Which group is at risk and should be tested with a PPD greater than 10 mm?

The correct choice reflects the guidelines for tuberculosis (TB) testing using the purified protein derivative (PPD), particularly for populations at risk. A PPD test greater than 10 mm is recommended for individuals who have certain risk factors that increase their susceptibility to latent TB infection or progression to active disease.

Injecting drug users known to be HIV negative are considered a high-risk group for TB, especially in areas with higher incidence rates of the disease. Substance use can lead to increased exposure to TB, either through close contact with infected individuals or through other risk behaviors that could compromise health or hygiene standards.

On the contrary, individuals without risk factors typically do not require PPD testing unless they present with symptoms or have had significant exposures. Healthy children under the age of four are also generally seen as lower risk for TB and would not need testing unless they have had significant exposure or other risk factors. Healthcare workers, while they face occupational exposure risks, are usually evaluated using a different threshold based on their specific risk of exposure to TB.

Thus, acknowledging the particular vulnerability of injecting drug users, especially in the context of the guidelines, underscores the rationale for recommending a PPD test greater than 10 mm for this group.