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.
- Send inoculated containers or blood-culture bottles only according to the local microbiology protocol, because sample requirements and validation differ.
- Gram stain has limited sensitivity, and synovial white-cell thresholds overlap; neither a negative stain nor a lower count safely excludes septic arthritis.
- Monosodium urate crystals are needle-shaped and strongly negatively birefringent; CPP crystals are commonly rhomboid and weakly positively birefringent when examined by trained staff.
- Crystals and infection may coexist, so every clinically suspicious aspirate still needs microbiological culture.
- Interpret a bloody aspirate using trauma, anticoagulation, bleeding disorder, fracture and procedural blood contamination rather than labelling it traumatic by appearance alone.
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.
Thresholds, sample number and culture strategy differ around a prosthesis; coordinate aspiration and antibiotic timing with orthopaedics and microbiology.
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.
- Start locally recommended treatment and arrange drainage according to clinical risk rather than waiting for every result, then narrow therapy as culture and response clarify the cause.