Joint Fluid Analysis Health Article

Physical characteristics

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.

Microscopic analysis

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 of the plasma level) are seen in rheumatoid arthritis. The total protein of synovial fluid is normally below 2.0 g/dL. Increased total protein is seen in rheumatoid and hemorrhagic arthritis. In rheumatoid arthritis, complement will be low owing to chronic consumption and local immunoglobulin production will cause the ratio of synovial to serum IgG to be greater than 0.5. Rheumatoid factor in joint fluid is positive in about 60% of persons with rheumatoid arthritis and in a lesser number of persons with other autoimmune diseases.

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.