A joint aspiration (arthrocentesis) is a test that involves withdrawing (aspirating) a small sample of joint fluid from a joint using a needle and syringe.

Joints are where two bones meet, allowing our bodies to move — the hips, knees, ankles, elbows, shoulders, knuckles, etc. Joints contain synovial fluid, which acts as a lubricant to help them move easily.

In a joint aspiration, a needle is carefully inserted into a joint space to collect a sample of synovial fluid.

Doctors perform joint aspiration and examine the synovial fluid to evaluate for suspected diseases or conditions in a joint.

Joint aspirations are most often done to help in the diagnosis and cause of arthritis (inflammation of a joint). Arthritis can cause redness, swelling, warmth, and pain in and around the joint, and difficulty moving the joint.

Septic arthritis is a type of arthritis caused by an infection in the joint. It is usually due to a bacterial infection in the joint. Joint aspiration helps to diagnose this condition. Septic arthritis causes symptoms like joint pain, swelling, redness, and fever. It is a serious illness that requires prompt diagnosis and treatment.

Other noninfectious causes of arthritis that can occur in kids and teens include juvenile idiopathic arthritis (or JIA, formerly called rheumatoid arthritis, or JRA), systemic lupus erythematosus (SLE), and Lyme disease.

Joint aspiration is diagnostic but it also can be therapeutic, helping to relieve pain and swelling caused by a buildup of joint fluid. Removing some of the fluid decreases pressure in the joint and improves joint movement.