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Septic arthritis
A hot swollen joint is septic until proven otherwise. Aspirate before antibiotics (unless the patient is septic), send the fluid properly, start antibiotics, and refer for washout. Crystals don't exclude infection, and a prosthetic joint should be aspirated by orthopaedics. Built for revision, not live patient decisions.
Settle them, and check the basics
aspirate · then antibioticsRed flags — what each should make you think
Finding crystals doesn't exclude infection: gout and sepsis can coexist. A normal temperature and CRP don't exclude it either. The synovial fluid Gram stain and culture decide, so get the sample before antibiotics whenever it's safe.
First actions
- 💉Aspirate before antibiotics
- 🧪Send synovial fluid
- cxBlood cultures
- ℞Empirical IV antibiotics
- 💊Analgesia + splint
- ☎Orthopaedics
Understand the patient
who · which jointRisk factors
- Age over 80, diabetes, rheumatoid arthritis, existing joint disease
- Prosthetic joint, recent joint surgery or injection
- IVDU, skin infection, immunosuppression
- Sexually active (gonococcal)
Differentials of a hot joint
- Gout — negatively birefringent needle crystals; first MTP, ankle, knee
- Pseudogout (CPPD) — positively birefringent rhomboid crystals; knee, wrist; elderly
- Reactive arthritis, inflammatory arthritis flare
- Haemarthrosis — trauma, anticoagulants, haemophilia
- Cellulitis / bursitis — superficial, joint movement preserved
Prosthetic joint infection
- Pain, effusion, wound problems, sinus tract, fever
- Don't aspirate in ED — refer to orthopaedics (aspiration in theatre/sterile conditions)
- Withhold antibiotics if stable until samples taken by orthopaedics
Work A–E — assess and act as you go
The diagnoses you must not miss
tap to open eachWhat points toward it, what would rule it in, and how to manage it.
Acute hot swollen joint, painful movement, ± fever.
Synovial fluid Gram stain/culture; very high WCC (often above 50,000) supports.
IV antibiotics after aspiration, orthopaedic washout (arthroscopic or open).
Hypotension, high lactate, confusion.
Clinical.
Sepsis bundle — antibiotics immediately, then aspirate.
Painful prosthetic joint, fever, wound issues.
Orthopaedic aspiration, bloods (CRP, ESR), X-ray.
Orthopaedics (debridement, implant retention or revision); antibiotics guided by cultures.
Migratory arthralgia, tenosynovitis, pustular skin lesions, sexually active.
Joint aspirate, urethral/cervical/rectal/throat swabs (NAAT + culture).
IV ceftriaxone per guidance; GUM referral, contact tracing.
Investigate — what to order, when, and what it tells you
test with a question in mindSynovial fluid
Bloods
Imaging
How the plan comes together
disposition · handoverhome
Confirmed crystal arthritis with negative Gram stain, well, clear follow-up — with culture results checked and patient told to return if worsening.
ortho admission
Suspected septic arthritis or PJI: IV antibiotics and washout.
Hand over: aspirate results, antibiotic time, joint, risk factors.
Aspirate first, send it properly, then antibiotics and washout. Crystals and a normal CRP don't rule out sepsis, and prosthetic joints belong to orthopaedics. Nicely done getting here.
Clerking template
copy or downloadSeptic arthritis — documentation structure
A structure and prompt list to help you document and think systematically. It is not a completeness check and cannot guarantee nothing is missed — use your trust's own proforma, your seniors, and your clinical judgement.
Test yourself
active recallCover the answer, say it out loud, then reveal. Retrieval beats re-reading.
0 / 10 knownFrequently asked questions
quick answersWhat are the red flags for septic arthritis?
- Hot, swollen, very painful joint — think septic arthritis
- Can't move the joint at all — think septic arthritis
- Fever, sepsis — think septic arthritis ± bacteraemia
- Prosthetic joint — think PJI — don't aspirate in ED
- IVDU, diabetes, RA, immunosuppressed — think high-risk host
- Sexually active young adult, rash, tenosynovitis — think gonococcal arthritis
What is the initial management of septic arthritis?
- Aspirate before antibiotics — unless septic shock — then antibiotics first
- Send synovial fluid — Gram stain, culture, crystals, (WCC) — in a sterile pot and blood culture bottles
- Blood cultures — two sets
- Empirical IV antibiotics — per local policy (e.g. flucloxacillin; adjust for MRSA risk, gonococcal risk, allergy)
- Analgesia + splint — rest the joint
- Orthopaedics — for washout
Always alongside senior support and your local guideline.
What diagnoses must you not miss in septic arthritis?
- Native joint septic arthritis — washout
- Septic shock from a joint — antibiotics first
- Prosthetic joint infection — ortho aspirate
- Gonococcal arthritis — young · rash · tenosynovitis
Open each one in the must-not-miss section for what points toward it, how to rule it in, and how to manage it.
What investigations should a junior doctor order for septic arthritis?
- Synovial fluid — Gram stain, culture, crystal microscopy, cell count.
- Bloods — FBC, CRP, ESR, U&E, urate (unreliable in acute gout), cultures.
- Imaging — Baseline, fracture, chondrocalcinosis, osteomyelitis.
Admit or discharge: how is the plan decided for septic arthritis?
- Home — Confirmed crystal arthritis with negative Gram stain, well, clear follow-up — with culture results checked and patient told to return if worsening.
- Ortho admission — Suspected septic arthritis or PJI: IV antibiotics and washout.
Is there a clerking template for septic arthritis?
Yes — there is a free clerking template for septic arthritis on this page, structured as presenting complaint, red flags asked, history, examination (A–E), investigations, differentials, plan and escalation. You can copy it or download it as a Word document. Jump to the template.
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