Aspiration of a joint (arthrocentesis) with subsequent analysis of the synovial fluid is a critical component in diagnosing arthritis. [1, 2] Analysis of the joint fluid can differentiate an inflammatory arthritis from a noninflammatory arthritis. A definitive diagnosis of crystalline arthritis or septic arthritis can be made only by means of joint aspiration.
Joint aspiration is a relatively quick and inexpensive procedure to perform. It can be done in an office setting or in a hospital. No particular certification is required to perform arthrocentesis; it can be done by any physician, physician’s assistant, or advance practice nurse who has the appropriate training and equipment. Initial analysis of the fluid can be performed in the office with the use of a polarized light microscope.
Joint aspiration should be considered for any patient with an inflamed joint or joints who does not have an established diagnosis. Inflamed joints are recognized by being red, warm, tender, swollen, and painful to bend.
A joint presenting as acute monoarthritis should always be aspirated if infection is suspected upon clinical evaluation. Patients with preexisting arthritis (eg, rheumatoid arthritis or gout) are at increased risk for the development of septic arthritis. Therefore, aspiration must be performed whenever there is suspicion of an infected joint in patients with known arthritis.
Repeated aspirations can be part of the management of a septic joint to relieve discomfort and prevent joint damage. Aspiration can be considered in cases of hemarthrosis to prevent adhesions. Aspiration can be performed immediately prior to injecting intra-articular medications such as corticosteroids to improve efficacy.
As a rule, if joint aspiration is being considered, it should probably be performed.
According to Infectious Diseases Society of America (IDSA) guidelines on management of prosthetic joint infection (PJI), diagnostic arthrocentesis should be performed for any suspected acute PJI unless the diagnosis is clinically evident, surgery is planned, and antibiotics can safely be withheld preoperatively.  It is also advised in patients with a chronic painful prosthesis who have unexplained elevations of erythrocyte sedimentation rate or C-reactive protein level or in whom PJI is clinically suspected (though it may not be necessary in all cases).
In this setting, synovial fluid analysis should include a total cell count and differential leukocyte count, as well as culture for aerobic and anaerobic organisms.  A crystal analysis can also be performed if clinically indicated.
No strict contraindications to arthrocentesis are recognized; however, caution is advised in certain situations.
A needle should not be passed through an area of infection (eg, overlying cellulitis) before entering a joint, because seeding infection into the joint capsule may occur.
Patients who are anticoagulated or have a bleeding diathesis (eg, hemophilia or thrombocytopenia) are at increased risk for hemarthrosis. When possible, aspiration should be delayed until the coagulopathy is reversed. When a delay is not possible, the physician should be prepared to treat bleeding such as with appropriate factor concentrates in a hemophiliac patient.
When aspiration of artificial joints is necessary, it is generally handled by an orthopedic surgeon.
When a clinical need to aspirate a joint is present in a patient with overlying infection or coagulopathy, the physician must weigh the risks and benefits of aspiration in their decision whether to proceed with arthrocentesis.
Any joint can be aspirated; however, some joint aspirations require the use of ultrasonographic or fluoroscopic guidance. Ultrasonography allows the clinician to confirm the presence of fluid before aspirating. It can also be helpful in aspirating deep or technically difficult joints. The hip joint should be aspirated under ultrasonographic guidance. Joints of the spine, including the sacroiliac joint, should be aspirated under fluoroscopic guidance. Guidance for aspiration is also recommended when blind attempts have failed to access any joint fluid.