## Distinguishing RA from Osteoarthritis **Key Point:** Morning stiffness duration is the single most reliable clinical discriminator between RA and OA. RA causes prolonged morning stiffness (typically >1 hour, often 2–3 hours or more) due to inflammatory synovitis, whereas OA causes brief stiffness (<30 minutes) that improves with activity. ### Comparative Features | Feature | Rheumatoid Arthritis | Osteoarthritis | |---------|----------------------|----------------| | **Morning stiffness** | >1 hour (often 2–3 hrs) | <30 minutes | | **DIP joint involvement** | Rare (swan-neck deformity) | Common (Heberden nodes) | | **PIP involvement** | Very common (early) | Common (Bouchard nodes) | | **MCP involvement** | Hallmark (early, symmetric) | Rare | | **Pattern** | Symmetric | Asymmetric or symmetric | | **Crepitus** | Fine, from synovitis | Coarse, from cartilage loss | | **Inflammation markers** | Elevated ESR, CRP | Normal | **High-Yield:** Morning stiffness >1 hour is part of the 2010 ACR/EULAR classification criteria for RA and is weighted heavily in diagnosis. **Clinical Pearl:** Ask patients: "How long after waking do your joints loosen up?" A reply of "2–3 hours" strongly suggests RA; "10–15 minutes" suggests OA. ### Why Other Options Are Suboptimal - **DIP involvement (option 1):** OA *preferentially* affects DIP joints (Heberden nodes); RA typically spares them. This is a feature of OA, not RA. - **Asymmetric involvement (option 2):** While OA is often asymmetric, RA can also present asymmetrically early; symmetry develops later. Not a reliable early discriminator. - **Crepitus (option 3):** Both conditions produce crepitus, though the quality differs (fine vs. coarse). Not discriminatory for diagnosis. [cite:Harrison 21e Ch 326]
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