From the Guidelines
No, a positive Rapid Plasma Reagin (RPR) test alone is not sufficient to diagnose syphilis. While the RPR is a useful screening test, it can produce false-positive results due to various conditions including autoimmune diseases, pregnancy, viral infections, and other inflammatory states 1. A positive RPR test should always be confirmed with a more specific treponemal test such as the Treponema pallidum particle agglutination (TP-PA), fluorescent treponemal antibody absorption (FTA-ABS), or various enzyme or chemiluminescence immunoassays.
This two-step approach is necessary because the RPR detects non-specific antibodies that react to cardiolipin rather than directly to the Treponema pallidum bacterium that causes syphilis. Some key points to consider in the diagnosis of syphilis include:
- The use of only one type of serologic test is insufficient for diagnosis, as false-positive nontreponemal test results may occur secondary to various medical conditions 1.
- Many high-volume clinical laboratories have reversed the testing sequence and begin the testing algorithm first with a specific treponemal test, such as an EIA or chemiluminescence immunoassay, and then retesting reactive results with a non-treponemal test, such as RPR, to confirm diagnosis 1.
- If the follow-up confirmation test (RPR) is negative, it requires the laboratory to perform a different treponemal-specific test to guide management decisions (ie, fluorescent treponemal antibody–absorbed) 1.
Once syphilis is properly diagnosed through both non-treponemal and treponemal tests, treatment typically involves penicillin G, with the specific regimen depending on the stage of infection. For primary, secondary, or early latent syphilis, a single intramuscular injection of benzathine penicillin G 2.4 million units is standard, while late latent or tertiary syphilis requires three weekly doses of the same amount. The two-step testing approach is crucial for accurate diagnosis and effective treatment of syphilis, as it helps to minimize false positives and ensure that patients receive the appropriate care 1.
From the Research
Diagnosis of Syphilis using RPR Test
- A positive Rapid Plasma Reagin (RPR) test is not sufficient to diagnose syphilis, as it can produce false-positive results 2, 3, 4, 5, 6.
- The RPR test is a non-treponemal test that detects antibodies against the syphilis bacteria, but it can also react with other antibodies, leading to false-positive results 3, 4, 6.
- Studies have shown that the sensitivity and specificity of the RPR test vary, and it is not reliable for diagnosing primary and late latent syphilis, especially in certain patient populations 2, 5.
Limitations of RPR Test
- The RPR test has a high false-positive rate, which can lead to misdiagnosis and unnecessary treatment 4, 6.
- The test is not suitable for screening purposes, especially in low-risk populations, due to its low positive predictive value 3, 5.
- The RPR test should be used in combination with treponemal tests, such as the Treponema pallidum hemagglutination assay (TPHA) or the fluorescent treponemal antibody absorption (FTA-ABS) test, to confirm the diagnosis of syphilis 2, 3, 4, 5, 6.
Diagnostic Approach
- A thorough physical examination and medical history should be taken to determine the stage of syphilis 6.
- A positive RPR test should be confirmed with a treponemal test to rule out false-positive results 2, 3, 4, 5, 6.
- Further testing, such as a lumbar puncture, may be necessary to rule out neurosyphilis in patients with a low-titer RPR test 6.