Musculoskeletal

Septic Arthritis

Bacteria in the joint space trigger a neutrophil response whose enzymes digest articular cartilage within days, so the inflammation destroys the joint faster than the organism does.

In a nutshell

Septic arthritis is bacterial infection of a joint that destroys cartilage within days, so any acutely hot, swollen, tender joint is septic arthritis until proven otherwise. Aspirate and take blood cultures first, then start the local empiric regimen; a common adult example is flucloxacillin 2 g intravenously four times daily.

Classic presentation

A 68-year-old with rheumatoid arthritis has two days of an exquisitely painful, hot, swollen knee that she cannot bend or weight-bear on, with a temperature of 37.8 degrees Celsius.

Key points

Staphylococcus aureus and streptococci dominate in every risk group, so empirical cover must be bactericidal against both.
Incidence is 2 to 12 per 100,000 a year; case fatality is about 11%, and up to a third are left with permanent joint damage.
Synovial white cell count above 50,000 cells per microlitre supports infection but does not prove it; below 25,000 lowers the probability without excluding it.
No synovial neutrophil percentage is recommended as a cut-off. Higher percentages only make infection more likely.
Gram stain is insensitive but highly specific, so a positive result is early proof of infection and guides the first antibiotic.
Half of confirmed disseminated gonococcal infections have negative gonorrhoea tests at genital, throat and rectal sites, so test the joint fluid too.
Prosthetic joints are excluded from the native-joint pathway. Drainage, sampling and the antibiotic plan are orthopaedic decisions.
Suspected hip sepsis usually needs ultrasound or an image intensifier to aspirate, and urgent open debridement is often necessary.
Tuberculous joint infection runs a subacute course over weeks to months and is treated for at least 6 months, not weeks.
The standing UK guideline dates from 2006; a 2017 statement found too little evidence to revise it, and a British Society for Rheumatology replacement is in development.

First-line investigation

Synovial fluid aspiration before antibiotics for Gram stain, culture, polarised microscopy and cell count, plus at least two sets of blood cultures.

Management

Samples first, unless the patient is septic

Any acutely hot, swollen, tender joint with restricted movement is septic arthritis until proven otherwise. Refer the same day and tell the on-call orthopaedic team.^1,9,8
Aspirate the joint and take two sets of blood cultures before the first antibiotic dose. Send fluid fresh for Gram stain, culture, crystals and cell count.^1,4,12
If the patient is septic, follow the sepsis pathway and give intravenous antibiotics within one hour of high risk being identified. Do not wait for a difficult aspiration.^5,6,7,4
Do not inject corticosteroid into a joint while infection is possible. A general practitioner who aspirates cloudy fluid sends the sample to the emergency department with the patient.^1,2

Empirical antibiotics, chosen by risk

No atypical risk factors: local policy governs. A common adult UK regimen is flucloxacillin 2 g IV four times daily; switch once improving and eating.^1,14,9,10
Penicillin allergy: clindamycin 450 to 600 mg intravenously four times daily. Oxford University Hospitals uses cefazolin 2 g intravenously three times daily for non-severe allergy.^1,15,9
MRSA risk: vancomycin, loading 25 to 30 mg/kg, then 15 to 20 mg/kg every 8 to 12 hours, maximum 2 g per dose, adjusted on levels.^1,16,10
Take the first vancomycin level on day 2 before a dose. Target a trough of 10 to 20 mg/litre, and 15 to 20 mg/litre for less susceptible organisms.¹⁶
Gram-negative risk from age, frailty, recurrent urinary infection or recent abdominal surgery: add gentamicin 5 to 7 mg/kg once daily, adjusted on levels.^1,10,17
Suspected gonococcal arthritis: ceftriaxone 1 g intramuscularly or intravenously every 24 hours for 7 days, with sexual health referral and testing of genital, throat and rectal sites.^13,18

Drain the joint and set the course

Aspirate the joint to dryness as often as needed, or arthroscopic washout. A suspected septic hip needs early orthopaedic referral and often open debridement.^1,2,4
An acutely unwell patient should reach drainage within 6 hours. A systemically stable patient needs consultant orthopaedic review within 48 hours of presentation.^7,8
Conventional course: intravenous for up to 2 weeks or until signs improve, then oral for about 4 weeks. Narrow to culture and sensitivities with microbiology.^1,2,3
Trusts differ: Oxford University Hospitals uses about 2 weeks in total, NHS Borders 4 to 6 weeks. Osteomyelitis and prosthetic infection need longer.^9,10

When it does not settle

Persistent fever, pain, effusion or a rising CRP means inadequate drainage until proven otherwise. Re-aspirate, repeat the washout and look for osteomyelitis on MRI.^4,1,9
A painful prosthetic joint is a different problem. Refer to orthopaedics, aspirate only in a sterile environment, and, if not septic, give no antibiotics until deep samples are taken.^7,3,1

Move the joint and finish the course

Start mobilisation as soon as the infection is controlled and drains are out. Prolonged immobilisation causes stiffness and contracture.^4,11
Document the organism, the antibiotic plan and its end date, the follow-up team and the return advice: recurrent pain, swelling, fever or systemic illness.^11,9

Exam traps

Absence of fever does not exclude septic arthritis. If suspicion is high, treat for it anyway.
Serum urate has no diagnostic value in an acute hot joint: it can be normal in gout and raised in sepsis.
Crystals in the fluid do not exclude infection, and gout can raise the synovial white cell count into the septic range.
Warfarin does not contraindicate joint aspiration. Anticoagulation is not a reason to skip the sample.
A plain radiograph cannot diagnose early septic arthritis. Take it as a baseline, not as a rule-out test.
Normal CRP, ESR and white cell count do not exclude septic arthritis, particularly in immunosuppressed patients.
Sepsis is the only reason to give antibiotics before the aspirate, and even then blood cultures come first.
A negative culture taken after antibiotics have started proves nothing, which is why the sample must come first.
Do not add gonococcal cover routinely. It goes in only when there is a specific clinical indicator.

Illustrations

Turbid synovial fluid from a septic jointClinical photograph of a hot, swollen knee with an effusion in a patient with suspected septic arthritis.James Heilman, MD, Wikimedia Commons · CC-BY-SA-4.0

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.