What laboratory test is the diagnostic test of choice for primary syphilis?

The diagnostic test of choice for primary syphilis is darkfield microscopy. This method allows for the direct visualization of Treponema pallidum, the bacterium responsible for syphilis, from a sample taken from a primary chancre. Darkfield microscopy is particularly useful because it can identify the organism in its active form, providing immediate results. This is crucial in the context of primary syphilis, where lesions may be present, allowing for a definitive diagnosis based on direct observation rather than relying solely on serological tests.

While other tests like the rapid plasma reagin and enzyme-linked immunosorbent assay are valuable in diagnosing syphilis, particularly in its later stages or for screening purposes, they are not specific to primary syphilis and may not provide timely results. Fluorescent antibody tests can also assist in diagnosing syphilis but are generally not the first-line choice for primary infection. Therefore, darkfield microscopy is the preferred modality when directly assessing the presence of Treponema pallidum at the site of an active primary lesion.