Which type of blood specimen is primarily analyzed for infectious disease screening?

Serum is the correct choice for blood specimens primarily analyzed for infectious disease screening due to its composition and the processes involved in its preparation. When blood is collected and allowed to clot, the liquid component that remains once the blood cells and clot are removed is known as serum. This matrix is rich in antibodies, which are crucial for detecting infections and diagnosing diseases.

In infectious disease screening, tests such as enzyme-linked immunosorbent assays (ELISA) or other serological tests are commonly performed on serum. These tests are designed to identify specific antibodies produced in response to infections or identify pathogens directly, making serum the most suitable specimen type for these analyses.

In contrast, whole blood includes both cellular components and the liquid portion, making it less efficient for tests that require specific components like antibodies. Plasma, while also rich in some proteins and containing clotting factors, is typically used for different types of tests that require those factors, such as coagulation studies. Packed cells are primarily used for transfusion purposes and do not contain the liquid medium needed for infectious disease testing. Thus, serum is the ideal choice for screening of infectious diseases.