Will a Rapid Plasma Reagin (RPR) test show a positive result if an individual has been exposed to syphilis?

RPR Test Performance in Syphilis Detection

Yes, a Rapid Plasma Reagin (RPR) test will show a positive result in most cases of syphilis infection, but its sensitivity varies significantly depending on the stage of infection and whether the patient has been treated. 1

RPR Sensitivity by Stage of Syphilis

The sensitivity of RPR testing varies across different stages of syphilis:

Untreated 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

Treated Syphilis

• Primary treated: 75.9% sensitivity 1
• Secondary treated: 80.8% sensitivity 1
• Latent treated: 66.7% sensitivity 1
• All phases treated: 74.4% sensitivity 1

Important Clinical Considerations

False Negative Results

Several factors can lead to false negative RPR results:

1. Early Primary Infection: RPR may be negative in very early primary syphilis before antibody development 2

2. Prozone Phenomenon: In some cases (0.5% of reactive samples), high antibody titers can cause a false negative result due to the prozone effect 3

3. Late Latent Syphilis: RPR sensitivity drops to 61-64% in late latent syphilis 1

4. Age Factor: Patients over 35 years have higher rates of non-reactive RPR results in both primary syphilis (OR 3.55) and late latent syphilis (OR 4.30) 2

False Positive Results

RPR can produce biological false positive (BFP) results in several conditions:

• HIV infection: 10.7% BFP rate (vs 4.2% in HIV-negative) 1
• Hepatitis B: 8.3% BFP rate (vs 3.7% in HBV-negative) 1
• Hepatitis C: 4.5% BFP rate (vs 3.8% in HCV-negative) 1
• Other conditions: Autoimmune diseases, leprosy, malaria, and pregnancy can also cause false positives 1

Clinical Recommendations

1. For suspected syphilis with negative RPR: Request dilution testing to overcome potential prozone phenomenon 3

2. For older patients (>35 years): Consider treponemal testing even with negative RPR results 2

3. For monitoring treatment response: Automated RPR testing may detect declining titers earlier than manual card testing 4

4. For high-risk populations: Consider using both treponemal and non-treponemal tests for screening 5

Bottom Line

RPR testing is highly sensitive (95-100%) for untreated secondary and latent syphilis but has lower sensitivity in primary syphilis (88.5%) and treated syphilis (74.4% overall). False negatives can occur due to early infection, prozone phenomenon, late disease, or older age. False positives are more common in patients with certain conditions like HIV, viral hepatitis, and autoimmune diseases. When clinical suspicion is high but RPR is negative, additional testing including diluted specimens and treponemal-specific tests should be performed.