Tuberculin purified protein derivative (PPD)

Mechanism of action

PPD is a standardised mixture of tuberculin proteins. Injected into the skin of someone already infected (or vaccinated with BCG, or exposed to other mycobacteria), memory T cells recognise it and call in other immune cells, which produces a firm, raised area (induration) over two to three days. The response appears only 2–8 weeks after infection. A positive test shows probable infection but does not tell latent infection from active disease.

1. PPD is injected into the skin of the forearm
2. Memory T cells of a sensitised person recognise it
3. Immune cells gather: induration over 48–72 hours
4. Induration (not redness) is measured in millimetres

Indications

• Testing for infection with M. tuberculosis, such as in contacts of a person with infectious TB Tuberculosis →
• Screening before immunosuppression, in people with HIV and in health workers (baseline two-step testing)
• Children under 5, in whom the skin test is the recommended test

Adverse effects

Common

• Itching or discomfort at the site
• A firm red swelling in people who test positive

Contraindications

• A previous severe reaction to a tuberculin skin test (necrosis, blistering, ulceration, anaphylaxis)
• Not useful when there is a documented earlier positive test or previous TB treatment
• Pregnancy, infancy and HIV are not contraindications

Key interactions

Live-virus vaccines (such as measles vaccine)

Can cause a false-negative result; the test is done the same day as the vaccine or at least one month later

BCG vaccine

Earlier BCG can cause a false-positive result

Immunosuppressants (high-dose corticosteroids, TNF-α antagonists)

A weakened immune response can make a real infection test negative, so a lower cut-off is used

What is monitored

• Reading of induration (not redness) across the forearm at 48–72 hours
• After a positive result: chest X-ray and sputum tests to look for active TB

Administration and safety

• Given intradermally on the inner forearm with a tuberculin syringe, bevel up (Mantoux technique); a pale wheal should form.
• If no wheal forms or the depth is wrong, the test is repeated at a site well away from the first.
• A test not read within 72 hours is repeated.

Pregnancy and breastfeeding

The skin test is safe in pregnancy and is not contraindicated.

Nursing care

• Make sure the patient can return for the reading in 48–72 hours.
• Measure only the induration in millimetres and record the exact size.
• Ask about earlier BCG, earlier positive tests, recent live vaccines, HIV and immunosuppressive drugs.
• Explain that a negative result does not rule out TB, especially in severe illness or HIV.

Pharmacokinetics

Onset and duration

Induration develops over 48–72 hours, the window in which the test is read

Test yourself

Think of the answer, then tap the question to see it.

Add to my cards (4)

More Pharmacology questions →

Connections

← Back to the class: Diagnostic agents

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Clinical Testing Guidance for Tuberculosis: Tuberculin Skin Test | Tuberculosis (TB) | CDC (U.S. Centers for Disease Control and Prevention (CDC), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Send

Representative model, for educational purposes only.