The Rapid Plasma Reagin (RPR) test is a blood screening tool used routinely during prenatal care to detect potential syphilis infection. This test is a standard part of a pregnant person’s first set of blood work. It identifies individuals who may be infected with the bacterium Treponema pallidum, which causes syphilis. Prompt identification and treatment are necessary to prevent severe complications for both the mother and the developing fetus.
How the RPR Test Works
The RPR test is classified as a non-treponemal test, meaning it does not directly look for the syphilis-causing bacterium itself. Instead, the test detects non-specific antibodies, also called “reagin,” that the body produces as a reaction to tissue damage caused by the infection. The procedure involves a simple blood draw, with no special preparation needed beforehand.
The test reagent contains an antigen made of cardiolipin, a lipid substance that reacts with non-treponemal antibodies found in the blood serum. If the antibodies are present, they cause a visible clumping or flocculation reaction with the cardiolipin antigen, which is interpreted as a reactive result. Because this test detects general markers of infection, it offers a rapid and cost-effective screening method. This non-specificity means that a reactive result requires further, more specific testing to confirm the diagnosis.
Importance of Screening During Pregnancy
Routine syphilis screening with the RPR test is a public health requirement and a standard practice during the first prenatal visit. The infection can be transmitted from the pregnant person to the fetus at any stage, leading to a condition known as congenital syphilis. The consequences of untreated syphilis during pregnancy can be severe, significantly increasing the risk of adverse outcomes.
Specific risks include miscarriage, preterm delivery, and stillbirth; up to 40% of babies born to untreated mothers may be stillborn or die shortly after birth. For infants who survive, the infection can cause long-term complications, such as bone deformities, neurological damage, severe anemia, deafness, or blindness. Screening is often repeated in the third trimester (around 28 weeks’ gestation) and again at delivery for high-risk individuals. Timely identification and treatment can effectively prevent these outcomes.
Understanding Test Results and Titers
RPR results are reported as either Non-Reactive or Reactive. A Non-Reactive result suggests that syphilis antibodies were not detected, indicating no likely infection, though very early infection may not yet show a reaction. A Reactive result means the test detected the non-specific antibodies and suggests a current or past syphilis infection, requiring immediate follow-up.
When a result is Reactive, a quantitative titer is also reported, often shown as a ratio, such as 1:8 or 1:32. This titer measures the concentration of antibodies in the blood and is a measure of disease activity. A higher number, such as 1:32, indicates a more active infection or a heavier bacterial load. The titer is used to monitor the effectiveness of treatment, as a successful response should show a fourfold decrease.
A Reactive RPR result does not always equate to an active infection, as false positives are possible. Various conditions can cause the body to produce the same non-specific antibodies that the RPR detects, including autoimmune disorders like lupus, certain viral infections, malaria, and even pregnancy itself. Therefore, a positive RPR must always be confirmed with a more specific test.
Next Steps Following a Reactive Result
The immediate action following a Reactive RPR result is to order a confirmatory test, such as the Fluorescent Treponemal Antibody Absorption (FTA-ABS) or the Treponema pallidum Particle Agglutination (TP-PA) test. These tests detect antibodies specific to the Treponema pallidum bacterium, confirming whether the person has ever been infected. If the confirmatory test is also positive, treatment is initiated immediately, regardless of the stage of pregnancy.
The standard treatment for syphilis during pregnancy is a course of Benzathine Penicillin G, which is the only antibiotic that reliably crosses the placenta and treats the fetus. The specific regimen, whether a single dose or a series of weekly injections, depends on the stage and duration of the infection. Following treatment, the RPR test is repeated at regular intervals to ensure the antibody titers are decreasing, indicating a successful therapeutic response.