Tuberculin Skin Testing What is It? How is the TST Read?
The Mantoux tuberculin skin test (TST) is the
The skin test reaction should be read between 48
standard method of determining whether a person is
and 72 hours after administration. A patient who
infected with Mycobacterium tuberculosis. Reliable
does not return within 72 hours will need to be
administration and reading of the TST requires
standardization of procedures, training, supervision, andpractice.
The reaction should be measured in millimeters ofthe induration (palpable, raised, hardened area orswelling). The reader should not measure erythema
How is the TST Administered?
(redness). The diameter of the indurated area shouldbe measured across the forearm (perpendicular to
The TST is performed by injecting 0.1 ml of tuberculin
purified protein derivative (PPD) into the inner surfaceof the forearm. The injection should be made with atuberculin syringe, with the needle bevel facing upward. How Are TST Reactions Interpreted?
The TST is an intradermal injection. When placedcorrectly, the injection should produce a pale elevation of
Skin test interpretation depends on two factors:
the skin (a wheal) 6 to 10 mm in diameter.
Measurement in millimeters of the induration Person’s risk of being infected with TB and of
Classification of the Tuberculin Skin Test Reaction
An induration of 5 or more
An induration of 10 or more
An induration of 15 or more millimeters is considered positive in millimeters is considered positive in millimeters is considered positive in
– Persons with clinical conditions that
day of prednisone for 1 month orlonger, taking TNF-alpha antago-
What Are False-Positive Reactions? What is a Boosted Reaction?
Some persons may react to the TST even though they
In some persons who are infected with M. tuberculosis,
are not infected with M. tuberculosis. The causes of
the ability to react to tuberculin may wane over time.
these false-positive reactions may include, but are not
When given a TST years after infection, these persons
may have a false-negative reaction. However, the TST
Infection with nontuberculosis mycobacteria
may stimulate the immune system, causing a positive, or
boosted reaction to subsequent tests. Giving a second
Incorrect method of TST administration
TST after an initial negative TST reaction is called two-
Why is Two-Step Testing Conducted? What Are False-Negative Reactions?
Two-step testing is useful for the initial skin testing of
Some persons may not react to the TST even though
adults who are going to be retested periodically, such as
they are infected with M. tuberculosis. The reasons for
health care workers or nursing home residents. This
these false-negative reactions may include, but are not
two-step approach can reduce the likelihood that a
boosted reaction to a subsequent TST will be
Cutaneous anergy (anergy is the inability to
misinterpreted as a recent infection.
react to skin tests because of a weakenedimmune system)
Can TSTs Be Given To Persons
Recent TB infection (within 8-10 weeks of
Vaccination with live viruses may interfere with TST
reactions. For persons scheduled to receive a TST,
Recent live-virus vaccination (e.g., measles and
Either on the same day as vaccination with live-
Some viral illnesses (e.g., measles and chicken
At least one month after smallpox vaccination
Incorrect method of TST administration Incorrect interpretation of reaction
American Thoracic Society and CDC. Diagnostic
Who Can Receive a TST?
standards and classification of tuberculosis in adultsand children. Am J Respir Crit Care Med 2000; 161.
contraindicated only for persons who have had a severereaction (e.g., necrosis, blistering, anaphylactic shock, or
CDC. Guidelines for preventing the transmission of
ulcerations) to a previous TST. It is not contraindicated
Mycobacterium tuberculosis in health-care settings,
for any other persons, including infants, children,
2005. MMWR 2005; 54 (No. RR-17). www.cdc.gov/
pregnant women, persons who are HIV-infected, or
persons who have been vaccinated with BCG.
CDC. Mantoux Tuberculin Skin Test: Training Materi-als Kit (2003). How Often Can TSTs Be Repeated?
CDC. Targeted tuberculin testing and treatment of
In general, there is no risk associated with repeated
latent tuberculosis infection. MMWR 2000; 49 (No.
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, unlessa previous TST was associated with a severe reaction.
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