Synopsis
Identify native-joint infection as a cartilage and sepsis emergency, aspirate correctly without delaying antibiotics in an unstable patient, distinguish crystals from exclusion, drain infected synovium promptly, and verify clinical and microbiological response.
- Suspect septic arthritis with acute severe joint pain, restricted active and passive movement and an effusion, but recognise that fever and inflammatory markers may be absent.
- Run the sepsis pathway and obtain at least two blood-culture sets. In shock or high-risk sepsis, start intravenous antibiotics immediately after cultures and never delay for aspiration.
- In a stable patient, aspirate before antibiotics using sterile technique and image guidance for a deep or difficult joint; record volume, appearance and any preceding antimicrobial exposure.
- Send synovial fluid urgently for Gram stain, aerobic and anaerobic culture, white-cell count with differential and crystal analysis; inoculate blood-culture bottles when local laboratory policy supports it.
- A synovial white-cell count above 50,000 cells per microlitre increases probability but is not diagnostic; lower counts, negative Gram stain and identified crystals cannot exclude infection.
- Plain radiographs provide a baseline and find fracture or advanced joint damage; ultrasound confirms and guides access to effusion, while MRI assesses deep joints, abscess and adjacent osteomyelitis.
- Drain a large infected native joint urgently by arthroscopic or open washout. An acutely unwell patient needs experienced emergency surgery as soon as safe and generally within six hours.
- Tailor antibiotics to culture, host and source, follow pain, fever, CRP and function, and repeat aspiration or washout when clinical response is inadequate.
Key red flags
A hot painful joint with severe movement restriction, systemic toxicity, hypotension, confusion or lactate elevation requires immediate sepsis and orthopaedic action rather than waiting for clinic aspiration results.
Hip, shoulder or deep-joint infection may have little visible swelling; refusal to move, fixed posture, referred knee pain or unexplained bacteraemia should lower the threshold for ultrasound-guided aspiration and drainage.
Immunosuppression, older age, diabetes, inflammatory arthritis, recent injection or surgery, skin infection, intravenous drug use and bacteraemia can blunt fever or white-cell response and increase atypical organisms.
Crystals in synovial fluid do not exclude concurrent infection, and normal serum CRP or a synovial white-cell count below a traditional threshold cannot independently clear a convincing presentation.
Investigation priorities
Obtain direct microbiological and inflammatory evidence before antibiotics when stable.
Identify fracture, foreign body, gas, chronic joint destruction and a baseline for follow-up.
Management branches
A suspected infected joint accompanies shock, organ dysfunction or high-risk sepsis.
- Obtain blood cultures without delaying treatment, start sepsis resuscitation and protocol broad-spectrum intravenous antibiotics immediately.
- Call orthopaedics, anaesthesia and microbiology together and plan emergency drainage by an experienced team within six hours when safe.
- Aspirate before antibiotics only if it is immediately available and does not defer either drug delivery or theatre.
- Search concurrently for endocarditis, skin, spine, urinary and other joint sources guided by examination and bloodstream organism.
Septic arthritis is suspected without current sepsis or organ compromise.
- Obtain blood cultures and sterile image-guided or landmark-guided aspiration before starting antimicrobial therapy.
- Send cell count, Gram stain, aerobic and anaerobic culture and crystals and notify microbiology of special exposures.
- Begin local empiric intravenous therapy after sampling and arrange urgent arthroscopic or open washout rather than awaiting final culture.
- Narrow to susceptibilities and define route and duration through microbiology, source control and any bone or cardiac involvement.