Synopsis
Obtain and interpret synovial fluid as a time-sensitive specimen, prioritising microbiology and crystal identification while recognising overlap between septic, crystal, inflammatory, haemorrhagic and non-inflammatory effusions.
- Synovial-fluid analysis answers two immediate questions: could this be infection, and are pathogenic crystals present? It does not replace the clinical assessment.
- Before aspiration confirm identity, joint, indication, consent, allergy and anticoagulant context; use aseptic non-touch technique and document local skin infection or prosthesis.
- When volume is limited, preserve fluid for bacterial culture first; negotiate additional cell count, crystal microscopy and specialised tests with the laboratory.
Key red flags
Cloudiness, reduced viscosity and neutrophil predominance indicate inflammation but cannot distinguish bacterial infection from crystal or immune-mediated arthritis without culture and context.
Investigation priorities
Detect and identify organisms and provide susceptibility information for definitive antimicrobial treatment.
Management branches
A new painful effusion with infection in the differential and no haemodynamic reason to delay antibiotics.
- Take blood cultures, prepare the skin aseptically and aspirate adequate fluid before antimicrobials, using ultrasound guidance when anatomy or a small effusion makes blind sampling unreliable.
- Prioritise bacterial culture, then cell count and crystal examination; label the exact joint, time, antibiotic exposure and relevant host factors for the laboratory.