A 42-year-old woman presents with a 3-month history of symmetrical polyarthritis affecting the small joints of her hands and feet, morning stiffness lasting 2 hours, and constitutional symptoms. Clinical examination reveals soft tissue swelling over the PIP and MCP joints bilaterally. Which investigation is most appropriate to confirm the diagnosis of rheumatoid arthritis?

Explanation

## Investigation of Choice for RA Diagnosis **Key Point:** Anti-CCP antibody is the single most specific and sensitive serological marker for rheumatoid arthritis, with >95% specificity and superior prognostic value over rheumatoid factor. **High-Yield:** Anti-CCP positivity predicts: - Erosive disease - More aggressive joint damage - Poor functional outcomes - Presence even in seronegative RA (RF-negative, anti-CCP positive) ### Why Anti-CCP is Superior | Feature | Anti-CCP | RF | ESR/CRP | X-ray | |---------|----------|----|---------|---------| | **Specificity** | >95% | 80–85% | Low (non-specific) | Late finding | | **Sensitivity** | 70–80% | 70–80% | Variable | 0% in early RA | | **Prognostic value** | Excellent | Moderate | Moderate | Confirms damage | | **Seronegative RA** | Can be positive | Negative | May be elevated | May show changes | | **Early detection** | Yes (pre-clinical) | Yes | Yes | No | **Clinical Pearl:** Anti-CCP can appear 5–10 years *before* clinical symptoms (in asymptomatic relatives), making it a true prognostic marker, not just a diagnostic one. ### Role of Other Investigations - **ESR/CRP:** Markers of inflammation, not diagnostic. Non-specific; elevated in many conditions. - **Rheumatoid Factor:** Older test; lower specificity than anti-CCP. Still useful but not first-line. - **Plain radiographs:** Show erosions and joint space narrowing only *after* disease has caused structural damage (typically >3 months into disease). Not useful for early diagnosis. **Mnemonic — RA Serology Hierarchy:** **ACE-RF** - **A**nti-CCP (most specific, prognostic) - **C**RP/ESR (non-specific inflammation markers) - **E**arly diagnosis (anti-CCP detects pre-clinical disease) - **RF** (Rheumatoid Factor — older, less specific) [cite:Harrison 21e Ch 313]