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Interpreting conflicting syphilis tests has been an active subject recently on the EIN listserve, in part because of recent changes in procedures at testing laboratories.

In one recent discussion, an EIN member was referred a patient with no known risk factors for syphilis who tested positive when she tried to donate blood. A rapid plasma reagin (RPR) nontreponemal test was negative, but fluorescent treponemal antibody (FTA) and microhemagglutination assay (MHA-TP) treponemal tests were positive. The member asked if the two treponemal tests were likely false positives.

Some members said the results could be false positives and the patient might have a connective-tissue disease, or possibly Lyme disease rather than syphilis. Or they could be due to another treponemal infection. Another possibility is that the test results reflect previous asymptomatic or congenital syphilis that resolved without treatment, or antibiotics given for some other reason may have cleared the infection. “One thing to remember is that if a child has congenital syphilis, the FTA would remain reactive for life despite treatment,” one respondent said.

However, others suggested the results could be a true positive test and the patient should be treated for latent syphilis. “My approach would be to carefully examine the patient for evidence of active syphilis, with particular attention to the neurological examination,” one respondent said. Lumbar puncture is probably not necessary unless signs or symptoms are present. “If all were normal, I would treat her with benzathine penicillin as recommended for late latent syphilis, then follow her with repeat syphilis serology annually.”

The Centers for Disease Control and Prevention (CDC)—and several EIN respondents—have noted that conflicting treponemal and nontreponemal tests are becoming more common because of changes in the testing algorithm at many commercial testing laboratories. Until recently, the nontreponemal RPR test was performed first. If this test was positive, a treponemal test was performed to confirm the diagnosis. However, with the introduction of low-cost treponemal enzyme immunoassays (EIA), many commercial laboratories have reversed the procedure.

“Unfortunately,” one member noted, “the change in testing procedures has not yet been accompanied by sufficient epidemiologic and clinical research to provide a clear understanding of the true spectrum of outcomes” of these tests, including “the mix of inactive/cured syphilis, latent but potentially active disease, or even false positive results that don’t indicate past treponemal infection at all.”

CDC made recommendations for dealing with these test results in an editorial note to a recent Morbidity and Mortality Weekly Report, including a recommendation to offer treatment to patients with two positive treponemal tests. EIN recently conducted a survey on syphilis to help inform CDC as it works on revising its syphilis guidelines. Revised guidelines are expected in early 2010.

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The Emerging Infections Network (EIN) is a provider-based sentinel network designed to help the public-health community detect trends in emerging infectious diseases.

A joint project of IDSA and the Pediatric Infectious Diseases Society (PIDS) and supported by funding from the Centers for Disease Control and Prevention, EIN tracks emerging infectious diseases and keeps the public-health community up to date with issues that are currently affecting or may soon affect members’ clinical practices.The Network provides a great opportunity for members to share knowledge quickly across large geographical distances. Both IDSA and PIDS members are eligible to join. The EIN listserve allows members to discuss new disease trends and difficult cases. Click here for more information or to join EIN.

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