Which test is most appropriate for diagnosing late latent or tertiary syphilis?

The test that is most appropriate for diagnosing late latent or tertiary syphilis is the FTA-ABS (Fluorescent Treponemal Antibody Absorption) test. This is a treponemal test that detects antibodies specific to Treponema pallidum, the bacterium that causes syphilis. FTA-ABS is highly sensitive and specific, making it an ideal choice for confirming syphilis, particularly in cases where the disease is in its later stages, such as late latent or tertiary syphilis.

In late latent and tertiary stages, non-treponemal tests (like RPR and VDRL) may yield false-negative results due to lower antibody levels. Non-treponemal tests are useful for screening and monitoring, but they are not as reliable for definitive diagnosis in advanced stages of the disease. The FTA-ABS, however, remains positive in these later stages, ensuring accurate identification of the infection.

A Pap smear is irrelevant in this context as it is primarily used for cervical cancer screening and does not have any diagnostic value for syphilis. Thus, using FTA-ABS provides a reliable diagnosis for late latent or tertiary syphilis.