Syphilis Serologic Testing: EIA, RPR, and TP-PA
Overview of Test Types
Syphilis diagnosis requires both nontreponemal and treponemal tests used together, as relying on a single test type is insufficient for accurate diagnosis. 1
Enzyme Immunoassay (EIA)
What It Detects
- EIA is a treponemal test that detects specific antibodies against Treponema pallidum antigens and is commonly used as an automated screening test in modern laboratory algorithms. 2, 3
Key Characteristics
- Remains reactive for life in most patients (85-100%) regardless of treatment or disease activity, making it unsuitable for monitoring treatment response. 2, 1
- High sensitivity (96-100%) across all stages of syphilis, including late latent disease where nontreponemal tests may be negative. 4, 3
- Can be automated for high-throughput screening, which has led many laboratories to adopt "reverse sequence" algorithms starting with EIA. 3
Clinical Interpretation
- A reactive EIA requires confirmation with a nontreponemal test (RPR) to determine disease activity, and if discordant, a second treponemal test like TP-PA should be performed. 1, 3
- 15-25% of patients treated during primary syphilis may revert to serologically nonreactive after 2-3 years, but most remain reactive indefinitely. 2, 1
Rapid Plasma Reagin (RPR)
What It Detects
- RPR is a nontreponemal test that detects antiphospholipid antibodies (not specific to T. pallidum) produced by the host in response to phosphatidylcholine released from damaged cells during infection. 2, 5
Key Characteristics
- Titers correlate with disease activity and must be reported quantitatively (e.g., 1:16,1:32) to monitor treatment response. 2
- A fourfold change in titer (two dilutions, e.g., 1:16 to 1:4) indicates clinically significant change in disease activity or treatment response. 2, 1
- Sensitivity varies by stage: 70-80% in primary syphilis, 97-100% in secondary syphilis, 61-76% in late latent syphilis. 1, 5
Clinical Interpretation
- RPR becomes nonreactive after successful treatment in most patients, though some maintain low titers indefinitely (serofast reaction). 2
- Sequential tests must use the same method (RPR or VDRL) by the same laboratory, as RPR titers are typically slightly higher than VDRL and cannot be directly compared. 2
- False-positive results can occur with HIV infection, pregnancy, autoimmune diseases, hepatitis, and illicit drug use. 2, 5
Critical Pitfall
- RPR has highest specificity (100%) but lower sensitivity in late-stage disease, so a negative RPR does not exclude syphilis, particularly in late latent or tertiary stages. 1, 6
Treponema pallidum Particle Agglutination (TP-PA)
What It Detects
- TP-PA is a treponemal test that detects specific antibodies against T. pallidum using gelatin particle agglutination technology. 2, 4
Key Characteristics
- Among the most sensitive (96.5%) and specific (95.5%) treponemal tests available, making it ideal for confirmatory testing. 7, 4
- Remains reactive for life in most patients regardless of treatment, similar to other treponemal tests. 1
- Used as the second treponemal test in reverse algorithm discordant resolution when EIA is reactive but RPR is nonreactive. 4, 3
Clinical Interpretation
- TP-PA is the gold standard confirmatory treponemal test when there is discordance between initial screening tests. 4
- Cannot be used to monitor treatment response because titers correlate poorly with disease activity. 2
- A reactive TP-PA with nonreactive RPR typically indicates late latent syphilis, prior treated syphilis, or rarely a false-positive treponemal test. 1
Practical Testing Algorithm
Traditional Algorithm
- Screen with RPR first, then confirm reactive results with treponemal test (EIA or TP-PA). 2
Reverse Algorithm (Increasingly Common)
- Screen with automated treponemal EIA first, then test reactive samples with RPR. 3
- If EIA reactive but RPR nonreactive, perform TP-PA as second treponemal test to resolve discordance. 1, 3
- If both EIA and TP-PA are reactive but RPR is nonreactive, treat as late latent syphilis (benzathine penicillin G 2.4 million units IM weekly × 3 weeks). 1
Monitoring Treatment
- Use RPR titers exclusively to monitor treatment response, repeating at 6,12, and 24 months after treatment. 1, 5
- A fourfold decline in RPR titer indicates successful treatment. 1, 5
Special Considerations
HIV-Infected Patients
- Standard serologic tests remain accurate for most HIV-infected patients, though atypical results (unusually high, low, or fluctuating titers) can occur. 2
- False-positive nontreponemal tests may be more common in HIV-infected persons. 2, 5