The Rapid Plasma Reagin (RPR) test is a common blood test used to screen for syphilis, a sexually transmitted infection. It identifies specific antibodies the body produces in response to Treponema pallidum, the bacterium that causes syphilis. This test detects antibodies, not the bacteria directly.
Purpose of the RPR Test
The RPR test detects non-treponemal antibodies, often called “reagin” antibodies, which the body produces when Treponema pallidum is present. Testing for syphilis is important for public health because untreated syphilis can lead to serious health complications, including damage to internal organs, and can be transmitted to others.
The RPR test is used in various scenarios. It is often part of general health screenings, particularly for individuals with symptoms of sexually transmitted infections or those considered at higher risk. Pregnancy screening also commonly includes the RPR test to prevent congenital syphilis, which can have severe consequences for newborns. The test additionally helps monitor the effectiveness of syphilis treatment, as antibody levels should decrease after successful therapy.
How the Test Works and What Results Mean
The RPR blood test involves drawing a blood sample, which is then sent to a laboratory for analysis. In the lab, the patient’s serum is mixed with a reagent containing carbon particles coated with cardiolipin, lecithin, and cholesterol. If reagin antibodies are present in the serum, they bind to these coated particles, causing them to clump together in a visible reaction called flocculation.
Test results are reported as “non-reactive” or “reactive.” A non-reactive result suggests no current infection or detectable antibodies, while a reactive result indicates the presence of antibodies, suggesting a possible syphilis infection. Reactive results also include a “titer,” which represents the highest dilution of the blood sample that still shows a reactive result. A higher titer generally suggests a more active or recent infection, reflecting a greater concentration of antibodies.
Next Steps After Receiving Your Results
If an RPR test result is reactive, further confirmatory testing is necessary to definitively diagnose syphilis. This is because the RPR test detects non-specific antibodies, and other conditions can sometimes cause a reactive result. Confirmatory tests, known as treponemal tests, specifically detect antibodies against Treponema pallidum itself.
Common confirmatory tests include the Fluorescent Treponemal Antibody Absorption (FTA-ABS) test or the Treponema Pallidum Particle Agglutination (TP-PA) assay. A diagnosis of active syphilis is made when both the RPR test is reactive and a treponemal confirmatory test is also reactive. Syphilis is treated with antibiotics. After treatment, follow-up RPR tests are performed to monitor the effectiveness of the therapy; successful treatment should lead to a fourfold or greater decrease in antibody titers over time.
Common Factors Influencing RPR Results
While the RPR test is a useful screening tool, it can sometimes produce results that do not accurately reflect a syphilis infection. A “false positive” RPR result can occur when the test is reactive, but the person does not have syphilis. This can happen due to other medical conditions that cause the body to produce similar antibodies, such as autoimmune diseases like lupus, certain infections (e.g., mononucleosis, malaria, Lyme disease), or even pregnancy.
Conversely, a “false negative” RPR result means the test is non-reactive even though syphilis is present. This is more common in the very early stages of infection, before enough antibodies have developed, or in very late-stage syphilis where antibody levels may be low. A phenomenon known as the “prozone effect” can also lead to a false negative, particularly in cases of very high antibody concentrations, where the excess antibodies interfere with the test’s ability to form visible clumps.