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Syphilis in Pregnancy
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4. Use of Serologic Tests for Syphilis
There are two types of serologic tests for syphilis,
non-treponemal tests (VDRL and RPR), and treponemal tests (FTA-ABS and TP-PA). The former are screening tests, but the diagnosis of syphilis must be confirmed with the latter because false positives do occur (more on this later). Non-treponemal test titers usually correlate well with disease activity as well as response to treatment. A fourfold (“two-tube”) change in titer (e.g., 1:4 to 1:16 or, 1:32 to 1:8) is considered significant. Sequential titers should be performed using the same method (VDRL or RPR) because quantitative results from the two tests cannot be compared directly. Non-treponemal tests may become negative if the disease is treated early, however they may remain positive at low titers for the life of the patient (“serofast”), especially if the disease is not treated until the latent phase. On the other hand, treponemal tests almost always remain positive for the life of the patient, regardless of whether the patient is successfully treated, and therefore they cannot be used to assess disease activity or treatment response.
Who needs a spinal tap?
Neurosyphilis implies syphilitic infection of the central nervous system. It may be found in any stage of syphilis, not just tertiary syphilis. In the pre-penicillin era it was common to see patients with advanced neurologic findings, such as meningitis, tabes dorsalis, or general paresis with dementia. Cognitive dysfunction, cranial nerve signs, and uveitis are signs of less advanced disease. In the current era however, it is usually diagnosed on the basis of cerebrospinal fluid (CSF) analysis. CSF abnormalities include pleocytosis (>10 cells/microliter), elevated protein (>45 mg/dL), and a positive serologic test (the CSF-VDRL or RPR is more specific, and the CSF-FTA is more sensitive, so both should be ordered on CSF specimens). Patients who have been treated for any stage of syphilis who have not had a fourfold decrease in titer in after appropriate treatment, or who remain “serofast” at a titer >1:32 six months after appropriate treatment, are candidates for lumbar puncture. This should be done to rule out occult neurosyphilis because the treatment regimen for neurosyphilis needs to be more intensive. Patients (without neurologic signs or concomitant HIV infection) whose titers regress four-fold as expected after treatment, need not be subjected to this investigation.
