A 48-year-old woman with a 6-year history of rheumatoid arthritis (RA) presents with progressive joint deformities. Her serum RF is 120 IU/mL and anti-CCP is 85 U/mL. Which single clinical feature best distinguishes RA from seronegative spondyloarthropathy (SpA) in this patient?

## Discriminating RA from Seronegative SpA ### Key Clinical Distinction **Key Point:** Symmetric polyarticular involvement of small joints (MCPs, PIPs, wrists) is the hallmark discriminator of RA; seronegative SpA typically presents with asymmetric, oligoarticular disease and axial predominance. ### Comparative Features | Feature | RA | Seronegative SpA | |---------|----|-----------| | **Joint pattern** | Symmetric polyarticular (MCPs, PIPs, wrists) | Asymmetric oligoarticular (knees, ankles, hips) | | **Axial involvement** | Rare (cervical spine late) | Prominent (sacroiliitis, spondylitis) | | **Morning stiffness** | Common (>1 hr) | Common (>1 hr) | | **Acute phase reactants** | Often elevated | Variable | | **RF/anti-CCP** | Positive in ~80% | Negative | | **HLA-B27** | Negative | Positive in ~70% | | **Enthesitis** | Absent | Hallmark feature | ### Why This Matters **High-Yield:** The symmetric small-joint polyarticular pattern is so characteristic of RA that it is part of the 2010 ACR/EULAR classification criteria. Seronegative SpA, even when seropositive for RF in rare cases, maintains its asymmetric, oligoarticular, and axially-predominant phenotype. **Clinical Pearl:** A patient with symmetric MCPs/PIPs + wrists is RA until proven otherwise. If you see asymmetric knees/ankles + sacroiliitis, think SpA regardless of serology. ### Why Other Options Are Traps - **Morning stiffness >1 hour:** Both RA and SpA present with prolonged morning stiffness; not discriminatory. - **Elevated ESR/CRP:** Both inflammatory arthropathies elevate acute phase reactants; not specific to RA. - **Axial involvement:** This is the hallmark of SpA, not RA—choosing this reverses the logic.