From the Guidelines
A negative RPR 1:1 with a negative treponema antibody test typically indicates no evidence of syphilis infection. The RPR (Rapid Plasma Reagin) is a non-treponemal test that can sometimes yield low-positive results (1:1 titer) due to biological false positives from other conditions like autoimmune diseases, pregnancy, or recent infections, as noted in the 2018 update by the Infectious Diseases Society of America and the American Society for Microbiology 1. When this occurs but the more specific treponema antibody test is negative, it confirms that the RPR result is likely a false positive rather than true syphilis.
Key Points to Consider
- The two-test approach (non-treponemal plus treponemal) is used precisely because it helps distinguish true infections from false positive screening results, as outlined in the guide to utilization of the microbiology laboratory for diagnosis of infectious diseases 1.
- If there were clinical concerns about very early infection, a repeat test in 2-4 weeks might be considered, but generally, these results indicate no syphilis is present.
- The treponema-specific tests are highly sensitive and specific for syphilis infection, making a negative result with a low-positive RPR highly reassuring.
Clinical Implications
- No treatment for syphilis is needed in this case, given the negative treponema antibody test and the low-positive RPR result.
- Clinicians should be aware of the potential for false-positive RPR results and the importance of confirming with a treponemal test to guide management decisions, as emphasized in the 2018 update 1.
From the Research
Interpretation of RPR and Treponema Antibody Test Results
- A Rapid Plasma Reagin (RPR) test result of 1:1 indicates a low level of non-treponemal antibodies in the blood, which may suggest a past or current infection with Treponema pallidum, the bacterium that causes syphilis 2, 3.
- A negative Treponema antibody test result indicates that the test did not detect the presence of treponemal antibodies, which are specific to Treponema pallidum 2, 4.
- The combination of a low RPR titer (1:1) and a negative Treponema antibody test result may be seen in several scenarios, including:
- Early primary syphilis, where the non-treponemal test (RPR) may be positive before the treponemal test becomes positive 5.
- Late latent syphilis or syphilis of unknown duration, where the non-treponemal test (RPR) may be positive at a low titer, but the treponemal test is negative 3.
- False-negative treponemal test result, which can occur in some cases, especially in early infection or in patients with certain medical conditions 4.
Clinical Implications
- The interpretation of RPR and Treponema antibody test results should be done in the context of the patient's clinical presentation, medical history, and risk factors for syphilis 2, 3.
- A single test result should not be used to diagnose or exclude syphilis, and repeat testing may be necessary to confirm the diagnosis or to monitor treatment response 3, 6.
- The use of non-treponemal tests (e.g., RPR) and treponemal tests (e.g., Treponema antibody test) in combination can provide a more accurate diagnosis of syphilis and help guide treatment decisions 2, 3, 4.