What are the RPR (Rapid Plasma Reagin) and RFx (Rheumatoid Factor) lab tests used for?

RPR and RF Laboratory Tests: Clinical Applications

RPR (Rapid Plasma Reagin) is a nontreponemal test used primarily for syphilis screening, while RF (Rheumatoid Factor) is an antibody test used to help diagnose rheumatoid arthritis and other autoimmune conditions.

RPR (Rapid Plasma Reagin)

Purpose and Clinical Application

• RPR is a nontreponemal serological test used for screening and monitoring syphilis infection 1
• It detects antibodies to cardiolipin, a component released during Treponema pallidum infection
• Sensitivity varies by stage of infection:
  • Highest in secondary syphilis (97-100%)
  • Moderate in early infection (50-92.7%)
  • Lower in late stages (61-75%) 1

Performance Characteristics

• Specificity: 95-100% 1
• RPR is more sensitive than VDRL in some studies (174/200 vs 167/200) 2
• Titers of 1:8 and greater are highly associated with infectious syphilis (accuracy 97.2%) 3

Interpretation

• A positive RPR requires confirmation with a treponemal-specific test (e.g., FTA-ABS, TP-PA) 1
• Interpretation matrix:
  • RPR positive + Treponemal test positive = Confirmed syphilis (current or past)
  • RPR positive + Treponemal test negative = Likely false-positive RPR
  • RPR negative + Treponemal test positive = Possible very early infection, previously treated syphilis, or late-stage syphilis
  • RPR negative + Treponemal test negative = No evidence of syphilis 1

Common Pitfalls

1. False positives can occur in:

   • Autoimmune diseases
   • Pregnancy
   • Viral infections
   • Drug use (23% false positive rate reported in methadone clinic patients) 2

2. Negative results despite infection:

   • Very early primary syphilis
   • Late latent or tertiary syphilis
   • "Prozone phenomenon" in high-titer specimens 1

RF (Rheumatoid Factor)

Purpose and Clinical Application

• RF is an antibody (usually IgM) that targets the Fc portion of IgG
• Primary use is in diagnosis of rheumatoid arthritis
• Also useful in evaluating other autoimmune conditions:
  • Sjögren's syndrome
  • Mixed cryoglobulinemia
  • Systemic lupus erythematosus (SLE)

Interpretation

• Positive RF is found in approximately 70-80% of patients with rheumatoid arthritis
• RF titers often correlate with disease severity and prognosis
• Higher titers are associated with:
  • More aggressive joint disease
  • Extra-articular manifestations
  • Poorer long-term outcomes

Common Pitfalls

1. False positives can occur in:

   • Elderly patients (up to 20% of healthy elderly may be RF positive)
   • Chronic infections (endocarditis, tuberculosis, hepatitis)
   • Other autoimmune diseases
   • Chronic lung diseases

2. False negatives:

   • Early rheumatoid arthritis (RF may develop later in disease course)
   • Seronegative rheumatoid arthritis

Testing Algorithms

Syphilis Testing

1. Traditional algorithm:

   • Screen with nontreponemal test (RPR/VDRL)
   • Confirm positive results with treponemal test

2. Reverse algorithm (increasingly common):

   • Screen with treponemal test (EIA/CIA)
   • Confirm positive results with nontreponemal test (RPR)
   • If discordant (treponemal+ but RPR-), perform second treponemal test 4

Monitoring Syphilis Treatment

• Quantitative RPR should be performed at 3,6,9,12, and 24 months after treatment
• Treatment success indicated by fourfold decline in titer within 6-12 months 1

Key Considerations

• RPR titers of 1:8 and greater are highly associated with infectious syphilis and should prompt immediate treatment 3
• RPR test results should always be interpreted in conjunction with clinical findings 1
• RF testing should be part of a broader autoimmune workup when rheumatoid arthritis is suspected, often alongside anti-CCP antibodies which offer higher specificity