IM7.11-13 | Rheumatologic Diagnostic Testing — Summary & Reflection

KEY TAKEAWAYS

Rheumatologic diagnostic testing follows a three-principle framework: hypothesis first, integration not isolation, and specificity for confirmation.

Key autoantibody facts:
- RF: sensitivity 70–80% for RA; specificity ~80%; positive also in Sjögren, hepatitis C, healthy elderly
- Anti-CCP: sensitivity 67–75%; specificity 95–98% for RA; the superior specificity test; high titre = erosive disease prognosis
- ANA: sensitivity 95–98% for SLE; specificity 57–74%; screening test only — always follow positive ANA with anti-dsDNA and anti-Sm
- Anti-dsDNA: sensitivity 57–70%, specificity 97% for SLE; titres track disease activity; rising titre + falling complement = impending flare
- Anti-Sm: sensitivity 25–30%, specificity 99% for SLE; essentially diagnostic when positive in clinical context

Synovial fluid interpretation:
- Normal: <200 WBC/μL; Group I (OA): 200–2,000; Group II (inflammatory RA/crystal): 2,000–50,000; Group III (septic): >50,000
- Gout: needle-shaped, NEGATIVELY birefringent MSU crystals (yellow when parallel to compensator)
- Pseudogout: rhomboid, POSITIVELY birefringent CPPD crystals (blue when parallel to compensator)

Key X-ray features:
- RA: periarticular osteopenia, marginal erosions at bare area, uniform joint space narrowing
- OA: osteophytes, non-uniform joint space narrowing, subchondral sclerosis, subchondral cysts; NO erosions
- Gout: punched-out erosions with overhanging margins, preserved joint space
- Ankylosing spondylitis: bilateral sacroiliitis (sclerosis, erosion, fusion), bamboo spine

REFLECT

Return to Dr Meera from the opening hook. Her error was not ordering the wrong tests — ANA, RF, and anti-CCP are the right tests for their respective conditions. Her error was ordering them without a pre-test hypothesis. Now consider three patients: (1) A 35-year-old woman with symmetric MCP polyarthritis and prolonged morning stiffness — should you order ANA first or RF/anti-CCP first, and why? (2) A 24-year-old woman with malar rash, oral ulcers, and lymphopenia — should you order RF or ANA first, and why? (3) A 62-year-old man with an acute hot knee and fever — should you order an autoimmune panel or perform arthrocentesis first, and why? The answers reveal the principle: the investigation selected should reflect the diagnostic hypothesis generated by the clinical assessment — not a blanket screen that generates more confusion than clarity.