What is joint fluid?

Joint fluid, also known as synovial fluid, lubricates the joints and helps maintain cartilage health. Normally, the fluid is thick, clear, and contains very few cells; however, this changes when the joint becomes diseased.

When might joint fluid analysis be recommended?

Your veterinarian may recommend assessing your pet’s joint fluid if they find joint pain or swelling caused by accumulation of excess fluid, or if they’re suspicious that your pet may have septic (infectious) arthritis.

How does joint fluid analysis help?

Joint fluid analysis can help identify the cause of lameness, which allows your veterinarian to provide targeted and often more effective treatment.

In the laboratory, the amount of protein in the fluid is measured. The fluid is also examined microscopically by a veterinary pathologist to see how many cells are present and to identify what types of cells are there. If the protein content of the joint fluid is higher than normal or if there are too many cells in the fluid, then joint inflammation is likely present.

The types of cells found in the fluid may explain the cause of inflammation. For example, high numbers of neutrophils (a type of white blood cell) suggest either bacterial infection of the joint or immune-mediated joint disease; increased numbers of mononuclear cells (white blood cells seen in longstanding inflammation) are associated with degenerative joint disease. Occasionally, there may be evidence of damaged cartilage or previous bleeding in the joint, and sometimes infectious organisms or tumor cells may be seen.

What additional diagnostic tests might be done on joint fluid?

If bacterial infection is suspected, a culture and sensitivity test of the joint fluid may be recommended. This test helps to identify bacteria that might be causing lameness and indicates which antibiotic should be used to treat the infection.

How is joint fluid obtained?

Your pet may need to be sedated or anesthetized for this procedure to prevent abrupt movement and maintain sterility of the joint. Before collecting joint fluid, the skin over the joint is shaved and carefully scrubbed, as if for surgery. Joint fluid is collected by passing a fine, sterile needle through the skin, into the joint space, and then attaching a small syringe and pulling out (aspirating) a small amount of joint fluid.

Immediately after collection, a small amount of joint fluid is spread thinly on one or more microscope slides and quickly air-dried. Any remaining fluid is placed in a sterile sample tube and sent along with the slides to the laboratory for analysis.