Synovial fluid analysis and knee aspiration
When and how to aspirate a knee, which tubes to send, how to read the fluid step by step (appearance, cell count, Gram stain and culture, polarised microscopy), the likelihood ratios for sepsis, the crystals and their mimics, prosthetic joint thresholds, and when the result changes management.
Tier 1Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- Aspirate every acute hot joint unless it is a typical flare in someone with established gout. The fluid separates sepsis, crystals, haemarthrosis and non-inflammatory effusions faster than any blood test.
- Send fluid for cell count with differential, Gram stain, culture and polarised microscopy, every time.
- The white cell count is a probability shift, not a verdict. Over 50 × 10^9/L favours sepsis but gout reaches it; partly treated sepsis can sit under 25.
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Sources
- Therapeutic Guidelines: Rheumatology (acute monoarthritis; gout; calcium pyrophosphate deposition)AU/NZTherapeutic Guidelines Limited (eTG complete), current online version, accessed November 2026
- Therapeutic Guidelines: Antibiotic. Bone and joint infectionsAU/NZTherapeutic Guidelines Limited (eTG complete), current online version, accessed November 2026
- Coakley G et al. BSR and BHPR, BOA, RCGP and BSAC guidelines for management of the hot swollen joint in adults. Rheumatology 2006;45:1039-41British Society for Rheumatology, 2006; still current, update in development
- Management of septic arthritis in adults with a hot swollen joint: British Society for Rheumatology guideline scope. Rheumatol Adv Pract 2025British Society for Rheumatology, 2025 scope; full guideline not published as of November 2026
- Margaretten ME et al. Does this adult patient have septic arthritis? JAMA 2007;297:1478-88JAMA, 2007
- Parvizi J et al. The 2018 definition of periprosthetic hip and knee infection: an evidence-based and validated criteria. J Arthroplasty 2018;33:1309-14Journal of Arthroplasty, 2018
- McNally M et al. The EBJIS definition of periprosthetic joint infection. Bone Joint J 2021;103-B:18-25European Bone and Joint Infection Society, 2021
- Ahmed I, Gertner E. Safety of arthrocentesis and joint injection in patients receiving anticoagulation at therapeutic levels. Am J Med 2012;125:265-9American Journal of Medicine, 2012
- Abhishek A et al. The 2023 ACR/EULAR classification criteria for calcium pyrophosphate deposition disease. Ann Rheum Dis 2023;82:1248-57ACR and EULAR, 2023
- Last updated
- 14 November 2026
What changed
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