The Mantoux tuberculin skin test, also known as the purified protein derivative (PPD) test, is a primary screening method used to detect a prior infection with the bacteria that causes tuberculosis (TB). This test is frequently required for employment in healthcare, school enrollment, immigration purposes, or following contact with an infected individual. This guide provides information on reliable testing sites and the precise two-visit procedure required for accurate results.
Understanding the Mantoux Test
The Mantoux test screens for the presence of Mycobacterium tuberculosis infection, which often exists in a non-active, or latent, state. The procedure involves injecting a small amount of PPD, a sterile protein derivative of the TB bacteria, just beneath the top layer of skin on the forearm. The body’s immune system recognizes this protein if it has been previously exposed to the bacteria. Since the test does not introduce live bacteria, there is no risk of contracting TB from the injection itself. A reaction to the PPD suggests the body has mounted an immune response, indicating an infection, though it does not confirm active disease.
Common Testing Locations
Finding a location for a Mantoux test depends on the urgency of the requirement and whether you have an existing primary care provider. Your family doctor is a convenient option for established patients, though appointments may be required days or weeks in advance. Occupational health services or employee health departments often administer the test directly if it is required for employment, particularly in high-risk environments like healthcare.
Local public health departments and community health centers are reliable resources for screening, often providing the test at a low cost or free of charge. Urgent care centers and retail clinics, such as those found within large pharmacies, may also offer the PPD test for quicker access. Always call any testing location beforehand to confirm the test is offered and to verify their specific hours for both the initial injection and the required follow-up reading.
The Two-Step Testing Procedure
The Mantoux test requires two separate visits to a healthcare provider to be considered valid and complete. During the first visit, a trained professional administers a precise dose of 0.1 milliliter of PPD solution into the inner surface of the forearm. A correctly performed injection creates a small, pale, raised blister, known as a wheal, which usually disappears within a few hours. Following the injection, the patient must keep the site uncovered and avoid scratching or scrubbing the area.
The second visit must occur within a specific window of 48 to 72 hours after the injection; failure to return within this timeframe invalidates the test. During this follow-up, a healthcare professional visually inspects and physically palpates the injection site. They are looking for induration, which is a firm, raised, hardened area of swelling, not merely redness or bruising.
Interpreting Your Results
The test result relies entirely on the measured diameter of the induration, which is measured in millimeters across the forearm, not the surrounding redness. A negative result means there is no significant induration, suggesting the individual has not been exposed to the TB bacteria. A positive result is determined by a specific threshold of induration that varies based on the individual’s risk factors for infection.
For individuals with no known risk factors, an induration of 15 millimeters or more is considered positive. For high-risk groups, such as those with HIV or recent close contact with an active TB case, an induration of 5 millimeters or more is considered positive. A positive result suggests latent TB infection but does not confirm active disease. If the test is positive, the physician will typically recommend a chest X-ray or a blood test, known as an Interferon Gamma Release Assay (IGRA), for further evaluation.