Septic arthritis refers to infection in a joint. It can occur at any age, but is most common in children under 4 years. The infection can rapidly destroy the joint and cause systemic illness.
Presentation
Septic arthritis usually only affects a single joint, often a knee or hip. It presents with a rapid onset of:
- Hot, red, swollen and painful joint
- Inability or refusal to weight bear
- Stiffness and reduced range of motion
- Systemic symptoms, such as fever, lethargy and sepsis
Septic arthritis can be subtle in young children and may present without fever and other systemic symptoms. A high index of suspicion is required in a child presenting with acute joint pathology.
Causes
Staphylococcus aureus is the most common causative organism.
Other bacterial causes of septic arthritis include:
- Kingella kingae (particularly in children under 5 years)
- Neisseria gonorrhoeae in sexually active individuals
- Group A streptococcus (Streptococcus pyogenes)
- Streptococcus pneumoniae
- Haemophilus influenzae
- Salmonella (in sickle cell disease)
TOM TIP: In a sexually active adolescent presenting with a single acutely swollen joint, consider gonococcal septic arthritis. The Gram stain may show Gram-negative diplococci. NAAT testing may be appropriate to exclude gonococcal infection. Urinary or genital symptoms may also suggest reactive arthritis (once gonococcal septic arthritis is excluded).
Differential Diagnosis
The key differential diagnoses of an acute painful joint in a child or adolescent include:
- Fracture or soft tissue injury
- Transient synovitis
- Osteomyelitis
- Perthes disease
- Slipped upper femoral epiphysis
- Juvenile idiopathic arthritis
- Reactive arthritis
Investigations
Joint fluid testing is the key diagnostic investigation. A sample may be obtained by needle aspiration or during joint washout. It should ideally be performed before starting antibiotics, although it should not delay treatment in an acutely unwell child. The sample is sent for Gram staining, culture and sensitivity testing. The joint fluid may be purulent (full of pus). The Gram stain result is available quickly and may give a clue about the organism. Culture and antibiotic sensitivities take longer.
Other investigations include:
- Bloods (including FBC, CRP and ESR)
- Blood cultures
- X-ray
- Ultrasound (may show joint effusion)
- MRI (second-line)
Management
A low threshold is required for suspecting septic arthritis, particularly in immunosuppressed patients. Delayed treatment has significant consequences.
Management involves emergency admission for input from orthopaedics, paediatrics and microbiology.
Joint washout is usually required for septic arthritis. This surgical procedure involves draining pus and irrigating the joint to help clear the infection.
Empirical IV antibiotics should be given until sensitivities are known, as per local guidelines. These may be switched to oral antibiotics as the infection improves (e.g., symptoms improve and CRP falls). Antibiotics are usually continued for 2-3 weeks in total for uncomplicated septic arthritis (longer if complex).
Last updated July 2026
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