Joint is aspirated under sterile conditions
Can be used to evaluate joint symptoms – especially if joint effusion is present. Most important indication is to check for joint infection.
Normally joint fluid is transparent and almost colourless (straw coloured).
Analysis of joint fluid:
1) Gross inspection (colour, clarity, viscosity, opacity, presence of blood)
2) Microscopy (white cell count; presence of fragment (eg cartilage), presence of crystals – using polarised light)
3) Microbiology (gram stains and acid fast methods, culture)
4) Mucin clotting (normally forms clot when acetic acid is added – get poor clot if inflammatory)
5) Other (lactic acid level, glucose, rheumatoid factor, complement, protein content)
Classification of synovial fluid inflammation:
Group one fluids:
• clear, transparent, may have some white blood cells, may be slightly turbid
• Total WBC count – 200-2000 mm3 (normally <200) • Polymorphonuclear leucocytes <20% (normally <10%) • eg osteoarthritis, joint trauma, avascular necrosisGroup two fluids: • inflammatory • colour not as clear – translucent, higher white cell count • Total WBC count – 2000-50 000 mm3 • Polymorphonuclear leucocytes 20-70% • eg gout, rheumatoid arthritis, psoriatic arthritis, SLE, sarcoidosisGroup three fluids: • pyarthrosis • purulent, high white cell count, very turbid • Total WBC count – >50 000 mm3
• Polymorphonuclear leucocytes >70%
• eg infection