What is the confirmatory test for diagnosing syphilis?

The confirmatory test for diagnosing syphilis is the Fluorescent treponemal antibody absorption (FTA-ABS) test. This test is highly sensitive and specific for the Treponema pallidum bacteria, which causes syphilis. It detects antibodies that are generated in response to the infection, making it a reliable tool for confirming a syphilis diagnosis after initial screening tests, such as the Rapid Plasma Reagin (RPR), which is a non-treponemal test and can produce false positives.

Other methods such as dark field microscopy can visualize the spirochete directly from lesions, but it's not commonly used due to sensitivity and skill requirements. The penicillin skin test is not relevant in the context of syphilis diagnosis but is instead used to determine allergies to penicillin. Therefore, the FTA-ABS serves as the established confirmatory test that solidifies the diagnosis of syphilis after initial screening.