The Tuberculin Skin Test: How to Give It, Read It, and Avoid Its Pitfalls

The TB skin test — also called the Mantoux tuberculin skin test (TST) — is one way to find out whether someone is infected with TB bacteria. Giving and reading it reliably takes standard procedures, training, supervision and practice.

Image from CDC's page on the tuberculin skin test

How it works

A standardized solution of purified protein derivative (PPD) — made from tuberculin, proteins released by M. tuberculosis grown under standard conditions — is injected under the skin. In most people with TB infection, the immune system recognizes it and mounts a T-cell–mediated delayed-type hypersensitivity reaction. PPD comes diluted to a standard strength in FDA-approved solutions. The immune system needs 2 to 8 weeks after infection before the test can detect it.

Who gives it: a designated, trained health care provider — state and local public health authorities decide who is authorized. CDC offers free training materials, including a fact sheet, wall chart and video.

When to use it

• CDC guidelines prefer TB blood tests in most cases; the skin test is an acceptable alternative when a blood test is unavailable, too costly or too burdensome.
• Children under 5: the guidelines recommend the skin test, though some experts use blood tests.
• Almost anyone can have it — including infants, children, pregnant women and people with HIV. The only contraindication is a severe reaction to a previous test (such as necrosis, blistering, anaphylactic shock or ulceration).
• Don't test people with a documented previous positive TB test or past treatment for TB disease: most stay positive, and another test adds nothing.

BCG vaccine

• BCG can cause a false-positive skin test, and there is no reliable way to tell a BCG reaction from true infection — so vaccinated people should be evaluated as if unvaccinated, with results interpreted by risk.
• BCG reactivity fades over time, but repeated skin testing can boost it. In babies given BCG at birth, reactivity is likely for the first two years; multiple BCG doses increase and prolong it.
• TB blood tests are not affected by BCG and are preferred for anyone who has had it, children included.

Boosting and two-step testing

In older adults infected long ago, and in people given BCG or infected with other mycobacteria, a skin test may at first show little or no reaction — but it can stimulate the immune system so a later test is positive (a boosted reaction). Blood tests do not boost.

Two-step testing keeps a boosted reaction from being mistaken for new infection. Use it for the baseline test of anyone who will be retested periodically, including U.S. health care personnel:

1. If the first test is negative, repeat it 1 to 3 weeks later.
2. If the second is positive, it is probably boosting: classify the person as previously infected — not a new infection — and a candidate for latent TB infection treatment.
3. If the second is also negative, the person is classed as uninfected.

Vaccines: live-virus vaccines (measles, mumps, rubella, oral polio, varicella, yellow fever) can interfere — test the same day or at least one month after. Inactivated vaccines don't interfere, and COVID-19 vaccination should not be delayed for TB testing.

Placing the test

Inject 0.1 ml of PPD into the inner forearm with a disposable 27-gauge tuberculin syringe, bevel up, into the skin (intradermally). Done correctly, it raises a pale wheal 6 to 10 mm across. A wrong dose or an injection too deep to form a wheal invalidates the test: report it under facility policy and repeat it at least 2 inches away.

On the first visit, a trained provider places the test. Image from CDC's page on the tuberculin skin test

Reading the test

• Read it 48 to 72 hours after placing it; a patient who misses that window needs a new test (repeat tests carry no risk, though they can cause boosting).
• Measure the induration — the firm swelling — in millimeters, across the forearm. Don't measure redness.
• If someone has symptoms of TB disease, don't wait for the result before starting other tests.

A trained provider reads the test 48 to 72 hours after placement. Image from CDC's page on the tuberculin skin test

False results

Serious illnesses — protein-calorie malnutrition, advanced cancer, and TB itself, notably miliary TB and TB meningitis — can also cause false negatives; if TB is a concern, consult a TB expert.

After the result

• A positive test or TB symptoms calls for evaluation for TB disease: a chest radiograph and sputum examinations.
• Latent TB infection can be treated with short, convenient regimens that are 90% effective at preventing TB disease.
• A negative skin test does not rule out TB disease, especially with severe TB illness or HIV.
• Report all active or presumed TB disease to the local or state health department; latent infection is reportable in some places.

Sources

Based on "Clinical Testing Guidance for Tuberculosis: Tuberculin Skin Test," Centers for Disease Control and Prevention; a work of the United States government in the public domain.