What does a reactive RPR (Rapid Plasma Reagin) test with a titer of 1:2 indicate?

Interpretation of Reactive RPR with Titer 1:2

A reactive RPR with a titer of 1:2 indicates confirmed syphilis infection (either current or past) when accompanied by a reactive confirmatory treponemal test. 1

Understanding RPR Test Results

RPR (Rapid Plasma Reagin) is a nontreponemal test used for syphilis screening with the following characteristics:

• Sensitivity: 50-92.7% (varies by stage of infection)
• Specificity: 95-100% 1
• A titer of 1:2 is considered a low titer (defined as <1:8) 2

Interpretation Framework

RPR Result	Treponemal Test Result	Interpretation
Positive	Positive	Confirmed syphilis (current or past)
Positive	Negative	Likely false-positive RPR

Clinical Significance of Low RPR Titers

Low RPR titers (1:2) may indicate:

1. Early primary syphilis: The beginning of antibody development
2. Late latent syphilis: RPR titers decline over time without treatment
3. Previously treated syphilis: Titers may remain low after successful treatment
4. False-positive result: Requires confirmatory treponemal testing 3, 1

Important Considerations

• Low titers are more common in certain populations:

  • Patients over 35 years of age (OR 3.55 [1.39-9.07] for primary syphilis; OR 4.30 [2.28-8.12] for late latent syphilis) 4
  • Patients with late latent syphilis 3

• Recent research shows that 52.63% of confirmed secondary syphilis cases had VDRL titers <1:8, with some being completely non-reactive despite active infection 5

Next Steps in Management

1. Confirmatory testing: A reactive treponemal test (TP-PA, FTA-ABS) is essential to confirm the diagnosis 1

2. Clinical evaluation: Determine the stage of syphilis based on:

   • History of previous syphilis diagnosis and treatment
   • Presence of symptoms (primary chancre, secondary rash, etc.)
   • Duration of infection if known

3. Treatment based on staging:

   • Early syphilis (primary, secondary, early latent): Benzathine penicillin G 2.4 million units IM as a single dose
   • Late latent syphilis or unknown duration: Benzathine penicillin G 2.4 million units IM once weekly for 3 consecutive weeks
   • Neurosyphilis: Aqueous crystalline penicillin G 18-24 million units IV daily for 10-14 days 1

4. Follow-up testing: Monitor treatment response with quantitative nontreponemal testing at 3,6,9, and 12 months 1

Potential Pitfalls

1. False-positive RPR: More common in:

   • Women (0.27% vs 0.20% in men)
   • Patients over 60 years (0.34% vs 0.25% in younger patients) 1
   • Conditions such as malaria, HCV infection, and drug use 3

2. False-negative treponemal tests: Consider repeat testing in 1-2 weeks if clinical suspicion is high 1

3. Prozone phenomenon: High antibody titers can paradoxically cause false-negative or low-titer results 1

4. Relying solely on RPR: Using RPR alone for screening is insufficient, especially in middle-aged and elderly individuals 4

5. Ocular syphilis with low/negative RPR: Recent evidence shows that ocular syphilis can present with nonreactive or low-titer RPR, requiring treponemal testing for diagnosis 2

By understanding these considerations, clinicians can accurately interpret a reactive RPR with a titer of 1:2 and provide appropriate management for patients with suspected syphilis infection.