What is the diagnostic test of choice for a patient presenting with a suspected syphilitic chancre on his penis?

Dark-field examination of the chancre is the most specific test for the diagnosis of syphilis. This test is typically positive unless the patient has applied or ingested antibiotics. In addition to primary syphilis, dark-field microscopy can also be used to diagnose all of the mucocutaneous lesions of secondary syphilis. However, it is less reliable for examining specimens from the mouth or rectum because of the high prevalence of commensal, nonpathogenic treponemes in these locations that may be mistaken for Treponema pallidum, the agent of syphilis. The best specimens for dark-field examination are serous fluid expressed from the bases of the chancre following cleaning with sterile saline and clean gauze. The specimen then should be immediately evaluated for the organism’s characteristic corkscrew morphology and flexing, hairpin motility. If a patient is suspected of having a syphilitic chancre and a darkfield evaluation is negative, it should be repeated at least once before the diagnosis is excluded.

Fluorescent antibody microscopy, although not widely available, offers a sensitive alternative and has the advantage that it does not require live organisms and it can be done on fixed slides. This technique utilizes antibodies to T. pallidum.