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Professional Reference articles are designed for health professionals to use. They are written by UK doctors and based on research evidence, UK and European Guidelines. You may find one of our health articles more useful.

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The presentation of one or more hot swollen joints raises a number of diagnostic possibilities but septic arthritis should always be suspected. This is the most important diagnosis to exclude as, if left untreated, the sequelae include permanent joint damage, impairment of function and even death. The elderly and very young are at particular risk of severe joint infection, complications and poor outcome.

Typically a patient with septic arthritis will present with a single hot and extremely painful joint and a reluctance to move and put weight on that joint. This scenario can occur with other pathologies and diagnosis may be particularly difficult in patients with pre-existing inflammatory arthritis. Consider septic arthritis in any patient with inflammatory arthritis who presents with a joint flare, particularly if one joint has flared more often than others.

It is important to ascertain the following:

Was the onset of joint swelling sudden or gradual?

Septic arthritis and crystal-induced arthropathies tend to present acutely (typically within two weeks) as opposed to rheumatoid arthritis or osteoarthritis which have a more insidious onset. Sudden-onset swelling or excessive swelling of one joint compared with others in patients with inflammatory arthritis should raise the possibility of septic arthritis.

How many joints are involved?

  • Gout and septic arthritis generally present as a monoarthritis (although more than one joint can be involved), whereas reactive arthritis and rheumatoid arthritis tend to affect several joints.

Which joints are affected?

  • Although any joint can be affected, the most usual sites for septic arthritis are the knee in adults and the hip in children.
  • In gout up to 70% of attacks first occur in the big toe but it can also occur elsewhere in the foot as well as the ankle, knee, wrist, elbow and small joints of the hands.[1] Atypical presentations of gout are more common in patients aged over 60 years. See the separate Gout article.

Are there any constitutional symptoms?

  • A patient with septic arthritis may have general malaise and a history of fever and rigors; however, bear in mind that systemic symptoms occur in less than half of adults with confirmed septic arthritis. The development of constitutional symptoms may be a helpful pointer to the onset of sepsis in patients with inflammatory arthritis.
  • A history of a gastrointestinal or genitourinary infection raises the possibility of reactive arthritis.[2, 3]
  • Lyme disease, which is associated with a typical rash (erythema migrans) is often accompanied by general symptoms of fever, malaise, arthralgia, myalgia and headache and is worth considering if the patient has had a potential exposure to deer ticks in an endemic area.[4]

Is there any history of trauma?

  • Haemarthrosis usually develops rapidly after an episode of trauma and results in a swollen painful joint.
  • A puncture wound caused by a recent animal or human bite over the affected joint may be the source of septic arthritis.

Is there loss of function?

  • Septic arthritis commonly presents with monoarticular joint pain with erythema, warmth, swelling, and pain on palpation and movement. Fever is present in many patients, though most are low grade. Therefore loss of function of a joint caused by pain on movement may be a feature of septic arthritis.[5]
  • Loss of joint function may also be a feature of any cause of joint arthritis or trauma.


  • Consider the general appearance of the patient and note any pyrexia.
  • Inspect the joints for swelling, redness and any deformity.
  • Redness is relatively uncommon and if present should raise suspicions of septic arthritis or gout. If sudden redness develops in patients with inflammatory arthritis, sepsis should be excluded.
  • Warmth is best elicited using the back of the fingers and suggests the presence of an inflammatory process. However, beware of a recently removed bandage or application of a hot compress.
  • Swelling is an important sign of active current inflammation. It may be due to an effusion in the joint or oedema of surrounding tissues. For detail on examining for a knee effusion see the separate Knee Pain article. Swelling over a bursa or tendon indicates inflammation of that structure and bony swelling would be in keeping with osteoarthritis.
  • Tenderness should be elicited by gentle palpation over the affected joint. Point tenderness may be the result of inflammation in periarticular soft tissue and suggests bursitis or muscle injury.
  • Passive and active ranges of movement are usually significantly diminished in septic arthritis.
  • Also look out for any associated signs of joint disease, such as rheumatoid nodules or gouty tophi.

Includes the following:

See also the separate Acute Monoarthritis article.

If septic arthritis or trauma cannot be excluded on clinical assessment then the patient should be referred immediately to hospital for further assessment, investigations and management.

Laboratory tests

  • FBC - a raised white cell count would be in keeping with an infective process but can also occur in the crystal arthropathies. It is not always a reliable sign of septic arthritis, particularly in children.
  • Inflammatory markers - ESR and CRP are generally higher in septic arthritis than in gout but this cannot be relied upon for diagnostic purposes. They are useful markers of response to treatment. ESR has largely been replaced by CRP.
  • Electrolytes and liver function - these should be measured, as any end-organ damage may affect antibiotic choice.
  • Urate - this is not a specific test, as hyperuricaemia frequently occurs in patients without gout.[1] Also uric acid levels can actually fall to within the normal range during an acute attack of gout and so should not be used as a diagnostic test in the acute setting. Urate levels can be useful for monitoring medication such as allopurinol in between attacks of gout.
  • Rheumatoid factor - this should only be requested if there are strong clinical suspicions of a systemic rheumatic disease. It would be of little value in investigating a single hot swollen joint but may be appropriate in the context of a polyarthritis.
  • Antinuclear antibodies (ANAs) - similarly, these should not be requested routinely for patients presenting with swollen joints but reserved for cases of suspected connective tissue disease. ANA titres tend to be positive in most lupus patients and can also be raised in rheumatoid arthritis.
  • Blood cultures - these should always be taken in suspected cases of septic arthritis. This would normally be done in secondary care when the patient is referred.


  • X-rays - a plain film should be done if there is a history of trauma. In septic arthritis the diagnostic value of X-rays is debatable but they may show evidence of chondrocalcinosis. In gout, radiographs are often normal in the acute setting apart from soft tissue swelling but subsequently erosive changes and tophi may be seen.
  • MRI scan - the imaging modality of choice in secondary care, as it can identify osteomyelitis that may require a surgical approach.
  • Ultrasound scan - this, or an image intensifier, may be required to assist in the aspiration of a suspected septic hip.
  • Computerised tomography (CT) scan - this is a more expensive option than ultrasound but may be used to assist in difficult aspirations or where cartilage or bony abnormalities need to be visualised.

Joint aspiration

The only joint that can realistically be aspirated in primary care is the knee. However, any person with a hot, swollen joint should be suspected of having septic arthritis in the absence of any other confirmed diagnosis.

Therefore any joint aspiration should be performed in secondary care, with joint aspirate sent for microscopy and Gram staining, microscopy for crystals, and for culture and sensitivities if applicable. Aspiration of a prosthetic joint should only be attempted in a clean environment - eg, in an operating theatre.

See the separate article on Joint Injection and Aspiration.

Any suspicion of septic arthritis or history of recent trauma mandates immediate referral to hospital. Otherwise the treatment is that of the underlying condition.

  • The great toe metatarsophalangeal joint is the most common to present as a hot swollen joint; this is almost always due to gout, which can be diagnosed clinically.
  • Any patient with possible septic arthritis (short history of hot, swollen, tender joint(s) with restriction of movement) should be referred as an emergency to an orthopaedic surgeon or rheumatologist for joint aspiration and synovial fluid analysis.
  • Suspected infection of a prosthetic joint should always be referred to an orthopaedic surgeon.
  • Similarly, if there is an acute injury and evidence of haemarthrosis, the patient should be referred, as there may be significant joint damage.[8]

Dr Mary Lowth is an author or the original author of this leaflet.

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Further reading and references

  1. Gout; NICE CKS, June 2022 (UK access only)

  2. Pathak H, Marshall T; Post-streptococcal reactive arthritis: where are we now. BMJ Case Rep. 2016 Aug 122016. pii: bcr-2016-215552. doi: 10.1136/bcr-2016-215552.

  3. Carlin E, Flew S; Sexually acquired reactive arthritis. Clin Med (Lond). 2016 Apr16(2):193-6. doi: 10.7861/clinmedicine.16-2-193.

  4. Lyme disease; NICE Guidance (April 2018 - last updated October 2018)

  5. Long B, Koyfman A, Gottlieb M; Evaluation and Management of Septic Arthritis and its Mimics in the Emergency Department. West J Emerg Med. 2019 Mar20(2):331-341. doi: 10.5811/westjem.2018.10.40974. Epub 2018 Dec 6.

  6. Rheumatoid arthritis; NICE CKS, April 2020 (UK access only)

  7. Coates LC, Helliwell PS; Psoriatic arthritis: state of the art review. Clin Med (Lond). 2017 Feb17(1):65-70. doi: 10.7861/clinmedicine.17-1-65.

  8. Lombardi M, Cardenas AC; Hemarthrosis. StatPearls, January 2022.