1. Glucose. Determination of synovial fluid glucose, when interpreted with a simultaneous serum value, is helpful in diagnosing infectious arthritis. In bacterial infection or tuberculosis, the synovial fluid glucose will be less than half the serum value. Occasionally, low values may be seen in rheumatoid arthritis (RA).
2. Protein determination does not provide additional useful information and should not be routinely ordered.
3. Complement may be decreased in RA, but the test is rarely helpful for diagnosis because synovial fluid complement is usually normal in early RA.
II. Diagnosis by fluid group (Iabie,.5:1). Synovial fluid can be divided into three groups based on the degree of inflammation.
A. Group 1 fluids are clear and transparent and have few white cells on cell count. They include normal, osteoarthritic, and systemic lupus erythematosus (SLE)
B. Group 2 fluids generally have a higher white cell count and are not as clear as group 1 fluids; they appear translucent. This group includes fluids from most noninfectious, inflammatory arthritic conditions such as gout, pseudogout, psoriatic arthritis, Reiter's syndrome, and RA. Leukemia or lymphoma occasionally presents in this category, but the differential count reveals more than 90% mononuclear cells.
C. Group 3 fluids are opalescent or purulent. Group 3 fluids include those from bacterial infections and tuberculosis (although joint fluid from gonococcal arthritis can be either group 2 or group 3). Group 3 fluids typically have 50,000 to 300,000 white blood cells per milliliter; these are mostly neutrophils. Occasionally, the synovial fluid from a patient with an inflammatory arthritic condition such as RA may have as many as 50,000 to 75,000 white cells per milliliter and appears opalescent or even purulent. As Table5-1 shows, there is considerable overlap between the various arthritic diseases; this table is meant to serve as a guideline rather than provide a rigid set of criteria.
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