Septic Arthritis

Septic arthritis is where an infection occurs within a joint. This could be in a native joint, meaning the persons own joint, or in a joint replacement. Infection in a joint is an emergency as the infection can quickly begin to destroy the joint and cause systemic illness. Septic arthritis has a mortality of around 10%.

It is a common and important complication of joint replacement. It occurs in around 1% of straight forward hip or knee replacements. This percentage is higher in revision surgery.



Usually, it only affects a single joint. This is often a knee. It presents with a rapid onset of

  • Hot, red, swollen and painful joint
  • Stiffness and reduced range of motion
  • Systemic symptoms such as fever, lethargy and sepsis

Common Bacteria

Staphylococcus aureus is the most common causative organism.

Other bacteria:

  • Neisseria gonorrhoea (gonococcus) in sexually active individuals
  • Group A Streptococcus (most commonly Streptococcus pyogenes)
  • Haemophilus influenza
  • Escherichia coli (E. coli)


TOM TIP: In a young patient presenting with a single acutely swollen joint always think of gonococcus septic arthritis until proven otherwise. Gonorrhoea infection is common and delaying treatment puts the joint in danger. In your exams it might say the gram stain revealed a “gram-negative diplococcus”. The patient may have urinary or genital symptoms to trick you into thinking of reactive arthritis but remember that it is important to exclude gonococcal septic arthritis first as this is the more serious condition.


Differential Diagnosis

  • Gout (fluid shows urate crystals that are negatively birefringent of polarised light)
  • Pseudogout (fluid shows calcium pyrophosphate crystals that are rod-shaped intracellular crystals positively birefringent of polarised light)
  • Reactive arthritis typically triggered by urethritis or gastroenteritis and associated with conjunctivitis
  • Haemarthrosis (bleeding into the joint)



Have a low threshold for treating a patient for septic arthritis until it has been excluded with examination of the joint fluid. Be particularly cautious with immunosuppressed patients.

There will be a local hot joint policy at your hospital to guide what team admits the patient (orthopaedics, rheumatology or infectious diseases), what antibiotics to use and for how long.

Aspirate the joint prior to antibiotics and send the sample for gram staining, crystal microscopy, culture and antibiotic sensitivities. The joint fluid may be purulent (full of pus). The gram stain will come back quite quickly and may give a clue about the organism. The full culture will take longer.

Empirical IV antibiotics should be given until the sensitivities are known. Antibiotics are usually continued for 3 – 6 weeks in total. Choice of antibiotic depends on the local guidelines. Example regimes are:

  • Flucloxacillin plus rifampicin is often first line
  • Vancomycin plus rifampicin for penicillin allergy, MRSA or prosthetic joint
  • Clindamycin is an alternative


Last updated March 2019