From the Guidelines
The RPR test is not fully reliable for diagnosing active syphilis in patients with a history of previous infection. After successful treatment, RPR titers typically decline but may remain positive at low levels (known as a serofast state) for months or even years, making it difficult to distinguish between past treated infection and new active infection 1. For patients with a history of syphilis, clinicians should compare current RPR titers with previous results, as a fourfold or greater increase in titer suggests reinfection or treatment failure. Some key points to consider when interpreting RPR test results in patients with a history of syphilis include:
- A single positive RPR test result does not necessarily indicate active infection, as it may reflect a past treated infection
- Comparison of current and previous RPR titers is essential to assess for potential reinfection or treatment failure
- Additional testing with treponemal-specific tests like TP-PA or FTA-ABS may be recommended, although these tests remain positive for life after infection and cannot distinguish between current and past infection 1
- Clinical evaluation for symptoms and exposure history is crucial in determining the need for further testing or treatment. In unclear cases, cerebrospinal fluid examination or empiric treatment may be necessary to prevent potential complications of untreated syphilis, which can affect multiple organ systems.
From the Research
Reliability of RPR Test for Diagnosing Active Syphilis
The reliability of the Rapid Plasma Reagin (RPR) test for diagnosing active syphilis in patients with a history of previous syphilis infection is a topic of interest.
- The RPR test is a nontreponemal test used to screen for syphilis, but its reliability can be affected by various factors, including previous syphilis infection 2.
- A study comparing automated RPR tests with conventional methods found that the concordance rate between RPR and VDRL (Venereal Disease Research Laboratory) tests was 67.5%, with 164 discrepant cases being VDRL reactive but RPR negative 2.
- Among these discrepant cases, 82 out of 106 specimens were from patients who had already been treated for syphilis, suggesting that RPR may not be reliable for diagnosing active syphilis in patients with a history of previous infection 2.
- Another study found that RPR sensitivity was 90% and specificity was 98.5% compared to an RPR-based algorithm, but the positive predictive value was only 47.4%, indicating that RPR may not be reliable for confirming active syphilis in all cases 3.
- The use of treponemal tests, such as the Treponema pallidum particle agglutination (TPPA) assay, may be more reliable for diagnosing syphilis, especially in patients with a history of previous infection 4.
Factors Affecting RPR Test Reliability
Several factors can affect the reliability of the RPR test, including:
- Previous syphilis infection: Patients with a history of previous syphilis infection may have a false-negative RPR result, making it less reliable for diagnosing active syphilis 2.
- Treatment status: Patients who have been treated for syphilis may still have a positive RPR result, which can make it difficult to distinguish between active and past infection 2.
- HIV status: Patients with HIV may have a poorer serologic response to syphilis treatment, which can affect the reliability of the RPR test 5, 6.
Alternative Testing Methods
Alternative testing methods, such as treponemal tests, may be more reliable for diagnosing syphilis, especially in patients with a history of previous infection.
- The TPPA assay has been shown to have high sensitivity and specificity for diagnosing syphilis, making it a reliable alternative to the RPR test 4.
- Other treponemal tests, such as the fluorescent treponemal antibody absorption (FTA-ABS) test, may also be used to confirm syphilis diagnosis, but their sensitivity and specificity may vary depending on the stage of infection 4.