Joint Fluid Analysis Health Article

Definition

Joint fluid analysis, also called synovial fluid analysis, or arthrocentesis, is a procedure used to assess joint-related abnormalities, such as occur in the knee or elbow. Synovial or joint fluid is an ultrafiltrate of plasma formed in the synovial membrane of movable joints. The fluid lubricates the bone and cartilage tissues of the joint.

Purpose

The purpose of joint fluid analysis is to diagnose arthritis, an inflammation of the joint, and identify its cause. In addition, removal of the fluid can decrease pain in the joint. Diseases which may cause joint swelling include rheumatoid arthritis, systemic lupus erythematosus, gout, gonococcal arthritis (caused by the bacteria that causes gonorrhea), other types of bacterial arthritis, and viral inflammation of the synovial lining.

Precautions

Universal precautions for the prevention of transmission of bloodborne pathogens should be observed when collecting synovial fluid. Arthrocentesis should not be performed on a patient who is uncooperative, especially if the patient cannot or will not keep the joint immobile throughout the procedure. Sampling of a joint may be contraindicated when there is evidence of infection in overlying skin or tissue. The joint space should be accessible. Therefore, a poorly accessible joint space, such as in hip aspiration in an obese patient, should not be subjected to this procedure.

Arthrocentesis

The removal of synovial fluid, arthrocentesis, is also called a joint tap, or closed joint aspiration. The procedure is done by passing a needle into a joint space and aspirating synovial fluid using aseptic technique. The joint must be cleaned thoroughly with iodine before inserting the needle to prevent any infection. The size of the needle and volume of fluid withdrawn depends on the size of the joint. The patient is asked to lie on their back and remain relaxed. A local anesthetic, typically a sub-cutaneous injection of lidocaine, xylocaine, or ethyl chloride, is then administered. As the needle enters the joint, a "pop" may be felt or heard; this is normal. Correct placement of the needle in the joint space is normally painless. At this point, the clinician slowly drains some of the fluid into the syringe. The syringe may contain a small amount of sodium heparin. The needle is then withdrawn and an adhesive bandage is placed over the puncture site. The sample is transferred to one or more tubes containing liquid heparin or liquid EDTA anticoagulant.

The procedure takes about 10 minutes. The physician may need to prioritize the tests that he or she orders since fluid yields from this procedure may be very small. Only a drop of fluid is needed for culture and microscopic examination and these two procedures are given top priority because of their diagnostic importance. Some tests, such as glucose and total hemolytic complement must be evaluated with respect to blood levels. Therefore, a blood sample should be collected at the same time.

Causes of arthritis

Arthritis can be classified by cause into five categories. Noninflammatory or osteoarthritis is the most common form and results from loss of cartilage covering the bone. Inflammatory arthritis results from damage to the joint caused by immune complexes that deposit in the joint or autoantibodies that attach to and destroy the synovial membrane. The most common cause of inflammatory arthritis is rheumatoid arthritis. Septic arthritis is caused by bacterial infection of the joint, The most commonly implicated organism in sexually active persons is Neisseria gonorrhoeae. Gout is joint inflammation caused by deposition of uric acid crystals. When other crystals such as calcium pyrophosphate are the cause, the condition is called pseudogout. Hemorrhagic arthritis, bleeding into the joint, is caused by trauma, hemophilia, or other bleeding disorder such as thrombocytopenia (low platelet count).

Laboratory tests

Laboratory analysis of joint fluid should include determination of color, transparency, volume, and viscosity; red and white blood cell counts with a differential; examination of wet mounts for synovial crystals; micro-biological culture; and tests for glucose, total protein, complement, rheumatoid factor, and mucin clot.