Tuberculin skin test reaction
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Tuberculin skin test reaction

Contributors: Medical staff writer
Other Resources UpToDate PubMed


The Mantoux tuberculin skin test (TST), also known as the purified protein derivative (PPD) skin test, is a method for diagnosing Mycobacterium tuberculosis infection. Interpretation (positive or negative) depends on 2 factors: 1) measurement in millimeters of the reaction and 2) risk of infection or risk of progression to tuberculosis (TB) disease if infected. The reaction is not measured visually (by redness) but by feel (induration). Only the part of the reaction that can be felt (the induration) is measured. Reading of the TST requires standardization of procedures, training, supervision, and practice.

The following information is from the US Centers for Disease Control and Prevention (CDC) Tuberculin Skin Testing Fact Sheet.

See also the Mantoux Tuberculin Skin Test Video, available from the CDC, which demonstrates the steps involved in administering and reading the Mantoux tuberculin skin test.

Check your institution's policy for evaluation and referral procedures.

For an overview of basic concepts related to tuberculosis prevention and control in the United States, refer to TB 101 for Health Care Workers.

How is the tuberculin skin test read?
The TST reaction should be read between 48 and 72 hours after administration by a health care worker trained to read TST results. 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 (firm 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 tuberculosis (TB) infection or the risk of progression to TB disease if infected
STEPS for READING the TST reaction (based on the PALPATION METHOD)
  1. Collect supplies:
    • A small plastic flexible ruler marked in millimeters to measure the test
    • A pen to mark the edges of the induration
    • An alcohol pad to clean off the pen marks
    • Patient record or other appropriate forms for documentation
    • Patient education information, available in different languages
  2. Inspect the test site:
    • Inspect the arm in a good light and on a firm surface.
    • Turn the arm up, support it, and slightly flex it at the elbow.
    • Sometimes the site will have erythema (reddening of the skin, perhaps with swelling); the erythema should not be measured.
    • Only the part of the reaction that can be felt (the induration) is measured.
    • There might not be an induration.
    • Reactions can range from no induration to a large, well-defined induration.
  3. Palpate the induration:
    • The induration is not always visible; you will rely on palpation with your fingertips to discover if there is induration at the site.
    • With your fingers together, touch the area lightly with the pads of your fingertips; using a light, gentle motion, sweep your fingertips over the surface of the forearm in a 2-inch diameter in all 4 directions to locate the margins, or edges, of the induration.
    • If induration is present, use a zig-zag feather-like touch over the area to outline the margins of induration. Determining margins helps define the edges, which will be measured later.
    • When palpating, be careful not to confuse a margin of induration with a margin of muscle of the forearm; to check this, raise the patient's arm to a 45-degree angle and palpate again – you should still be able to palpate the margins of induration.
    • The diameter of the induration is measured across the forearm from the thumb side of the arm to the little finger side of the arm, or vice versa (eg, parallel to a watch band).
  4. Mark the induration:
    • To mark the edges of the induration, hold your palm over the injection site with your fingertips at the outer edge of the patient's forearm.
    • Without lifting, move the fingertips from the outer edge of the forearm toward the induration.
    • Rest one fingertip firmly against the induration border on one side before marking the margin. The fingertip should remain in contact with the skin at all times.
    • Mark lightly with a fine dot at the widest edge of the induration using the fingertip as a guide.
    • Repeat the procedure from the other side of the patient's forearm and place the second mark on the margin of induration.
    • Palpate again to double check that the induration was marked correctly.
    • If the margins are not equally clear all the way around the induration, it's still necessary to mark the margins on each side of the induration. Palpate around the induration from the easily-felt margin to the not-so-easily-felt margin.
    • If the margins of induration are irregular, mark and measure the longest diameter across the forearm.
  5. Measure the induration:
    • To measure the diameter of the induration, use the millimeter ruler.
    • Place the zero ruler line inside the left-dot edge, and read the ruler line inside the right-dot edge.
    • If the measurement falls between 2 divisions on the millimeter scale, record the lower mark.
    • Reactions to the skin test will vary.
    • Make sure to record blistering even if no induration is present. Palpate gently, as it may be painful. Measure only the induration.
    • There may be redness and swelling, but no induration. The redness and swelling should not be mistakenly measured.
    • Without induration, the measurement is zero millimeters.
  6. Record the measurement:
    • Immediately after the test is measured, write the exact measurement in millimeters of induration on the patient's record.
    • Do not simply record the interpretation of the results as negative or positive.
    • Do not record the results in centimeters.
    • Also record the date and time the test was read and the name and signature of the reader.
    • Also record the presence or absence of adverse effects.
CLASSIFICATION of the TST reaction (≥ 5 mm, ≥ 10 mm, ≥ 15 mm)

An induration of 5 or more millimeters is considered positive in:
  • People living with HIV
  • A recent contact of a person with infectious TB disease
  • People with chest x-ray findings suggestive of previous TB disease
  • People with organ transplants
  • Other immunosuppressed people (eg, patients on prolonged therapy with corticosteroids equivalent to / greater than 15 mg per day of prednisone or those taking TNF-alpha antagonists)
An induration of 10 or more millimeters is considered positive in:
  • People born in countries where TB disease is common, including Mexico, the Philippines, Vietnam, India, China, Haiti, and Guatemala, or other countries with high rates of TB
  • People who abuse drugs
  • Mycobacteriology laboratory workers
  • People who live or work in high-risk congregate settings (eg, nursing homes, homeless shelters, or correctional facilities)
  • People with certain medical conditions that place them at high risk for TB (eg, silicosis, diabetes mellitus, severe kidney disease, certain types of cancer, and certain intestinal conditions)
  • People with a low body weight (< 90% of ideal body weight)
  • Children younger than 5 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:
  • People with no known risk factors for TB
Refer to the Tuberculin Skin Testing Fact Sheet for more information on the following:
  • What are false positive reactions?
  • What are false negative reactions?
  • Who can receive a TST?
  • How often can TSTs be repeated?
  • What is a Boosted Reaction?
  • Why is 2-step testing conducted?
  • Can TSTs be given to persons receiving vaccinations?
  • Are there alternative tests to the TST?
  • What does a positive TST mean for the diagnosis of latent TB infection and TB disease?
  • What are treatment options for latent TB infection?


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Last Reviewed:01/14/2024
Last Updated:01/30/2024
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Tuberculin skin test reaction
Copyright © 2024 VisualDx®. All rights reserved.