A PPD (Purified Protein Derivative) test screens for exposure to tuberculosis (TB) bacteria. It involves injecting a small amount of tuberculin, a sterile extract, under the skin. The body’s immune response, a raised, firm area, indicates potential prior infection. Understanding how to read these results is important for identifying possible infection and ensuring timely medical guidance.
Initial Visual Cues
Approximately 48 to 72 hours after the PPD injection, inspect the test site on the forearm. Look for any reaction, including redness, swelling, or a raised area. Redness alone does not indicate a positive result. The key indicator is induration: a firm, raised, or hardened area at the injection site. This signifies a reaction if TB bacteria have been present.
Accurate Measurement Techniques
Accurate measurement of the induration is crucial. Use a flexible ruler, focusing solely on the induration’s diameter (the firm, raised area), not surrounding redness or swelling. Measure across the forearm, perpendicular to its long axis. To define the edges, gently sweep fingertips over the forearm, then place the “0” mark at one edge and read the measurement at the opposite edge. If the measurement falls between two millimeter divisions, record the smaller number for accuracy; record zero millimeters if no induration is present.
Interpreting Induration Sizes
The interpretation of the induration size depends on an individual’s specific risk factors for tuberculosis. A positive result does not necessarily mean active TB disease, but rather indicates exposure to the bacteria. Further tests, such as a chest X-ray, are typically needed to determine if active disease is present. The Centers for Disease Control and Prevention (CDC) provides guidelines for different thresholds.
5 mm or More
An induration of 5 millimeters (mm) or more is considered positive for individuals with heightened risk factors. These include people with HIV, recent close contacts of TB patients, organ transplant recipients, and those with chest X-ray findings suggestive of prior TB infection. This threshold also applies to individuals with suppressed immune systems due to medical conditions or medications.
10 mm or More
An induration of 10 mm or more is considered positive for several other groups. This includes recent immigrants from countries where TB is common, individuals who misuse drugs, and residents or employees in high-risk congregate settings like nursing homes or homeless shelters. Children under four years old and those with certain medical conditions such as silicosis, diabetes, or severe kidney disease, also fall into this 10 mm positive threshold.
15 mm or More
For individuals with no known risk factors for TB, an induration of 15 mm or more is considered a positive reaction. A positive PPD test in someone who has received the BCG vaccine may be a false positive, though BCG vaccination history should not automatically exclude further evaluation for TB infection.
What to Do After Reading
After you have measured and noted your PPD test result, it is important to communicate this information to your healthcare provider. Record the exact measurement of the induration in millimeters. Even if the result appears negative, or if you have any questions, contacting your doctor is a necessary next step. Your healthcare provider will interpret the result in the context of your medical history and risk factors. For a positive result, further evaluations like a chest X-ray or sputum tests may be ordered to determine if you have active TB disease or latent TB infection; if the result is negative, it typically means you have not been infected with TB bacteria, but your provider may still want to document it.