Arthrocentesis is performed under local anesthetic and involves inserting a needle with a syringe attached to it into the affected joint. In order to ensure greater precision and more effective results, arthrocentesis may be performed under ultrasound guidance. In many cases, if infection is not suspected, corticosteroid medication may be injected into the joint during this procedure in order to further reduce inflammation and alleviate pain. Fluid from the joint is aspirated and sent for laboratory testing.
This testing can sometimes determine the cause of the joint swelling in addition to helping patients achieve symptom relief. In the laboratory, synovial fluid is examined visually, microscopically and chemically to check for abnormalities. If the fluid appears cloudy or bloody, it may be a sign of infection or injury. If there are uric acid crystals present, the patient likely has gout. Other abnormalities of synovial fluid may include the presence of glucose or protein.