Is the RPR (Rapid Plasma Reagin) test used to screen for syphilis?

RPR Test for Syphilis Diagnosis

Yes, the RPR (Rapid Plasma Reagin) test is a nontreponemal antibody test used to screen for syphilis, detect active infection, and monitor treatment response. 1

What RPR Detects

• RPR detects antiphospholipid antibodies produced by the host in response to phosphatidylcholine released from tissue damage caused by Treponema pallidum infection 2
• RPR correlates with disease activity and should always be reported quantitatively as titers (e.g., 1:4,1:16,1:64), not just as positive/negative 1
• A fourfold change in titer (equivalent to two dilutions) represents clinically significant change when monitoring treatment response 1, 2

Sensitivity by Disease Stage

The diagnostic performance of RPR varies substantially depending on syphilis stage:

• Primary syphilis: 62-78% sensitive - meaning approximately 22-38% of early primary cases may be missed 3, 4
• Secondary syphilis: 97-100% sensitive - nearly all cases detected 3, 4
• Early latent syphilis: 85-100% sensitive 3, 1
• Late latent syphilis: 61-75% sensitive - with 25-39% of cases potentially having non-reactive results 1, 4

Critical Distinction: RPR vs Treponemal Tests

RPR is fundamentally different from treponemal tests (FTA-ABS, TP-PA, treponemal EIA/CLIA) and both types are required for complete syphilis diagnosis 1:

• Nontreponemal tests (RPR/VDRL): Become nonreactive after successful treatment in most patients, making them ideal for monitoring treatment response 1
• Treponemal tests: Remain positive for life in 75-85% of patients regardless of treatment, making them unsuitable for monitoring disease activity 1

Diagnostic Algorithm

The CDC recommends performing both test types for accurate diagnosis 1:

• Traditional algorithm: Start with RPR screening, then confirm positive results with treponemal testing 2
• Reverse algorithm: Start with treponemal EIA/CLIA, then confirm with quantitative RPR 1
• A positive treponemal test alone is insufficient - nontreponemal tests must also be performed to distinguish active infection from past treated infection 1

RPR vs VDRL Comparison

RPR generally demonstrates higher sensitivity than VDRL across all stages of syphilis 4:

• In direct comparisons, serum RPR was consistently as sensitive or slightly more sensitive than VDRL 3, 4
• RPR and VDRL titers are NOT interchangeable - this is the most critical practical distinction 4
• Sequential monitoring must use the same test method (either RPR or VDRL), preferably by the same laboratory 1, 4

Common Pitfalls to Avoid

• Never switch between RPR and VDRL when following a patient - this is the single most common error that undermines treatment monitoring 4
• Do not compare titers between different test types as they are not directly comparable 1
• False-positive RPR results occur in 0.6-1.3% of the general population, with higher rates in autoimmune diseases, pregnancy, HIV infection, hepatitis B/C, IV drug use, and advanced age 1
• At titers ≥1:8, false-positive results are extremely rare 1

Special Populations

• HIV-infected patients: May have atypical serologic responses with unusually low, high, or fluctuating titers, though standard tests remain accurate for most 1, 4
• Pregnant patients: All pregnant women should have serologic status documented at least once during pregnancy, and in high-risk populations also at 28 weeks and delivery 1

Treatment Monitoring

• Use nontreponemal test titers (RPR or VDRL) to monitor treatment response - never use treponemal test titers 1
• Treatment success is defined as a fourfold decline in RPR titer within 6-12 months for early syphilis and 12-24 months for late latent syphilis 1
• Some patients remain "serofast" with persistent low-level positive titers (generally <1:8) despite adequate treatment, which does not necessarily indicate treatment failure 1