RPR Stands for Rapid Plasma Reagin

RPR stands for Rapid Plasma Reagin, which is a nontreponemal antibody test used in the diagnosis and monitoring of syphilis infection. 1

Understanding RPR Testing

RPR is one of the primary antiphospholipid antibody tests used to detect host antibodies made in response to phosphatidylcholine taken up from mammalian tissue by Treponema pallidum (the bacterium that causes syphilis) 1
RPR detects nonspecific or nontreponemal antibodies (more accurately termed "antiphospholipid" antibodies) that develop during syphilis infection 1
RPR is commonly used alongside treponemal-specific tests (such as FTA-ABS, TP-PA) to help diagnose syphilis infection in clinical settings 1
RPR test results are reported quantitatively as titers, which usually correlate with disease activity 1

RPR shows varying sensitivity depending on the stage of syphilis:
- Primary syphilis: 88.5% sensitivity 1
- Secondary syphilis: 100% sensitivity 1
- Latent untreated syphilis: 95.7% sensitivity 1
- All phases untreated: 95.0% sensitivity 1
- All phases treated: 74.4% sensitivity 1
The specificity of RPR is generally high, with studies showing specificity of 94.3% compared to treponemal tests 2

RPR and VDRL (Venereal Disease Research Laboratory) are equally valid assays, but quantitative results cannot be compared directly as RPR titers often are slightly higher than VDRL titers 1
Several high-quality studies have reported similar or higher sensitivity of RPR compared to VDRL, independent of syphilis stage 1
RPR has been shown to have higher specificity compared to VDRL in multiple studies 1

A fourfold change in titer (equivalent to a change of two dilutions, e.g., from 1:16 to 1:4) is considered clinically significant when monitoring treatment response 1, 3
Sequential serologic tests in individual patients should be performed using the same testing method (e.g., VDRL or RPR), preferably by the same laboratory 1
RPR titers are used to monitor treatment responses, with a fourfold decrease in titer indicating successful treatment 3
Automated RPR tests are now available and may be useful for monitoring treatment response, especially in patients with high RPR titers 4

False-positive RPR results can occur in various conditions:
- HIV infection (10.7% biological false positives in HIV+ vs 4.2% in HIV-) 1
- Hepatitis B (8.3% biological false positives in HBV+ vs 3.7% in HBV-) 1
- Hepatitis C (4.5% biological false positives in HCV+ vs 3.8% in HCV-) 1
- Autoimmune diseases 1
- Pregnancy (0.6% biological false positives) 1
A positive RPR test should always be confirmed with a treponemal-specific test to establish a definitive diagnosis of syphilis 3
In HIV-infected patients, serologic tests may present atypical results (unusually high, unusually low, or fluctuating titers) 1
Nontreponemal tests like RPR usually become nonreactive with time after treatment; however, in some patients, nontreponemal antibodies can persist at a low titer for a long period, sometimes for life (known as the "serofast reaction") 1