55 year old male presents to your emergency department with a warm, erythematous, swollen right knee.  You perform an arthrocentesis of the joint and synovial fluid analysis reveals a WBC 8,000 cells/µL with 50% PMNs and positive birefringent crystals.  Which of the following is the most likely diagnosis?  (scroll down for the answer) 

A) Gout

B) Pseudogout

C) Gonococcal Septic Arthritis 

D) Non-gonococcal Septic Arthritis 

The correct answer is B) Pseudogout 

A synovial fluid analysis with WBC 2,000 cells/µL to 50,000 cells/µL and >50% PMNs suggests an inflammatory arthritis, while an analysis with WBC > 50,000 cells/µL and >75% PMNs suggests a septic arthritis. The crystals associated with gout are negative birefringent, while the crystals associated with pseudogout are positive birefringent.  

Gout Pearls 

• Most common joint involved: 1st MTP
• Crystals: needle like, monosodium urate, negative birefringent
• XR findings: “rate bite” erosions
• Treatment: NSAIDs, colchicine, steroids

Pseuodogout Pearls

• Most common joint involved: Knee
• Crystals: rhomboid shape, calcium pyrophosphate, positive birefringent 
• XR findings: white lines of chondrocalcinosis 
• Treatment: NSAIDs, colchicine, steroids

Stapczynski, J. Stephan,, and Judith E. Tintinalli. Tintinalli's Emergency Medicine: A Comprehensive Study Guide. 7th ed. New York, N.Y.: McGraw-Hill Education LLC., 2011.