What is considered a positive PPD test for HIV infected persons or individuals in contact with a known case of TB?

A positive PPD test, also known as the Tuberculin skin test (TST), is measured by the induration that develops at the site of the injection after 48 to 72 hours. For individuals who are HIV-infected or have had close contact with someone who has active tuberculosis (TB), a reading of 5 mm or greater is considered positive. This lower threshold is due to the fact that HIV infection can impair the immune response, making it more difficult for individuals to mount a significant reaction to the PPD test.

In contrast, for individuals with no known risk factors or those with a healthy immune system, a PPD of 10 mm or more is typically deemed positive. For certain populations, such as high-risk individuals or those with underlying health conditions, even higher thresholds are necessary to determine a positive result. Thus, the criteria for interpreting the PPD test must take into account the individual's risk factors and overall health status, especially in the case of immunocompromised persons such as those living with HIV.