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Arthrocentesis Quick Look

Raeann Whitney

When in doubt, tap it!

  • Send synovial fluid for Gram stain/culture and cell count

  • Beware: Inflammatory / crystal arthropathies can have very high leukocyte counts!

    • The presence of crystals does NOT exclude septic arthritis
  • A fever or concern for infection in a prosthetic joint is a surgical emergency; ortho will come in the middle of the night to tap a joint

  • Contraindications to tap: overlying cellulitis or psoriatic plaque

    • Anticoagulation / DVT ppx is OK (no need to hold)
​Arthritis Type Appearance ​ Total WBC ​ PMNs ​
Non-inflammatory ​ Straw like / yellow ​ \<2000​ \<25%​
Inflammatory / Crystalopathies ​ Yellow / cloudy ​ >2000​ (often 20-50K+) >50%​
Septic ​ Purulent ​ >50,000​ (often >100K) >75%​


  • Monosodium urate: needle-shaped, negatively birefringent

    • Sensitivity is generally good (>90s%)
  • Calcium pyrophosphate dihydrate: rhomboid-shaped, positively birefringent (weakly)

    • Weak birefringence significantly reduced sensitivity

Last update: 2022-06-27 03:29:20