Evaluation of the synovial fluid from patients with clinical diseases that involve one or more joints is a valuable diagnostic tool in establishing the cause, treatment and prognosis of various arthropathies. The examination should include evaluation of physical features such as color, viscosity, and volume, and laboratory analysis including cell counts, examination of cytologic preparations from the centrifuged cell sediment, bacteriologic cultures, and other biochemical analysis as deemed necessary. The results are interpreted in light of the routine clinical, radiology and clinical pathology data to establish the underlying cause of the joint disease.

When a disease process affects the joint, the normal equilibrium and function of the synovial membranes are disturbed. For example, inflammation brings with it the cuncomitant increased fluid of an inflammatory effusion. This effusion decreases the joint fluid viscosity either by dilution or by the influence of inflammatory cell hyaluronidase that directly breaks down hyaluronic acid. Bacterial hyaluronidase has a similar action in septic arthropathies. The introduction of inflammatory cells into the joint space leads to further severe consequences. Lysosomal enzymes released from degenerating neutrophils act on the vulnerable articular cartilage to pro- duce erosive changes in the cartilage. This damage is further compounded by lost viscosity and compressibility of the joint fluid, which results in direct cartilage contact and abrasive v ear. Fibrin clots formed within the joint space can impede resorption of fluid and impair drainage of any particulate material via the venous or lymphatic channels of the synovial membrane.

Strict asepsis must be observed when performing joint fluid aspiration. Sterile preparation with surgical scrubbing of the effected joint is necessary to avoid infection. Local or full anesthesia is advised to avoid injury to intra-articular and periarticular structures. The joints most commonly aspirated include the carpal, elbow, shoulder, stifle, hock and hip. The anatomic sites used for fluid aspiration are the same as those used for intra-articular injections. The fluid is collected using a 3-5 ml syringe and a 20-22 gauge needle. For smaller dogs and cats, the use of a 25 gauge needle and tuberculin syringe may be more appropriate. The needle is gently advanced into the joint; when the joint space is entered, a loss of resistance is felt. Gentle, negative pressure is applied on the syringe to allow the fluid to enter the syringe barrel. Discontinue suction when flow stops or if frank blood appears. Gently but quickly remove the needle. The contents are immediately added to a tube containing EDTA anticoagulant (LTT) and residual fluid is added to a culturette swab. If the volume collected is small (less than 0.1 ml), prepare 2-3 smears on glass slides, rapidly air dry and submit them for cytology. The syringe with the residual fluid (minus the needle) is sealed and submitted for culture at an additional charge.

Routine laboratory examination of synovial fluid depends, to a large extent, on the volume of fluid obtained. In order of priority, the following tests should be performed:

1. Cytology;
2. Cell counts;
3. Chemical tests;
4. Bacteriology

Normal canine synovial fluid has generally fewer than 2,000 cells/mm³. Increased counts are observed in most types of arthropathy, the number varying with the severity of the reaction. Normal fluid contains mostly mononuclear cells, including lymphocytes, histiocytes, and shed synovial lining cells. Neutrophils and red cells are uncommon unless blood contamination occurred at the time of collection.