Individuals showing a positive purified protein derivative (PPD) skin test for M. tuberculosis are usually which of the following?

A positive purified protein derivative (PPD) skin test indicates that an individual has been exposed to the Mycobacterium tuberculosis bacteria and has developed an immune response to it. This test does not determine if the individual is currently infected or contagious. Instead, it reflects a prior exposure, which means that the individual may be latently infected with the bacillus. Latent tuberculosis infection (LTBI) is when a person carries the bacteria without showing symptoms or being infectious, as the bacteria remains dormant in the body.

In contrast, being infective would imply that the individual can spread the bacteria to others, which is not the case in latent infections. Symptomatic pulmonary disease would indicate active tuberculosis, which requires further clinical evaluation beyond the skin test to confirm. A false positive can occur due to several factors, such as prior vaccination with the Bacillus Calmette-Guérin (BCG) vaccine or cross-reactivity with non-tuberculous mycobacteria, but true positive results indicate the presence of latent infection rather than invalidity of the test. Thus, those who show a positive PPD skin test are typically latently infected with M. tuberculosis.