The tuberculosis skin test — also called the PPD test, Mantoux test, or TST — is a method of detecting TB infection by injecting a small amount of tuberculin protein under the skin of the forearm and measuring the body's reaction 48–72 hours later. It is one of two main TB screening methods (the other being the blood test/IGRA), is widely accepted for school and healthcare compliance, and costs $40–$100 at most locations. Unlike the TB blood test, the skin test requires two separate clinic visits.

How Does the TB Skin Test Work?

The Mantoux tuberculin skin test (TST) — called a PPD test after its reagent, purified protein derivative — works by triggering a localized immune reaction in people who have been infected with Mycobacterium tuberculosis.

Here is exactly what happens:

1. Injection (Visit 1): A trained healthcare provider injects 0.1 mL of PPD tuberculin solution intradermally (just under the surface of the skin) on the inner forearm. A small, pale blister-like bump called a wheal — about 6–10 mm in diameter — forms immediately at the injection site. This confirms correct placement. If no wheal forms, the injection was likely too deep and must be repeated.
2. The 48–72 hour window (Return Visit): The test is read — meaning the injection site is physically examined — between 48 and 72 hours after the injection. This window is critical. If you come back too early (before 48h) or too late (after 72h), the reading is unreliable and the test must be repeated from the beginning.
3. Reading the result: The provider measures any induration (a firm, raised area of skin — not just redness) at the injection site in millimeters. Redness alone does not count. The induration diameter is then interpreted against cutoff thresholds that depend on your risk category (see the table below).

PPD Test Results: What the Numbers Mean

The result is reported as a measurement in millimeters (mm), which is then interpreted based on your individual risk profile. There is no universal positive cutoff — different thresholds apply to different people.

TB Skin Test vs. TB Blood Test (IGRA): How to Choose

The TB blood test (also called an IGRA — brand names QuantiFERON-TB Gold and T-SPOT) is an alternative to the skin test. If you've been vaccinated with BCG or need only one clinic visit, the blood test may be a better fit. Here is a direct comparison:

Who Needs a TB Skin Test?

TB skin testing is typically required in the following situations:

• Nursing and allied health students (most programs require annual or baseline testing)
• Healthcare workers at hire and sometimes annually
• Nursing home and long-term care staff
• K–12 school employees in many states
• Immigration medical examinations
• Persons with recent exposure to someone with active TB
• Individuals with HIV or other immune-compromising conditions

Where to Get a TB Skin Test

TB skin tests are available at health departments (often low-cost or free), urgent care clinics, occupational health clinics, primary care offices, and travel medicine clinics. If you need documented results quickly for a school or employer, SchoolTiters offers the skin test at $79.88 with physician order included and compliance-ready documentation.

What to Expect at Your Appointment

1. Visit 1 — Injection: Roll up your sleeve. The provider cleans the inner forearm with alcohol, then injects PPD tuberculin just under the skin surface. A small raised bump (wheal) forms immediately. The appointment takes under 5 minutes.
2. The wait: Avoid scratching or rubbing the injection site. You can shower and go about normal activities. Some mild itching or redness is normal and not a positive result.
3. Visit 2 — Reading (48–72 hours later): Return to the clinic. The provider uses a ruler to measure any induration (raised lump). They record the mm measurement and tell you whether it meets the positive threshold for your risk category.

False Positives and False Negatives

False positives can be caused by prior BCG vaccination or infection with nontuberculous mycobacteria (NTM). False negatives can result from very recent TB infection (within 8–10 weeks), severe immunosuppression, incorrect injection technique, or reading the test outside the 48–72 hour window.

The 2-Step TB Skin Test

Some healthcare employers require a 2-step TB skin test for new hires who lack a documented prior TST. Two tests are administered 1–3 weeks apart to detect a booster effect in people with waned immune memory. The 2-step result establishes a baseline; future annual tests are single-step.

TB Skin Test Frequently Asked Questions

Can I shower after a TB skin test?

Yes. Normal water exposure will not affect the result. Avoid rubbing or applying lotions or bandages to the site.

Does a positive TB skin test mean I have TB disease?

No. A positive TST means your immune system has been exposed to TB bacteria — it does not mean you have active TB disease. Most positive reactors have latent TB infection (LTBI), which is not contagious and is treated with preventive antibiotics. A positive TST is always followed by a chest X-ray and clinical evaluation.

Is the TB skin test the same as a PPD test?

Yes. PPD stands for purified protein derivative — the reagent used in the test. TST, Mantoux test, and PPD test all refer to the same procedure.

Can I get a TB skin test if I've been vaccinated with BCG?

You can, but the result may be falsely positive. If you received BCG vaccine (common in many countries outside the U.S.), the TB blood test (IGRA/QuantiFERON) is preferred.

What happens if I miss my 72-hour reading window?

The result is unreliable and the test must be repeated from the beginning with a new injection.