The Rapid Plasma Reagin (RPR) test is a common blood test used as a screening tool for syphilis. This infection, caused by the bacterium Treponema pallidum, can lead to serious health complications if left untreated.
Purpose of the RPR Test
The RPR test functions as a non-treponemal test, meaning it detects antibodies that the body produces not directly against the syphilis bacteria, but rather in response to tissue damage caused by the infection. These non-specific antibodies, sometimes called “reagins,” are markers of active disease or recent infection.
Healthcare providers often order an RPR test in various situations. It may be part of routine check-ups, especially for individuals at higher risk of sexually transmitted infections, or as a component of general STD screening. Pregnant individuals are routinely screened with an RPR test to prevent congenital syphilis, which can cause severe health issues for the baby. The test is also used when individuals present with symptoms consistent with syphilis, such as unexplained sores or rashes. Additionally, the RPR test can help monitor the effectiveness of syphilis treatment, as antibody levels are expected to decrease after successful therapy.
How the Test is Performed
A healthcare professional typically collects a blood sample through a venipuncture, which involves inserting a needle into a vein, usually in the arm. No special preparation, such as fasting, is usually required before an RPR test. After collection, the blood sample is sent to a laboratory for analysis to detect specific antibodies.
Understanding Your RPR Results
RPR test results are typically reported as “reactive” or “non-reactive.” A “reactive” result indicates the presence of the non-specific antibodies associated with syphilis, suggesting a current or past infection. Conversely, a “non-reactive” result generally means these antibodies were not detected, indicating no active infection or no past exposure. However, a non-reactive result does not entirely rule out syphilis, particularly in very early stages of infection before the body has produced detectable antibodies.
When an RPR test is reactive, it often includes a quantitative result known as a “titer,” expressed as a ratio (e.g., 1:16, 1:32). The titer reflects the concentration of antibodies in the blood, with higher numbers indicating more antibodies. These quantitative results help monitor disease activity and assess treatment effectiveness; a decline in titer after treatment suggests a positive response.
A reactive RPR result can sometimes be a “biological false positive,” meaning the test is positive but syphilis is not present. This can occur due to other conditions such as autoimmune diseases (like lupus), certain infections (e.g., Lyme disease, malaria), or even pregnancy. False positives are generally at lower titers, typically less than 1:8.
Considerations and Next Steps
A reactive RPR test result necessitates further action, as it is a screening test and does not definitively confirm a syphilis diagnosis. A confirmatory test, typically a treponemal test such as the Fluorescent Treponemal Antibody Absorption (FTA-ABS) or T. pallidum Particle Agglutination (TP-PA) assay, is required. These confirmatory tests detect antibodies specific to the Treponema pallidum bacteria itself, helping to distinguish a true syphilis infection from a biological false positive.
If the confirmatory test is also reactive, a diagnosis of syphilis is confirmed, and treatment will be initiated. Penicillin is the treatment for syphilis, with dosage and duration depending on the infection stage. The RPR test can also show false negatives in very early or very late stages of infection. Therefore, if there is a strong clinical suspicion of syphilis despite a non-reactive RPR, repeat testing may be recommended after a few weeks.