There are four general modalities used in diagnostic testing for syphilis: histopathology, darkfield microscopy, serology, and CSF examination. Histopathology employing Warthin-Starry staining or with specific immunofluorescent antibody preparations is sometimes used on biopsy tissue of late syphilitic lesions if darkfield examination is unavailable. Otherwise, histopathology is largely reserved for autopsy material. Darkfield microscopy is considered the gold standard and quickest method of diagnosis with primary, secondary, and early congenital syphilis. Transudate from a moist primary chancre, condyloma latum, or mucous patch is examined under darkfield for the characteristic corkscrew appearance and spiraling motion. Aspirates from lymph nodes may also be useful, particularly in secondary syphilis. Oral lesions should not be used, as T. pallidum can be confused with nonpathogenic treponemes normally found in the oral cavity.
There are two types of serologic testing for syphilis: standard nontreponemal and specific treponemal antibody tests. Nontreponemal tests are quick and inexpensive, making them ideal for screening. The specific treponemal antibody tests are more specific for either current or past infection. Generally, a nontreponemal test is first used for screening and then infection is confirmed with a specific antibody test.
Four types of nontreponemal tests exist. These are all based on an antigen developed from the Venereal Disease Research Laboratory (VDRL), which consists of cardiolipin, cholesterol, and lecithin. They screen for IgG and IgM antibodies directed against this antigen, which is produced when host cells interact with T. pallidum. The antigen was originally generated from extracts of beef livers and hearts and was found to cross-react with antibodies produced with syphilis infection, albeit with many false-positives. The antigen has since been purified and is now more specific; however, false-positives may still be seen in association with pregnancy, other spirochete infections, multiple viral or bacterial infections, and various autoimmune diseases. Nonetheless, the tests remain 97% to 98% specific (2). In addition to the VDRL test, the other nontreponemal tests include the rapid plasma reagin (RPR), unheated serum reagin (USR), and toluidine red unheated serum test (TRUST). The nontreponemal tests do not turn positive until one to four weeks after development of a primary chancre. All of the tests are quantitative with the highest titers in secondary syphilis. In fact, particularly high antibody titers seen in secondary disease and pregnancy can produce a prozone phenomenon with false-negative testing. This can be corrected with serum dilution. Given the quantitative nature, the nontreponemal tests are quite useful in assessing response to treatment. After successful treatment, a nontreponemal assay should become nonreactive within one year in a case of primary syphilis or within two years when treating secondary disease. Tertiary syphilis may take up to five years before testing turns negative after adequate treatment (7).
In addition to the nontreponemal tests, there are several assays, which measure antibodies specific for T. pallidum. The two primary tests are the fluorescent treponemal antibody absorption (FTA-abs) and the microhemagglutination assay for antibodies to T. pallidum (MHA-TP). The MHA-TP is somewhat easier to use than the FTA-abs, but slightly less sensitive in early disease. There is no strong persuasion to use one test over the other, and both continue to be used throughout laboratories worldwide. These specific antibody tests are significantly more expensive than the nontreponemal tests and thus are generally only used for confirmation of the diagnosis. Once positive, the tests usually remain positive for life, and repeat testing is not indicated. However, about 10% of patients may revert to a nonreactive result after treatment.
If neurosyphilis is suspected, CSF examination can be used to confirm the diagnosis. CSF testing is not necessarily indicated for routine primary and secondary syphilis, although certain conditions may prompt it. These include treatment failure, evidence of tertiary disease including gumma or aortitis, HIV infection, or known neurologic, ophthalmic, or auditory symptoms. The most commonly employed test is the CSF-VDRL, which is highly specific though insensitive. A moderate mononuclear pleocytosis and elevated protein level in the CSF are also suggestive of neurosyphilis in the correct clinical setting. The FTA-abs test should not be used with CSF, as traces of blood may produce a false-positive result.
Diagnosis of otosyphilis remains somewhat problematic, especially in those patients with no known history of congenital or acquired syphilis. It is a diagnosis of exclusion, and has been defined as a positive serologic test for syphilis with otherwise unexplained hearing loss or vestibular disturbance (10). Confirmation as well as evaluation for active neurosyphilis can be achieved by CSF analysis including CSF-VDRL. The benefit of routine syphilis screening in patients with idiopathic sensorineural hearing loss is called into question by an investigation showing a positive result in only 1 of 182 patients (13). Other studies, however, have suggested otosyphilis to be the cause of 6.5% of previously unexplained sensorineural hearing loss and 7% of Meniere's disease (14,15). As syphilitic cochleovestibular dysfunction may be treatable, especially if early in the process, these statistics suggest screening may be worthwhile, especially in those patients with risk factors.
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