Arthrocentesis and Synovial Fluid Analysis
Mariana Gonzalez Trevino
Arthrocentesis should be performed in virtually every patient with acute monoarthritis. Also perform if infection is suspected or in diagnostic uncertainty.
Relative contraindications: Extensive cellulitis or psoriatic plaque around the site of interest (risk of introducing bacteria into a sterile space), coagulopathy, bacteremia, concern for infection of a prosthetic joint.
Consult orthopedics for consideration of aspiration.
| Category | Appearance | Viscosity | WBC Count (/μL) | PMN (%) | Crystals | Major Diagnoses |
|---|---|---|---|---|---|---|
| Normal | Clear, colorless | High (stringy) | 200 | 25% | None | Normal joint |
| Non-inflammatory | Clear to straw | High | <2,000 (usually 500) | 25% | None | Osteoarthritis, trauma, avascular necrosis |
| Inflammatory | Yellow, cloudy | Low | 2,000-50,000 | >50% | ± Crystals | RA, crystal arthritis (gout, CPPD), reactive arthritis, psoriatic arthritis, SLE |
| Septic | Yellow-green, opaque/purulent | Very low | >50,000 (often >100,000) | ~75-90% | None (crystals do not exclude infection) | Bacterial septic arthritis; always culture even if crystals present |
| Hemorrhagic | Bloody (xanthochromic if old) | Variable | Variable (RBCs predominate) | Variable | None | Trauma, coagulopathy, pigmented villonodular synovitis, tumor, fracture through joint |
Key Pearls for Synovial Fluid Interpretation
- Synovial lactate >10 mmol/L has positive LR of 7.9 for septic arthritis.
- Gram stain has only 22% sensitivity but 99.6% specificity—a positive result is reliable.
- Crystals + infection can coexist—always send culture even when crystals are identified.
- Negatively birefringent, needle-shaped crystals = gout.
- Weakly positive birefringent, rhomboid crystals = CPPD.