The Purified Protein Derivative (PPD) test, commonly known as the Tuberculin Skin Test (TST) or Mantoux test, is a standardized method used to screen for latent tuberculosis (TB) infection. This screening tool does not indicate if you have active TB disease but rather detects if your body has been exposed to the Mycobacterium tuberculosis bacteria. The test involves an immune reaction that checks for sensitivity to a protein derived from the bacteria.
Primary Testing Locations
A variety of medical and public health facilities offer the PPD test, often depending on whether the test is required for employment, school, or routine screening. Your primary care physician’s office is a reliable option, providing continuity of care and the ability to schedule both the injection and the required follow-up reading. You should call ahead to ensure they stock the tuberculin solution and can accommodate the two necessary appointments.
Urgent care centers and walk-in clinics offer greater convenience and flexibility, often accommodating same-day testing without a prior appointment. These locations are useful for individuals needing the test quickly for occupational or academic clearance. For those seeking lower-cost options, local public health departments often provide TB screening services, sometimes free or at a reduced fee, as part of community health initiatives.
Many large employers or educational institutions maintain Occupational Health or Employee Health Services, which administer the PPD test to employees or students as a mandatory screening measure. Regardless of the chosen location, it is essential to confirm the facility can administer the injection and perform the reading within the required 48 to 72 hours.
The PPD Test Procedure
The PPD test is an intradermal injection, meaning the substance is administered into the top layer of the skin, not into a vein or muscle. A healthcare professional injects a small amount, typically 0.1 milliliters, of the purified protein derivative solution into the inner surface of the forearm. This injection uses a fine-gauge needle and should create a slight, pale elevation on the skin, known as a wheal, which indicates proper delivery.
The first visit is solely for the administration of the solution, and the test site should not be rubbed, scratched, or covered with a bandage. The immune system’s reaction requires time to develop, making the second visit mandatory within a specific timeframe. You must return to the clinic for the results to be measured between 48 and 72 hours after the injection.
The professional reading determines the result. If you return outside of the 48-to-72-hour window, the immune response may have waned or become inaccurate, making the test result invalid. In this situation, the entire procedure, including the injection, must be repeated to obtain a reliable result.
Understanding Test Results and Next Steps
The healthcare provider reads the test by measuring the diameter of the induration—the firm, raised, and hardened area at the injection site. This measurement is taken across the forearm and is recorded in millimeters; surrounding redness (erythema) is not measured. The criteria for a positive result are not uniform and depend heavily on an individual’s specific risk factors for TB infection.
For individuals with a compromised immune system, recent close contact with an active TB case, or evidence of old TB on a chest X-ray, an induration of five millimeters or more is considered a positive result. A measurement of ten millimeters or more is generally considered positive for people who have recently immigrated from high-prevalence countries, those who work in high-risk settings, or children under four. For all others with no known risk factors, a positive result requires a larger induration, typically fifteen millimeters or more.
A positive PPD test only indicates exposure to the TB bacteria, not that the person has active, contagious disease. If the test is positive, further evaluation is required to differentiate between latent TB infection and active TB disease. This typically includes a chest X-ray and may involve a blood test, such as an Interferon-Gamma Release Assay (IGRA), to confirm the infection and determine the appropriate treatment plan.