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.
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.
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.
A new effusion without shock where infection, crystals and bleeding remain plausible.
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.