What Is the Tuberculosis (TB) Skin Test (PPD)? Complete Guide
What Is the Tuberculosis (TB) Skin Test (PPD)? Complete Guide
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:
- 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.
- 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.
- 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.
| Induration Size | Considered Positive For… |
|---|---|
| ≥ 5 mm | HIV-positive individuals; recent close contacts of TB case; organ transplant/immunosuppressed patients; chest X-ray showing prior TB |
| ≥ 10 mm | Recent immigrants from high-prevalence countries; injection drug users; residents/employees of high-risk congregate settings (prisons, nursing homes, shelters); mycobacteriology lab workers; children under 4; children exposed to high-risk adults |
| ≥ 15 mm | All other persons (no known risk factors) |
TB Skin Test vs. TB Blood Test (IGRA): How to Choose
The TB blood test (also called an IGRA, or Interferon Gamma Release Assay — brand names QuantiFERON-TB Gold and T-SPOT) is an alternative to the skin test. Here is a direct comparison:
| Feature | TB Skin Test (TST/PPD) | TB Blood Test (IGRA) |
|---|---|---|
| Visits required | 2 (injection + reading) | 1 (blood draw only) |
| Affected by BCG vaccine | Yes — BCG can cause false positive | No — not affected by BCG |
| Result time | 48–72 hours | 24–48 hours (blood processing) |
| Cost | $40–$100 | $100–$200+ |
| Accepted for school/healthcare compliance | Yes (most programs) | Yes (most programs) |
| Best for BCG-vaccinated individuals | No | Yes |
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
- Refugees and immigrants from TB-endemic countries
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/family medicine offices
- Travel medicine clinics
- Dedicated testing services (like School Titers — order online, walk in at a local lab partner)
Prices range from $40 to $100 depending on location. Some public health departments offer free testing for uninsured individuals.
What to Expect at Your Appointment
Here is a step-by-step of the patient experience:
- 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.
- 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.
- 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 (positive result without true TB infection) can be caused by:
- Prior BCG vaccination (common outside the U.S.)
- Infection with nontuberculous mycobacteria (NTM)
False negatives (negative result despite true TB infection) can be caused by:
- Recent TB infection (within 8–10 weeks — immune response not yet mounted)
- Very old or very young age
- Severe immunosuppression (HIV, chemotherapy, etc.)
- Incorrect injection technique
- Improper reading (after 72 hours)
- Improper tuberculin storage
The 2-Step TB Skin Test
Some employers — particularly in healthcare — require a 2-step TB skin test for new hires who do not have a documented prior TST result. The purpose is to detect the booster effect: in people who were infected with TB long ago (or vaccinated with BCG), the immune memory may have waned enough that the first TST gives a false negative. A second TST 1–3 weeks later “boosts” the immune response and produces a positive result if true exposure occurred.
The 2-step process:
- Step 1: TST administered and read at 48–72 hours.
- If Step 1 is negative: Step 2 TST is administered 1–3 weeks later and read at 48–72 hours.
- If Step 2 is positive: This is interpreted as a boosted reaction, not a new conversion — the person is considered to have had prior exposure.
- If Step 2 is also negative: The person is considered truly TB-negative at baseline.
The 2-step result serves as the baseline. Future annual tests are single-step, and a new positive result is then interpreted as a true conversion (new infection).
TB Skin Test Frequently Asked Questions
Can I shower after a TB skin test?
Yes. Normal water exposure (showering, handwashing) 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 due to cross-reactivity with BCG. If you received BCG vaccine (common in many countries outside the U.S.), the TB blood test (IGRA/QuantiFERON) is preferred.
How long does the TB skin test take?
The injection itself takes under 5 minutes. You then return in 48–72 hours for reading. Total time commitment: two brief clinic visits over 2–3 days.
What happens if I miss my 72-hour reading window?
If you miss the 48–72 hour window, the result is unreliable and the test must be repeated from the beginning (a new injection).
