Basic and Clinical Sciences (BCSE) Practice Exam

In a tuberculosis skin test (TST), the substance injected to assess for infection is PPD, which stands for purified protein derivative. This is a standardized extract derived from the culture of Mycobacterium tuberculosis. The purpose of injecting PPD intradermally is to provoke an immune response in individuals who have been sensitized to the tubercle bacillus, indicating either a current or past infection with tuberculosis.

When PPD is introduced into the skin, if the individual has been exposed to the tuberculosis bacteria, their immune system will recognize the proteins in PPD and initiate an inflammatory response. This reaction is measured by the induration (swelling and hardness) that develops at the injection site after 48 to 72 hours. A significant induration suggests a positive reaction, indicating that the individual has been exposed to the bacteria at some point, whether the infection is active or latent.

Other options, such as live bacteria or antibiotic solutions, are not used in this test because they do not provide a safe or accurate method for assessing tuberculosis exposure. Live bacteria could cause active infection, and antibiotics would interfere with any potential immune response that the test aims to measure. Saline solution is simply a neutral substance that would not provide any indication of