In which test for syphilis is antigen mixed with serum on a plastic-coated card?

The test that involves mixing antigen with serum on a plastic-coated card is the Rapid Plasma Reagin (RPR) test. This is a non-treponemal test used for syphilis screening, where the patient's serum is mixed with a card that contains cardiolipin antigen. The test is simple and results are typically obtained quickly, allowing for rapid screening and diagnosis of syphilis.

The RPR test detects nonspecific antibodies that are produced in response to the presence of cellular damage due to syphilis infection, rather than directly detecting the syphilis bacteria. If antibodies react with the antigen, visible agglutination occurs on the card, which indicates a positive result. This method's card format is especially practical for mass screenings as it allows for easy visualization of results.

In contrast, other tests such as the direct fluorescent antibody test, Western blot, and complement fixation test utilize different methodologies or reagents that do not involve a simple card system for mixing the serum with antigen. Therefore, the RPR test distinctly fits the description provided in the question.