Arthrocentesis is the removal of fluid from a joint.  It is indicated for diagnostic and therapeutic reasons.  It can help to differentiate infection, hemorrhagic, and rheumatologic etiologies of joint effusion.  Fluid should be sent for cell count and differential, gram stain, crystals, and culture. 

Gout:  monosodium urate (MSU) crystals and calcium pyrophosphate dihydrate (CPPD) crystals are negitavely birefrigent

Pseudogout: calcium pyrophosphate dihydrate (CPPD) crystals are rhomboidal or rectangular shape and have positive birefringence

Infection:  septic arthritis risk increases with leukocyte count in the synovial fluid: the positive likelihood for >25K, >50K. and >100K is 2.9 to 7.7 and to 28, respectively.  Increase in number of PMNs.

Keep in mind infectious etiologies other than bacterial: gonococcal, lyme, TB, fungal.  Additional testing of fluid may be needed to make these diagnoses.