Synovial fluid aspiration<unk>  = Negatively birefringent, needle-shaped monosodium urate crystals<unk>  = Positively birefringent, rhomboid-shaped, calcium pyrophosphate crystals
Serum uric acid levels are not helpful (30% of patients with<unk>  attack have normal levels)
Uric acid during attacks can be low due to the precipitation of<unk>  crystals
High uric acid is >6.8
ESR may be elevated
No bacteria on Gram Stain<unk> : X-ray of joint space may show radiolucent calcium pyrophosphate crystals<unk>  = Negatively birefringent, needle-shaped monosodium urate crystals<unk>  = Positively birefringent, rhomboid-shaped, calcium pyrophosphate crystals
Uric acid during attacks can be low due to the precipitation of<unk>  crystals
High uric acid is >6.8
>1,100 (prosthetic joint)
>25,000; LR=2.9
>50,000; LR=7.7
>100,000; LR=28
>64% (prosthetic joint)
>90%
Viscosity of synovial fluid may actually be decreased in inflammatory or infectious etiologies, as hyaluronic acid concentrations decrease
The presence of crystals does not rule out septic arthritis; however, the diagnosis is highly unlikely with synovial WBC < 50,000
Gout and pseudogout
Acute osteoarthritis
Boxer's fracture
Avascular necrosis
Colles' fracture
Crystal-induced 
Salter-Harris fractures
Gonococcal arthritis, arthritis-dermatitis syndrome