What I Have Learned About Infectious Diseases With My Sleeves Rolled Up

Karen L. Roos, MD


Semin Neurol. 2002;22(1) 

In This Article

The Most Common Reason for a False-Positive VDRL Test is Old Age

Present recommendations are that all patients with positive serology undergo CSF examination for neurosyphilis. The importance here is in defining "positive serology." A presumptive diagnosis of syphilis can be made from a nontreponemal test, such as the VDRL test and the rapid-plasma-reagin (RPR) test. These use cardiolipin-lecithin-cholesterol antigens to detect antibodies to Treponema pallidum.[16] Antibody titers measured by these reaginic tests vary during the natural history of untreated syphilis. They become detectable shortly after patients are infected and are at their highest levels during the secondary or early latent stages. Even without treatment antibody levels can decline, and in one fourth of untreated patients the VDRL becomes nonreactive.[16] Of note, the results of the VDRL and RPR tests may vary by one serial dilution on repeated testing, so titer changes of less than two serial dilutions (a fourfold change) are rarely significant.[16] For example, a change in the RPR from 1:2 to 1:4 is not significant. A change in the RPR from 1:2 to 1:8 is a meaningful change.

The use of only a nontreponemal test is insufficient for diagnosis because false-positive nontreponemal test results may occur. Most false-positive VDRL tests have a low titer of 1:8 or less. Ten percent of elderly individuals age 80 or older have a low titer false-positive VDRL test. Reactive nontreponemal-antibody tests are confirmed with one of the treponemal tests. These tests consist of the fluorescent-treponemal antibody absorption (FTA-ABS) test and the microhemagglutination assay for antibody to T. pallidum. Testing should be done initially with a nontreponemal test followed by a treponemal test when the nontreponemal test is positive. All patients with a positive treponemal test should undergo spinal fluid analysis. In neurosyphilis, the CSF leukocyte count is usually elevated (>5 WBCs/mm3), and there is a pleocytosis of mononuclear cells. The CSF VDRL when reactive, and in the absence of substantial contamination of CSF with blood, is diagnostic of neurosyphilis. The CSF VDRL test is very specific. The VDRL will be falsely positive only if blood contamination is sufficient to tinge the CSF pink.[16] The CSF FTA-ABS test is less specific but highly sensitive for the diagnosis, and a negative CSF FTA-ABS excludes the diagnosis of neurosyphilis.[16] In the absence of a reactive CSF VDRL, an elevated CSF white blood cell count and protein concentration and a reactive serum treponemal test are sufficient evidence to treat for neurosyphilis. Penicillin is the only antibiotic with documented efficacy for neurosyphilis. The Centers for Disease Control and Prevention recommends aqueous penicillin G 3 to 4 million units every 4 hours (18 to 24 million units per day) intravenously for 10 to 14 days for the treatment of neurosyphilis. Alternatively, procaine penicillin, 2.4 million units/d intramuscularly, plus oral probenecid, 500 mg four times daily for 10 to 14 days, can be used for treatment. The 4-hour interval is recommended to achieve consistent treponemicidal concentrations in CSF. The Centers for Disease Control and Prevention recommends that patients with a penicillin allergy be desensitized and treated with penicillin.[17]


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