• Clinical science

Septic arthritis (Infectious arthritis)


Septic (infectious) arthritis is a bacterial infection of the joint space. Contamination occurs either via the bloodstream, iatrogenically, or by local extension (e.g., penetrating trauma) and patients with damaged (e.g., patients with rheumatoid arthritis) or prosthetic joints have an increased risk. Patients usually present with an acutely swollen, painful joint, limited range of motion, and a fever. Suspected infectious arthritis requires prompt arthrocentesis for diagnosis. In addition to the immediate broad-spectrum antibiotic therapy, surgical drainage and debridement may be necessary to prevent cartilage destruction and sepsis.



Clinical features

  • Acute onset
  • Joint involvement
    • Usually monoarticular
    • Most commonly affected joints: knees; followed by hip, wrists, shoulders, and ankles
  • Classical triad of fever, joint pain, and restricted range of motion
  • Joint may be swollen, red, and warm


Subtypes and variants

Prosthetic joint infection

  • Etiology [4]
    • Early onset (< 3 months of placement); : most commonly S. aureus
    • Delayed onset (3–12 months of placement); : coagulase-negative staphylococci, particularly S. epidermidis
    • Late onset (> 12 months of placement) : most commonly S. aureus
  • Clinical findings
    • Usually prolonged, low-grade course
    • Minimal swelling, with or without a sinus that drains pus
    • Can present acutely (see “Clinical features” above)
  • Diagnostic findings
    • Conventional x-ray: loosening of the prosthesis, periosteal reactions
    • For other diagnostic tests see “Diagnostics” below
  • Therapy
    • Removal of the prosthesis (usually) and administer IV antibiotics for 6–8 weeks
    • Reimplantation of the prosthesis following antibiotic treatment

In order to avoid infection, strict sterile techniques should be ensured in any procedure that involves penetration of the joint space.

Bacterial coxitis (septic arthritis of the hip)

  • Uncommon condition that requires urgent treatment to avoid destruction of the joint
  • Etiology: S. aureus and group A streptococcus account for the majority of cases
  • Clinical findings
  • For diagnostics and therapy, see respective sections below.

Bacterial coxitis is an orthopedic emergency!

Gonococcal arthritis

See purulent gonococcal arthritis and arthritis-dermatitis syndrome.

Gonococcal arthritis is the most common form of arthritis in sexually active young adults! In a young, sexually active adult presenting with classic symptoms of septic arthritis, gonococcal infection must be ruled out!

Lyme disease

See Lyme arthritis.



  • If septic arthritis is suspected, arthrocentesis should be conducted for synovial fluid analysis. Imaging (e.g., X-ray, MRI) may be indicated to assess potential underlying diseases or differential diagnoses.
  • Ultrasound-guided arthrocentesis: Definitive diagnosis requires detection of bacteria in the synovial fluid.
    • To conduct synovial fluid analysis, gram stain, and culture
    • ↑ Synovial fluid WBC and dominance of polymorphonuclear (PMN) cells
      • Cell count: > 50,000 WBC/μl (neutrophil predominant) points to septic arthritis (can be as low as > 10,000 in early disease).
    • Fluid appears yellowish-green and turbid
    • Synovial fluid culture and gram stain is positive in most patients with bacterial arthritis.
    • Also alleviates pain by decreasing intraarticular pressure
  • Laboratory tests
    • CRP/ESR
    • Blood culture: at least 2 sets of blood cultures to rule out a bacteremic origin
  • Imaging: to look for signs of underlying osteomyelitis and concurrent joint disease and rule out possible differential diagnoses (see “Differential diagnosis” below)
    • Ultrasound: effusion, edema of the surrounding soft tissue, possible empyema
    • X-ray: unremarkable early in the course of septic arthritis; osteolysis usually becomes visible after 2–3 weeks.
    • MRI or scintigraphy for early detection
      • MRI provides early evidence of infectious involvement of the surrounding soft tissue
      • Scintigraphy is used for detection or exclusion of polyarticular involvement

Suspected septic arthritis requires aspiration of synovial fluid for analysis!


Differential diagnoses

Differential diagnosis based on synovial fluid analysis findings

Synovial fluid analysis: comprises a group of tests that examine joint (synovial) fluid to aid in definitively establishing the specific types of arthritis.

Appearance WBCs/μl (PMN %) Glucose levels Culture Crystals
Normal synovial fluid
  • Transparent
  • Clear and viscous
  • < 200 (< 25%)
  • Nearly equal to blood
  • Negative
  • None
Noninflammatory arthritis
  • Transparent
  • Yellow and viscous
  • 200–2000 (< 25%)
  • Nearly equal to blood
  • Negative
  • Translucent-opaque
  • Yellow and watery
  • > 2,000 (≥ 50%)
  • Lower than blood
  • Negative
  • Opaque
  • Yellow or green with variable viscosity
  • > 50,000 (≥ 75%)
  • Early: > 10,000 (≥ 75%)
  • Much lower than blood
  • Usually positive
  • None
  • Cloudy
  • Reddish with variable viscosity
  • 200–2,000 (50%–75%)
  • Nearly equal to blood
  • Negative
  • None

Further differential diagnoses to consider

Viral arthritis

Fungal arthritis

  • Etiology: Histoplasma species, Sporothrix schenckii, Blastomyces species, Coccidioides species
  • Clinical findings: very variable with acute and chronic courses, often with symptoms of disseminated infection (e.g., pulmonary symptoms)
  • Diagnostics
  • See “Overview of fungal infections” in the General mycology article for additional information

Non-infectious arthritis


The differential diagnoses listed here are not exhaustive.


Initial management

Following arthrocentesis and culture, simultaneous empiric antibiotic therapy (based on the Gram stain) and evacuation of purulent material should be performed.

Further management

Treatment of adults after culture has returned

Organism Antibiotics
S. aureus and other gram-positive cocci
Gram-negative cocci
Gram-negative rods
N. gonorrhea

Treatment of children




We list the most important complications. The selection is not exhaustive.

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last updated 09/17/2020
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