Cell Count and Differential, Synovial Fluid

What Is Synovial Fluid?

Synovial fluid is the thick liquid that lubricates your joints and keeps them moving smoothly. It’s on all of your joints, including in your knees, shoulders, hips, hands, and feet.

What Is a Synovial Fluid Analysis?

A synovial joint fluid analysis is a group of tests your doctor can use to diagnose problems with your joints.

Joint conditions like arthritis, gout, infections, and bleeding disorders can change how your synovial fluid looks and feels. A sample of this fluid taken during a procedure called an arthrocentesis can help your doctor figure out what’s causing your symptoms.

What’s The Purpose of a Synovial Fluid Analysis?

If you have joint symptoms like:

→ Pain
→ Redness
→ Swelling
→ Fluid buildup

Synovial fluid analysis can also help your doctor figure out if you have:

→ A condition that inflames your joints, like gout, rheumatoid arthritis, and lupus
→ An infection like septic arthritis
→ Bleeding disorders like hemophilia or von Willebrand disease
→ A disease that break down joints over time, like osteoarthritis

Your doctor might also use this test to see if your treatment for your joint condition is working.

A synovial fluid analysis may be ordered to diagnose or rule out the following conditions:

→ Autoimmune disorders:

Autoimmune disorders are conditions in which a person’s immune system mistakenly attacks healthy tissue. In some autoimmune disorders, including rheumatoid arthritis, lupus, and psoriatic arthritis, the immune system damages healthy tissues of the joints.

→ Septic arthritis:

Septic arthritis is an infection caused by microbes such as bacteria or fungi that lead to acute inflammation in one or more joints.

→ Gout or pseudogout:

Gout and pseudogout are painful forms of arthritis, caused by the accumulation of uric acid or calcium crystals in the joint.

→ Bleeding in the joint:

Also called hemarthrosis, an accumulation of blood in the joint can be caused by an injury to the joint, cancer, hemophilia, and other health conditions.

Synovial fluid analysis is helpful for determining the underlying cause of arthritis, particularly for septic or crystal-induced arthritis. The white cell count, differential count, cultures, Gram stain, and crystal search using polarized light microscopy are the most useful studies.

Arthrocentesis with synovial fluid analysis should be attempted in all patients who have a joint effusion or signs suggestive of inflammation within the joint, without a known cause. The most important reason to perform a synovial fluid analysis may be to evaluate for septic arthritis in a patient presenting with an acutely swollen joint that is painful and warm even in patients with a known arthritic disease. 

Synovial fluid analysis is also helpful in narrowing the differential diagnosis by distinguishing between categories of joint effusions (ie, inflammatory versus noninflammatory), and to diagnose hemarthrosis.