What are the most common indications for arthrocentesis and synovial fluid analysis
• Suspicion of infection: As little as 2 mL of fluid is sufficient for Gram stain, culture, and white blood cell (WBC) count and differential.
• Suspicion of crystal-induced arthritis: The sensitivity of polarizing microscopy in identifying birefringent crystals approaches 90% in acute gout and 70% in acute pseudogout.
• Suspicion of hemarthrosis: Bloody joint fluid is characteristic of traumatic arthritis, clotting disorder, and rare conditions such as pigmented villonodular synovitis and hydroxyapatite crystalline arthropathy.
• Differentiating inflammatory from noninflammatory arthritis: The degree of elevation of synovial fluid WBC count is useful in narrowing the list of possible diagnoses.