Synovial fluid (septic arthritis)
The most useful diagnostic laboratory test for identifying septic arthritis is an evaluation of the synovial fluid from the affected joint (culture, Gram stain, crystals analysis, white blood cell count with differential). Synovial fluid with a white blood cell count greater than 50,000 and 90% neutrophil predominance suggests a bacterial source. Identification of a bacterial organism in the synovial fluid confirms the diagnosis.
General points about synovial fluid in septic arthritis include:
- appearance: turbid
- viscosity: LOW
- WCC: 100,000 x 10^6/L Neutrophils
- crystals: NONE
- culture: positive
Reference
- Margaretten ME, Kohlwes J, Moore D, Bent S. Does this adult patient have septic arthritis? JAMA. 2007 Apr 04;297(13):1478-88
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