The Rapid Plasma Reagin (RPR) test is a common blood examination used as a screening tool. It identifies certain conditions. A blood sample is drawn and analyzed in a laboratory. The RPR test serves as an initial check to guide further diagnostic steps.
Understanding the RPR Test
The RPR test, or Rapid Plasma Reagin, primarily screens for syphilis, a sexually transmitted infection caused by the bacterium Treponema pallidum. It does not directly detect the bacteria, but measures non-treponemal antibodies, which are proteins the body produces in response to the infection.
The test works by observing the reaction of these antibodies with a specific antigen, typically a cardiolipin-lecithin antigen derived from beef heart. A blood sample is collected, typically from a vein in the arm.
Interpreting RPR Test Results
RPR test results are categorized as “reactive” or “non-reactive.” A non-reactive result suggests syphilis antibodies were not detected, indicating a low likelihood of infection. Conversely, a reactive result means these antibodies were found, suggesting a possible syphilis infection.
When a result is reactive, it is reported with a titer, such as 1:8 or 1:16. This titer indicates the concentration of antibodies in the blood, representing how much the sample could be diluted and still react. Higher titers can suggest a more active infection or a recent exposure. However, a reactive RPR result is preliminary and requires further confirmatory testing for a definitive diagnosis.
Factors Affecting RPR Results
RPR test results can be affected by various factors. A “false positive” can occur, showing a reactive result even when syphilis is not present. Conditions such as autoimmune diseases like lupus, other infections (e.g., Lyme disease, malaria, tuberculosis), recent vaccinations, or pregnancy can lead to false-positive RPR results.
Conversely, “false negatives” are also possible, particularly during the very early or very late stages of syphilis infection. In the initial phase, the body may not yet have produced enough antibodies for detection, a period known as the “window period.” Similarly, in late-stage untreated syphilis, antibody levels might be too low or too high (prozone effect), leading to an inaccurate non-reactive result.
Next Steps After an RPR Test
If an RPR test yields a reactive result, the next step involves confirmatory treponemal tests. These tests, such as Fluorescent Treponemal Antibody Absorption (FTA-ABS), Treponema pallidum Particle Agglutination (TP-PA), or Enzyme Immunoassay (EIA), detect antibodies against the Treponema pallidum bacterium. These tests help distinguish a true syphilis infection from a false positive.
If syphilis is confirmed, treatment, typically with antibiotics like penicillin, is administered. RPR titers then monitor treatment effectiveness, with a significant decrease (e.g., a fourfold drop) indicating a successful response. Follow-up testing is usually recommended at specific intervals, such as 6 and 12 months post-treatment, especially for individuals with HIV or those treated for late latent syphilis.