The Rapid Plasma Reagin (RPR) test is a type of nontreponemal blood test used to screen for and monitor syphilis, a sexually transmitted infection caused by the bacterium Treponema pallidum. Unlike tests that directly detect the bacteria, the RPR identifies antibodies your immune system makes in response to cellular damage caused by the infection.
When ordered “with reflex to titer,” the lab first performs a standard RPR screen. If this screen is positive, they automatically perform a titer, which quantifies how much antibody is present by diluting the blood sample in steps. This measurement is reported as a ratio, such as 1:4 or 1:32. Higher titers generally indicate more active disease, although clinical context matters.
The RPR titer is not static. Studies show that up to 15% of patients may experience a fourfold change (either increase or decrease) in titer before treatment even begins, especially if several days pass between diagnosis and therapy. Because of this variability, guidelines recommend repeating the RPR if treatment is delayed.
After syphilis treatment, most people show a fourfold decline in titer within about 3 months. This decline is a key indicator that treatment is working. However, in roughly 1 out of 5 patients, the titer may temporarily rise during the first two weeks after treatment—a short-term fluctuation that does not usually signal treatment failure.
Long-term, only about 17% of patients experience seroreversion, meaning the RPR becomes completely nonreactive within a year. A persistent low-level titer (like 1:2 or 1:4) is common and does not necessarily mean the infection is still active or that treatment failed. It often reflects lingering immune memory rather than ongoing infection.
Another important nuance is how the test is performed. Automated RPR systems can generate more low-level positive results than traditional manual methods. While automation helps with consistency and efficiency, it may increase the risk of false positives, especially at low titers, underscoring the importance of confirmatory testing (usually with a treponemal-specific test like FTA-ABS or TPPA).
Overall, the RPR with reflex to titer is a critical tool not only for detecting syphilis but also for monitoring how well a person is responding to treatment and identifying potential reinfections. Because titers can shift over time and after treatment, clinicians use both trends and timing to interpret results accurately.
