A PPD test, which stands for Purified Protein Derivative, is a common skin test designed to screen for tuberculosis (TB) infection. This test helps determine if an individual has been exposed to the bacterium Mycobacterium tuberculosis, which causes TB, by detecting an immune response to its components. The PPD test is a simple method to identify individuals who may have a latent TB infection, meaning the bacteria are present in the body but are not yet causing active disease.
Common Places to Get a PPD Test
Individuals seeking a PPD test have several accessible options, ranging from their regular healthcare providers to specialized clinics:
Primary care physician’s offices: Offer convenience and integrated health records. Call ahead to confirm testing.
Urgent care centers: Provide walk-in services, extended hours, and quick initial injection.
Retail clinics: Located in pharmacies, they offer a convenient and accessible choice.
Local public health clinics: Often provide testing at reduced cost or free. Check their website for details.
Employers or educational institutions: May offer on-site testing for health and safety protocols.
Travel clinics: Administer tests for international travel requirements.
The PPD Test Process
The PPD test involves two distinct visits to a healthcare professional. During the first visit, a small amount of purified protein derivative solution, typically 0.1 milliliters, is injected just beneath the surface of the skin, usually on the inner forearm. This injection creates a small, temporary raised bump, known as a wheal, at the site. The administration of the test is a quick procedure, generally causing only a brief sting.
The second visit is for reading the test results and is equally important. The injection site must be examined by a trained healthcare professional approximately 48 to 72 hours after the initial administration. Returning within this specific window is essential for accurate interpretation of the reaction. If the test is not read within 72 hours, it may need to be repeated.
Interpreting Your PPD Test Results
Interpreting a PPD test involves assessing the size of the induration, which is the firm, raised area or swelling at the injection site, not just any redness. A negative result means there is no or very minimal induration, indicating the individual has not been infected with the bacteria that cause TB.
A positive result is identified by a specific size of induration, though the threshold varies based on individual risk factors. For individuals with compromised immune systems, recent close contact with a TB patient, or organ transplant recipients, an induration of 5 millimeters or more can be considered positive. For other groups like healthcare workers or those from countries where TB is common, 10 millimeters or more may indicate a positive result. In people with no known risk factors for TB, 15 millimeters or more is considered positive.
What Happens After a Positive PPD Test
A positive PPD test result suggests latent TB infection (LTBI). The next steps involve further evaluation to determine if the infection is latent or has progressed to active TB disease.
This includes a chest X-ray to check for signs of active disease in the lungs. Sputum cultures may also be performed to confirm active TB. If active disease is ruled out, treatment for latent TB infection is recommended to prevent the bacteria from becoming active and causing illness. Treatment for LTBI involves a course of antibiotics, like isoniazid, taken for several months to reduce the risk of developing active TB disease.