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Synovial fluid analysis

Body Fluids · also called Joint fluid analysis, Arthrocentesis fluid

What it measures

Examines joint fluid for appearance, cell count, crystals and organisms. It distinguishes septic, crystal, inflammatory and non-inflammatory arthritis.

Specimen and preparation

Specimen: Synovial fluid in sterile, EDTA and heparin tubes
Tube: Other top
Preparation: Examine for crystals within a few hours; avoid powdered anticoagulants such as oxalate.

Reference range

Group	Range
Appearance	Clear, straw, viscous
WBC	< 200 cells/µL
Neutrophils	< 25 %
Crystals	None

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

WBC 200–2,000: non-inflammatory (osteoarthritis, trauma)
WBC 2,000–50,000: inflammatory (RA, gout, reactive)
WBC > 50,000 with neutrophils > 90%: septic arthritis until proven otherwise
Needle crystals, negative birefringence: gout (urate)
Rhomboid, weakly positive: CPPD (pseudogout)

Method

Cell count and differential, polarised light microscopy, Gram stain, culture

Good to know

Crystals do not exclude infection; always culture a hot swollen joint.

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