Joint Fluid Analysis
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.
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.
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.
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 The size of the needle and volume of fluid withdrawn depends on the size of the joint.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 The needle is then withdrawn and an adhesive bandage is placed over the puncture site.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 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.
Normal joint fluid has a volume between 0.15-3.5 mL. An increased volume is common in all five classes of arthritis. The fluid should be clear and pale yellow. Observation of the color of the supernatant after centrifugation helps to discriminate between a hemorrhagic fluid (bleeding from injury to the joint) and a traumatic tap (puncture of a blood vessel during arthrocentesis). Deep yellow or pink fluid points to a hemorrhagic process whereas a normal color points to a traumatic tap. A clot in the fluid also points to a traumatic tap because large protein molecules such as fibrinogen are not found in the fluid. The specific gravity is normally the same as plasma, but the viscosity is much greater. The viscosity is measured by inserting a wooden applicator stick into the fluid and removing it. The fluid should form a thread of at least 1.6 in (4 cm) at the end of the applicator stick. Failure to do so indicates a low level of mucoprotein which is most common in inflammatory arthritis.
The white blood cell (WBC) count is performed manually using a hemacytometer. The fluid is diluted in saline rather than a WBC counting fluid because the acetic acid will cause the formation of a mucin plug. The normal WBC count is very low, less than 200 per micro-liter. High counts are seen in septic arthritis, rheumatoid arthritis, and gout. Persons with osteoarthritis or hemorrhagic arthritis may or may not have a high WBC count. The highest counts with neutrophils predominating are associated with septic arthritis.
The normal differential shows 50-65% monocytes with neutrophils and lymphocytes each accounting for less than 25% of the WBCs. Neutrophils above 80% signal septic arthritis. A higher percentage of lymphocytes favors a diagnosis of rheumatoid arthritis. Neutrophils with dark staining cytoplasmic inclusions (ingested immunoglobulins) point to a diagnosis of rheumatoid arthritis. The red blood cell (RBC) count of synovial fluid is also performed manually. The count is normally less than 2000 per microliter. Higher counts especially in the presence of xanthochromia (abnormal color) indicate a synovial hemorrhage.
Microscopic examination of the fluid for crystals requires a polarizing microscope with a red compensating filter. This type of microscope transmits light in a single plane through the specimen. An analyzer filter is placed above the specimen before the ocular is aligned, so that it is out of phase with the polarizing filter. The analyzer filter blocks the light transmitted through the specimen, causing a dark background unless the light is rotated by the object on the slide. Uric acid and calcium pyrophosphate are the two most common crystals seen in joint fluid. They both rotate plane polarized light which causes them to be illuminated by the polarizing microscope. However, they can be differentiated using the red compensating filter. Uric acid crystals are seen as yellow needles when the long axis of the crystal is parallel to the slow vibrating light from the filter. Calcium pyrophosphate crystals are blue when the long axis of the crystal is parallel to the slow vibrating light.
Biochemical and immunological tests
Glucose in synovial fluid is normally within 10 mg/dL of the plasma level. Low glucose is seen in septic and rheumatoid arthritis. Very low levels (less than half
The mucin clot test is used to measure the amount of mucin in the fluid. This substance consists of repeating subunits of hyaluronic acid that will cross link forming a mucin clot when acetic acid is added to the fluid. In infection bacterial hyaluronidase may destroy the mucoprotein causing no clot. In rheumatoid arthritis, damage to the synovial cells results in deficient production of mucoprotein and the mucin clot is either absent or easily broken apart.
All samples of synovial fluid should be cultured and Gram-stained. The Gram stain results can be definitive for septic arthritis if neutrophils and bacteria are found, but is not always positive. The fluid should be inoculated on blood agar plates; on chocolate (heated blood) agar plates for gonococcus and Haemophilus; and in broth such as thioglycolate for the isolation of anaerobic bacteria. N. gonorrhoeae is responsible for about 75% of septic arthritis in young and middle aged adults. Staphylococcus spp. account for about 75% of septic arthritis in the elderly. Staphylococcus, Streptococcus, and Haemophilus are the most common genera isolated from children.
Prior to the procedure, any risks that are involved should be explained to the patient. The patient will be given a local anesthetic but no pain medications or sedatives are required. If the clinician requests a glucose test, the patient will be asked to fast for six to 12 hours before the procedure. If not, there is no special preparation required for a joint fluid analysis.
Some post-procedural pain may be experienced. For this reason, the patient should arrange to be driven home by someone else. Aftercare of the joints will depend on the results of the analysis.
Aspirate—The removal by suction of a fluid from a body cavity using a needle.
Bursae—A closed sac lined with a synovial membrane and filled with fluid, usually found in areas subject to friction, such as where a tendon passes over a bone.
Gout—A painful joint disease characterized by the deposition of uric acid crystals in the joint. These crystals cause swelling and redness.
Hematoma—A localized mass of blood that is confined within an organ or tissue.
Hyaluronic acid—A high molecular weight made of carbohydrate and glucuronic acid that is found in high concentrations in the synovial fluid. This provides thickness to the fluid so it can cushion the joint.
Joint—A moveable portion of bone found between two bones.
Lupus erythematosus—A multisystem disease with an autoimmune etiology. Lupus causes problems with the skin, kidneys, joints and the serosal membranes.
Rheumatiod arthritis—A chronic and progressive inflammation of the joints whose cause is unknown.
Synovial fluid—A transparent lubricating fluid secreted in a sac to protect an area where a tendon passes over a bone.
While joint fluid analysis is generally a safe procedure, especially when performed on a large, easily accessible joint, such as the knee, some risks are possible. Some of the complications to the procedure, although rare, include infection at the site of the needle stick, an accumulation of blood (hematoma) formation, local pain, injury to cartilage, tendon rupture, and nerve damage.
Normal values typical for synovial fluid are shown below:
- Volume: 0.15-3.5 mL.
- Transparency: clear.
- Color: straw or pale yellow.
- Viscosity: 1.6 in (4 cm) thread or greater.
- Mucin clot: firm.
- Glucose: different from plasma by 10 mg/dL or less.
- Total protein: less than or equal to 2.0 g/dL.
- White blood cell count: less than 200 per microliter.
- Differential: less than 25% granulocytes.
- Crystals: negative.
Health care team roles
The removal of synovial fluid from a joint should be done by a physician. Laboratory tests are performed by clinical laboratory scientists/medical technologists. Physicians interpret the results of laboratory tests.
Burtis, Carl A. and Edward R. Ashwood. Tietz Textbook of Clinical Chemistry. Philadelphia: W.B. Saunders Company, 1999.
Kaplan, Lawrence A. and Amadeo J. Pesce. Clinical Chemistry, Theory, Analysis and Correlation. St. Louis: Mosby Publishers, 1996.
Stobo, John D., David B. Hellman, Paul W. Ladenson,, Brent G. Petty and Thomas A. Traill. Principles and Practice of Medicine, 23rd Edition. Stamford, CT: Appleton and Lange, 1996.
Jane E. Phillips, PhD