From the Guidelines
The RPR test and treponemal antibody tests like TPHA and FTA-ABS are complementary diagnostic tools for syphilis that provide different information, with the RPR test indicating active infection and its titer helping assess disease activity and treatment response, while treponemal tests usually remain positive for life after infection, regardless of treatment 1. The RPR test is a non-treponemal test that detects antibodies to cardiolipin, becoming positive 1-4 weeks after infection.
- Its titer (the highest dilution giving a positive result) helps assess disease activity and treatment response, with higher titers generally indicating active infection.
- After successful treatment, RPR titers typically decline and may eventually become negative. Treponemal tests like TPHA and FTA-ABS detect antibodies specific to Treponema pallidum and usually remain positive for life after infection, regardless of treatment.
- The standard diagnostic approach uses both test types: a positive non-treponemal test (RPR) followed by confirmation with a treponemal test.
- This combination helps distinguish between active infection, previously treated infection, and false positives. For monitoring treatment effectiveness, serial RPR titers are used, with a fourfold decrease (e.g., from 1:32 to 1:8) indicating successful treatment 1. False positive RPR results can occur with conditions like autoimmune diseases, pregnancy, or viral infections, which is why treponemal confirmation is essential for accurate diagnosis 1. It's also important to note that some patients may have a serofast reaction, where nontreponemal antibodies persist at a low titer for a long period, sometimes for the remainder of their lives 1. In terms of the testing sequence, many high-volume clinical laboratories have reversed the traditional sequence and begin with a specific treponemal test, such as an EIA or chemiluminescence immunoassay, and then retest reactive results with a non-treponemal test, such as RPR, to confirm diagnosis 1.
From the Research
RPR Test
- The RPR (Rapid Plasma Reagin) test is a non-treponemal test used to screen for syphilis 2, 3, 4, 5, 6
- It detects the presence of antibodies against the syphilis bacterium, but not the bacterium itself
- The RPR test can produce false-negative results, especially in early stages of syphilis or in patients with a history of syphilis infection 2
- The prozone phenomenon, where a high concentration of antibodies interferes with the test, can also lead to false-negative results 2
RPR Titer
- The RPR titer is a measure of the level of antibodies against syphilis in the blood 3, 4, 5
- A higher titer indicates a more recent or active infection, while a lower titer may indicate a past infection or false-positive result 4
- The RPR titer can be used to monitor the effectiveness of treatment and to detect reinfection 2
Treponemal Antibody Tests
- Treponemal antibody tests, such as TPHA (Treponema pallidum Hemagglutination Assay) or FTA-ABS (Fluorescent Treponemal Antibody Absorption), are used to confirm a syphilis diagnosis 3, 4, 5, 6
- These tests detect the presence of antibodies against the syphilis bacterium and are more specific than non-treponemal tests like the RPR test
- Treponemal antibody tests can remain positive for life, even after successful treatment, making them useful for detecting past infections 2
Comparison of RPR and Treponemal Antibody Tests
- The RPR test is more sensitive than treponemal antibody tests in detecting early syphilis, but may produce false-negative results in later stages of the disease 3
- Treponemal antibody tests are more specific than the RPR test, but may remain positive for life, making it difficult to distinguish between current and past infections 2, 4
- A combination of RPR and treponemal antibody tests is often used to diagnose syphilis and to monitor the effectiveness of treatment 5, 6