection.

How the Skin Test Works

The Mantoux skin test uses something called tuberculin purified protein derivative  (PPD), a solution which contains certain components of the TB bacterium. When injected beneath the top layer of skin, a delayed localized skin reaction will develop if a person has been infected with TB.

The PPD causes specialized immune system cells, known as T-cells, to recognize injected derivative as a threatening invader, and in doing so, causes a dermatological response hours. The size of the response is interpreted to assess whether the person is negative (uninfected) for TB or positive (infected) for TB.

How Is the Test Administered?

A syringe with a small gauge needle is filled with 0.1 cc of the PPD solution. The solution is then injected subcutaneously on the forearm. Unlike a vaccination that is given intramuscularly, the PPD injection is so superficial that it leaves a raised area of skin called a wheal at the injection site (see photo).

If the injection doesn't create a wheal, the injection must be repeated. The wheal will eventually vanish over the course of several hours.

It is important that the injection site be free of lesions, wounds or veins.

Interpreting the Skin Test

The reaction reaches its peak about 48 to 72 hours after the PPD is injected.

Waiting too long or not long enough may cause an underestimation of the reaction and, therefore, affect how the test is read.A positive test will cause redness, itching and a raised, hardened area of skin called an induration.

Presence or absence of induration, as well as the size of the induration, are the key indicators of a positive result. By measuring the induration in millimeters, the doctor can make a pretty strong interpretation as to whether an infection has taken place. The extent of redness (erythema) is not included when making the measurement. 

Induration size indicates a positive result in the following circumstances:

  • An induration of 15 mm is considered positive in all individuals.
  • An induration of 10 mm is considered positive in recent immigrants from countries with high TB prevalence, injecting drug users, people with kidney disease or diabetes, children under 4 years of age, and anyone considered at high risk of TB infection.
  • An induration of 5 mm is considered positive in people with HIV, organ transplant recipients, persons identified as immune compromised, and anyone in recent contact with a person with an active TB infection.

    What Happen If I Test Positive?

    A positive test means that, at some point in your life, you were exposed to and infected with TB. If you are positive, talk with your doctor about the next step. It is likely that your TB is latent, meaning  that it is not active, contagious or making you sick. Your doctor will most likely prescribe a course of antibiotics to manage your infection.

    In some cases, a positive test may indicate an active infection. If this is the case, your doctor will discuss symptoms, order any other necessary tests (such as a chest x-ray), and set a course of therapy.


    U.S. Centers for Disease Control and Prevention (CDC). "Fact Sheet: Tuberculin Skin Test." Atlanta, Georgia, accessed January 17, 2016.

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