Indications
- Suspected septic arthritis — the hot, swollen, painful joint with reduced range of movement and systemic upset. Aspirate before giving antibiotics wherever possible, since antibiotics can sterilise culture and delay/obscure diagnosis — the only exception is a septic, unstable patient where empirical antibiotics should not be delayed for the sample.
- Suspected crystal arthropathy (gout, pseudogout) — aspiration for polarised light microscopy is the definitive diagnostic test and can be done alongside sepsis work-up when both are on the differential.
- Tense, painful effusion — therapeutic aspiration for symptomatic relief, e.g. large traumatic or degenerative effusions, once septic arthritis has been excluded or is not suspected.
In practice, most ED aspirations are done to answer one question: "is this septic arthritis?" Send fluid for Gram stain, culture and crystals every time you aspirate a hot joint.
Contraindications
Do not aspirate
- Through cellulitis or infected overlying skin — risk of seeding infection into a sterile joint. Choose a different, clean entry point, or defer to a specialist who can.
- A prosthetic joint — a hot prosthetic joint is an orthopaedic emergency requiring aspiration in theatre under strict aseptic conditions by the orthopaedic team, not in the ED. Refer urgently instead.
- Overlying skin/soft-tissue infection without an alternative safe approach.
Relative cautions
- Anticoagulation/coagulopathy is not an absolute contraindication to aspiration, but use a fine-gauge needle and apply firm post-procedure pressure; discuss significant coagulopathy with haematology/orthopaedics if time allows.
- Bacteraemia/endocarditis risk — no specific contraindication, but treat the joint as a possible secondary source.
Never aspirate a prosthetic joint in the ED. Refer immediately to orthopaedics — a periprosthetic joint infection needs theatre-level asepsis and a different management pathway from a native joint.
Consent & preparation
- Explain the procedure, discuss pain, bleeding, infection introduction (small but real risk), and the possibility of a dry or unsuccessful tap.
- Take a focused history — recent joint surgery/injection, prosthesis, anticoagulation, immunosuppression, prior gout/pseudogout.
- Position comfortably with the knee extended/slightly flexed, quadriceps relaxed.
- Full aseptic no-touch technique: skin preparation, sterile gloves, avoid repalpating the marked site once cleaned.
Equipment
Skin antiseptic, sterile gloves, drapes
Lidocaine 1% for local infiltration (optional)
21G needle (large-bore for a tense/thick effusion)
10–20 mL syringe(s)
Sterile specimen pots: microscopy/culture, crystals
Gram stain request form
Dressing
Bedside ultrasound (if available, for a difficult or small effusion)
Knee — superolateral landmark approach
Technique
Mark the landmark before prepping
Identify the superolateral (or superomedial) point 1–2 cm from the patellar border, with the knee extended and quadriceps relaxed. Mark it before sterile prep so you don't have to repalpate through your prepared field.
Aseptic no-touch technique
Full skin preparation, sterile gloves, and avoid contaminating the needle or re-touching the marked site.
Local anaesthetic (optional)
Infiltrate the skin and subcutaneous tissue with lidocaine if desired, particularly for a tense or anxious patient.
LAST rescue — AAGBI algorithm ↗Advance the aspiration needle
Insert the aspiration needle at the marked point, directing it posteromedially beneath the patella into the suprapatellar pouch, applying gentle negative pressure as you advance.
Aspirate fully
Withdraw as much fluid as is safely possible — this both aids diagnosis and provides symptomatic relief for a tense effusion. Milking fluid from the suprapatellar pouch can help if flow slows.
Distribute samples and dress
Send fluid for microscopy/Gram stain, culture and sensitivity, and polarised light microscopy for crystals. Apply a simple dressing and advise rest/elevation.
Synovial fluid interpretation
| Appearance | Likely cause | Key tests |
|---|---|---|
| Clear/straw-coloured, viscous | Normal / non-inflammatory (osteoarthritis) | Low white cell count, no organisms/crystals |
| Cloudy/turbid, purulent | Septic arthritis until proven otherwise | Markedly raised white cell count (neutrophil-predominant), Gram stain, culture |
| Cloudy, less purulent | Crystal arthropathy (gout/pseudogout) or inflammatory arthritis | Polarised microscopy for negatively (gout) or positively (pseudogout) birefringent crystals |
| Bloodstained | Traumatic tap, haemarthrosis (fracture, ligament injury, anticoagulation, bleeding disorder) | Look for fat globules (suggests intra-articular fracture) |
No single fluid appearance rules septic arthritis in or out with certainty — always send Gram stain and culture, and treat clinically pending results if suspicion is high.
Complications
- Introduction of infection (iatrogenic septic arthritis) — rare with proper aseptic technique.
- Bleeding/haemarthrosis, particularly in anticoagulated patients.
- Pain, vasovagal response.
- Dry tap — does not exclude septic arthritis; consider ultrasound guidance or repeat attempt, and treat empirically if clinical suspicion remains high.
- Nerve or vessel injury — rare with correct landmark technique.
DOPS pointers
RCEM curriculum: Intermediate/Higher Procedural Skill. What assessors look for at each entrustment level:
| 1 | Observed only — describes indications, contraindications (including prosthetic joints), and landmarks. |
| 2a | Performs with the supervisor present throughout: correct landmark, aseptic technique, appropriate sample distribution. |
| 2b | Performs with supervisor in the department: manages the whole sequence including interpretation and antibiotic-timing decisions. |
| 3 | Indirect supervision: manages a dry tap, atypical presentation, or a suspected prosthetic joint referral appropriately. |
| 4 | Independent; teaches and supervises trainees including fluid interpretation and escalation. |
References
- Coakley G, et al. BSR & BHPR, BOA, RCGP and BSAC guidelines for management of the hot swollen joint in adults. Rheumatology 2006;45(8):1039–1041.
- Guideline for the management of the hot swollen joint in adults with a particular focus on septic arthritis. J Antimicrob Chemother 2006;58(3):492–493.
- RCEMLearning. The hot swollen joint and joint aspiration.
- RCEM Curriculum 2021 — Emergency Medicine Specialty Learning Outcomes: intermediate/higher procedural skills (DOPS).