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Fact Sheets

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Tuberculin Skin Testing

What is it?

The Mantoux tuberculin skin test (TST) is the standard method of determining whether a person is infected with Mycobacterium tuberculosis. Reliable administration and reading of the TST requires standardization of procedures, training, supervision, and practice.

How is the TST Administered?

The TST is performed by injecting 0.1 ml of tuberculin purified protein derivative (PPD) into the inner surface of the forearm. The injection should be made with a tuberculin syringe, with the needle bevel facing upward. The TST is an intradermal injection. When placed correctly, the injection should produce a pale elevation of the skin (a wheal) 6 to 10 mm in diameter.

How is the TST Read?

The skin test reaction should be read between 48 and 72 hours after administration. A patient who does not return within 72 hours will need to be rescheduled for another skin test.

The reaction should be measured in millimeters of the induration (palpable, raised, hardened area or swelling). The reader should not measure erythema (redness). The diameter of the indurated area should be measured across the forearm (perpendicular to the long axis).

How Are TST Reactions Interpreted?

Skin test interpretation depends on two factors:

  • Measurement in millimeters of the induration
  • Person’s risk of being infected with TB and of progression to disease if infected
Classification of the Tuberculin Skin Test Reaction

An induration of 5 or more millimeters is considered positive in

-HIV-infected persons

-A recent contact of a person with TB disease

-Persons with fibrotic changes on chest radiograph consistent with prior TB

-Patients with organ transplants

-Persons who are immunosuppressed for other reasons (e.g., taking the equivalent of >15 mg/day of prednisone for 1 month or longer, taking TNF-a antagonists)

An induration of 10 or more millimeters is considered positive in

-Recent immigrants (< 5 years) from high-prevalence countries

-Injection drug users

-Residents and employees of high-risk congregate settings

-Mycobacteriology laboratory personnel

-Persons with clinical conditions that place them at high risk

-Children < 4 years of age

- Infants, children, and adolescents exposed to adults in high-risk categories

 >An induration of 15 or more millimeters is considered positive in any person, including persons with no known risk factors for TB. However, targeted skin testing programs should only be conducted among high-risk groups.

What Are False-Positive Reactions?

Some persons may react to the TST even though they are not infected with M. tuberculosis. The causes of these false-positive reactions may include, but are not limited to, the following:

  • Infection with nontuberculosis mycobacteria
  • Previous BCG vaccination
  • Incorrect method of TST administration
  • Incorrect interpretation of reaction
  • Incorrect bottle of antigen used

What Are False-Negative Reactions?

Some persons may not react to the TST even though they are infected with M. tuberculosis. The reasons for these false-negative reactions may include, but are not limited to, the following:

  • Cutaneous anergy (anergy is the inability to react to skin tests because of a weakened immune system)
  • Recent TB infection (within 8-10 weeks of exposure)
  • Very old TB infection (many years)
  • Very young age (less than 6 months old)
  • Recent live-virus vaccination (e.g., measles and smallpox)
  • Overwhelming TB disease
  • Some viral illnesses (e.g., measles and chicken pox)
  • Incorrect method of TST administration
  • Incorrect interpretation of reaction

Who Can Receive a TST?

Most persons can receive a TST. TST is contraindicated only for persons who have had a severe reaction (e.g., necrosis, blistering, anaphylactic shock, or ulcerations) to a previous TST. It is not contraindicated for any other persons, including infants, children, pregnant women, persons who are HIV-infected, or persons who have been vaccinated with BCG.

How Often Can TSTs Be Repeated?

In general, there is no risk associated with repeated tuberculin skin test placements. If a person does not return within 48-72 hours for a tuberculin skin test reading, a second test can be placed as soon as possible. There is no contraindication to repeating the TST, unless a previous TST was associated with a severe reaction.

What is a Boosted Reaction?

In some persons who are infected with M. tuberculosis, the ability to react to tuberculin may wane over time. When given a TST years after infection, these persons may have a false-negative reaction. However, the TST may stimulate the immune system, causing a positive, or boosted reaction to subsequent tests. Giving a second TST after an initial negative TST reaction is called two-step testing.

Why is Two-Step Testing Conducted?

Two-step testing is useful for the initial skin testing of adults who are going to be retested periodically, such as health care workers or nursing home residents. This two-step approach can reduce the likelihood that a boosted reaction to a subsequent TST will be misinterpreted as a recent infection.

Can TSTs Be Given To Persons Receiving Vaccinations?

Vaccination with live viruses may interfere with TST reactions. For persons scheduled to receive a TST, testing should be done as follows:

  • Either on the same day as vaccination with live-virus vaccine or 4-6 weeks after the administration of the live-virus vaccine
  • At least one month after smallpox vaccination

Additional Information

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