How does arthrocentesis help diagnose osteoarthritis?

Arthrocentesis -- removing lubricating fluid from inside a painful or swollen joint -- can help diagnose osteoarthritis by ruling out other conditions. To collect the fluid, a clinician will give a local anesthetic to numb the joint and insert a needle to take a sample. The fluid is sent to the lab to be checked for signs of other problems that could be causing the pain and swelling, such as gout or an infection. If other problems are ruled out, osteoarthritis is probably the diagnosis.

http://www.hss.edu/professional-conditions_13649.asp

http://www.arthritis.org/disease-center.php?disease_id=32&df=diagnosed