Synopsis
Assess one newly painful swollen joint as a time-critical diagnostic problem, obtain synovial fluid safely, treat suspected infection without avoidable delay, and distinguish crystal, traumatic and inflammatory causes.
- Treat acute monoarthritis as septic arthritis until infection has been actively considered and, where feasible, synovial fluid has been obtained.
- Examine whether pain is truly intra-articular: severe restriction of both active and passive movement supports joint pathology; focal bursal or tendon pain may preserve passive range.
- Take blood cultures before antibiotics when systemic infection is suspected, but never postpone antimicrobials for sampling in a haemodynamically unstable patient.
- Aspirate using aseptic technique and send sufficient fluid for Gram stain, bacterial culture, cell count where locally available and compensated-polarised-light microscopy for crystals.
- Neither a low synovial white-cell count nor identified urate or CPP crystals reliably excludes concurrent infection; culture and clinical trajectory remain essential.
- Serum urate may fall during a gout flare and cannot diagnose the cause of an acutely swollen joint by itself.
- A painful prosthetic joint, joint after recent procedure, or hot joint in an immunosuppressed patient needs early orthopaedic and microbiology involvement.
- After immediate danger is controlled, determine whether the episode is the first manifestation of gout, inflammatory arthritis, haemarthrosis or structural injury and plan prevention.
Key red flags
Acute severe pain, effusion and markedly painful passive movement with fever, rigors, skin infection or bacteraemia risk requires urgent aspiration and source-control planning.
New pain, swelling, wound change or systemic illness around a prosthesis may represent periprosthetic infection and needs urgent orthopaedic review before casual aspiration or oral antibiotics.
Investigation priorities
Obtain the decisive specimen for microbiology and crystal identification before treatment changes the yield.
Management branches
Acute monoarthritis with sepsis, systemic toxicity, rapidly worsening pain or high-risk host features.
- Perform ABCDE, obtain intravenous access, analgesia, blood cultures and organ-function tests, and contact orthopaedics and microbiology immediately.
- Aspirate the joint urgently unless this would delay life-saving treatment; start locally approved intravenous antibiotics after samples in a stable patient or immediately if unstable.
- Give orthopaedics explicit ownership of native-joint drainage and source control. Select repeated aspiration, arthroscopic washout or open drainage according to the joint, macroscopic or radiological damage, organism and response; review cultures and clinical progress with microbiology, then narrow treatment when results permit.
A new effusion without shock where infection, crystals and bleeding remain plausible.
- Document risk factors and full examination, withhold unnecessary antibiotics until prompt synovial and blood samples are secured, and avoid injecting corticosteroid into an undiagnosed hot joint.
- Request culture and crystal examination together, add imaging for trauma or a difficult deep joint, and provide analgesia while maintaining a clear escalation threshold.
Key medicines
Empirical intravenous antibacterial therapy for suspected native-joint septic arthritisThere is no single universal UK agent or dose. If the patient is stable, obtain synovial fluid and blood cultures first, then start the current local intravenous regimen; in sepsis or septic shock, take blood cultures promptly but do not delay time-critical intravenous antibiotics for joint aspiration. Microbiology must select and document the agent, dose and interval using Gram stain, likely source, prior cultures, allergy, renal and hepatic function, native versus prosthetic joint, and MRSA, resistant Gram-negative or gonococcal risk.Do not use a casual oral regimen before sampling in a stable undiagnosed hot joint. A prosthetic joint follows a separate implant-infection pathway. Review cultures, susceptibilities, renal function, toxicity and clinical response promptly; narrow, change route and set duration only with orthopaedic and microbiology oversight, reassessing source control if improvement is inadequate.