Many people mistakenly believe the PPD shot is a vaccine for tuberculosis, but it is not an immunization. The PPD shot is a diagnostic tool called the Tuberculin Skin Test (TST). It is used to determine if a person’s immune system has been exposed to the bacteria that cause tuberculosis and serves as a first step in screening for the disease.
The PPD Skin Test Procedure
The PPD skin test, also known as the Mantoux test, begins with a simple injection. A healthcare provider cleans a section of skin on the inner forearm and injects a small, standardized dose of 0.1 mL of tuberculin purified protein derivative (PPD) just under the top layer of skin. This substance is a protein extract from Mycobacterium tuberculosis, the bacterium that causes TB. The injection creates a small, pale bump that disappears within a few hours as the fluid is absorbed.
A follow-up visit is required between 48 and 72 hours after the test was administered. During this second visit, a trained professional examines the injection site to read the results. They look for a specific reaction known as induration, which is a palpable, raised, and hardened area of skin. The professional measures the diameter of this induration in millimeters to determine the test’s outcome.
Interpreting Test Results
A negative result, with no induration or only a very small one, means the person has likely not been infected with TB bacteria. However, medical conditions or medications that weaken the immune system can cause a false-negative result, where an infected person does not react to the test.
A positive result is an induration that meets a size threshold based on an individual’s risk factors. For instance, a smaller induration might be considered positive in someone with a compromised immune system. A positive PPD test does not mean a person has active, contagious tuberculosis; it only signifies exposure to the TB bacteria.
This exposure can lead to two states: latent TB infection or active TB disease. In latent TB, the bacteria are present but inactive, causing no symptoms and being non-contagious. In active TB disease, the bacteria are multiplying and causing illness.
Because the PPD test cannot distinguish between these states, a positive result requires further evaluation. Follow-up tests, such as a chest X-ray or a blood test, are necessary to determine if the infection is latent or active and to guide treatment.
The Actual Tuberculosis Vaccine (BCG)
The vaccine used to prevent tuberculosis is the Bacille Calmette-Guérin (BCG) vaccine. It is one of the most widely used vaccines in the world, administered to infants in countries with high rates of TB. The BCG vaccine is most effective at protecting young children from severe forms of tuberculosis, such as TB meningitis. It contains a live, weakened strain of Mycobacterium bovis, a bacterium related to the one that causes TB in humans.
The BCG vaccine is not routinely recommended in the United States. This is due to the low risk of TB infection in the country and the vaccine’s variable effectiveness in preventing adult pulmonary TB. Its limited impact on preventing the spread of infectious TB has made it a lower priority in low-prevalence nations.
A primary reason for not using the BCG vaccine in places like the U.S. is its interference with PPD test results. The vaccine creates an immune response that can cause a person to have a reaction to the PPD skin test, even if they have never been infected with Mycobacterium tuberculosis. This false-positive reaction complicates screening, making it difficult to distinguish between a vaccinated individual and someone with a true TB exposure.