What does a positive Rapid Plasma Reagin (RPR) result of 1:2 indicate?

Understanding RPR 1:2 Results in Syphilis Testing

A positive Rapid Plasma Reagin (RPR) result of 1:2 indicates a low-titer positive result that could represent either early/active syphilis infection, successfully treated past infection, or a biological false positive reaction. 1

Interpretation of RPR 1:2 Result

RPR is a nontreponemal test that detects antibodies to cardiolipin, which are produced during syphilis infection. The titer (1:2) represents the highest dilution of the patient's serum that still produces a positive reaction.

What this result means:

• Low-titer positive: 1:2 is considered a low titer
• Requires confirmation: All positive RPR results need confirmation with a treponemal-specific test (TPHA, FTA-ABS, or TP-PA) 1
• Possible interpretations:
  • Early primary syphilis (beginning of infection)
  • Late latent or previously treated syphilis with declining titers
  • Biological false positive reaction (0.32% of RPR tests) 2

Clinical Significance

Possible scenarios with RPR 1:2:

1. Early/Active Syphilis:

   • May represent early primary infection before antibody levels rise
   • In primary syphilis, approximately 16.5% of cases can be initially non-reactive by RPR despite active infection 3

2. Successfully Treated Syphilis:

   • After successful treatment, RPR titers gradually decline
   • A 1:2 titer may represent a previously higher titer that has decreased following treatment 1
   • Among patients with initial RPR titers <1:8, approximately 44-53% become RPR negative two years after therapy 4

3. Biological False Positive (BFP):

   • Low titers (≤1:4) are more commonly associated with biological false positives 2
   • BFPs occur in approximately 0.32% of RPR tests 2
   • More common in females, adults ≥80 years old, and individuals between 16-35 years old 2

Next Steps in Evaluation

1. Confirm with treponemal test:

   • Order a treponemal-specific test (TPHA, FTA-ABS, or TP-PA) with 95-100% sensitivity and specificity 1
   • If treponemal test is positive: confirms current or past syphilis infection
   • If treponemal test is negative: likely a biological false positive RPR

2. Clinical correlation:

   • Assess for signs/symptoms of syphilis
   • Review patient's risk factors and sexual history
   • Evaluate for history of previous syphilis treatment 1

3. Consider alternative testing:

   • If clinical suspicion remains high despite negative results, consider repeat testing
   • Rule out prozone phenomenon (false negative due to antibody excess) 1

Common Pitfalls in Interpretation

• Assuming all positive RPR results indicate active infection: A low-titer positive RPR with positive treponemal test may represent previously treated syphilis
• Relying solely on RPR: RPR testing alone is insufficient, especially in middle-aged and elderly individuals 3
• Ignoring clinical context: Interpretation must consider patient history, risk factors, and previous treatment
• Overlooking false positives: Low-titer RPR results (1:2) have higher rates of biological false positives 2
• Missing early infection: Some primary syphilis cases may present with low titers or even negative RPR 3

Conclusion

An RPR titer of 1:2 requires confirmation with treponemal testing and careful clinical correlation to determine whether it represents active infection requiring treatment, previously treated infection, or a biological false positive result.