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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. 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 (Reiter's syndrome)[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.
- Is there any history of trauma?
- Haemarthrosis usually develops rapidly after an episode of trauma and results in a swollen painful joint.
- Make note of a history of a recent animal or human bite over the affected joint, as a puncture wound can be the source of septic arthritis.
- Is there loss of function? Sudden loss of function in patients with inflammatory arthritis can indicate sepsis.
- 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 Assessment 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:
- Rheumatoid arthritis.
- Crystal arthropathies - gout and pseudogout.
- Reactive arthritis (now considered synonymous with Reiter's syndrome).
- Palindromic rheumatism (multiple and unpredictable episodes of arthritis which can be severe, affecting different joints at different times; joints tend to return to normal between attacks).
- Psoriatic arthropathy.
See also the separate Acute Monoarthritis article.
- If septic arthritis is suspected, the affected joint should be aspirated by an experienced clinician using an aseptic technique. Warfarin treatment does not contra-indicate needle aspiration.
- The synovial fluid is often turbid and purulent but the absence of pus does not exclude infection. GPs who aspirate such fluid should arrange for a same day hospital admission (see 'Management', below).
- Fluid should be sent for Gram staining and culture before starting antibiotics. Examination of crystals should also be made to exclude acute gout or pseudogout. The absence of organisms on Gram staining does not exclude infection.
- Synovial fluid white cell count and percentage of polymorphonuclear cells may be helpful in patients with monoarticular inflammation and may indicate the need for the treatment of septic arthritis before Gram staining and culture results are available.
- Polarising microscopy should always be carried out to look for crystals.
- If the patient has classic signs and symptoms of gout, the condition should be treated on clinical grounds. The only joint that can realistically be aspirated in primary care is the knee. If other joints require aspiration, the patient should normally be referred to a specialist.
- 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. 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.
- 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 suspected 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. Pus should be removed either by arthroscopy or needle aspiration.
- The synovial fluid must be aspirated, Gram-stained and cultured prior to starting antibiotics.
- GPs who aspirate joints should be prepared to arrange a same day admission if purulent fluid is obtained. Antibiotics should be commenced once the fluid has been obtained (this will usually be arranged by the admitting doctor) and there is no need to wait for the results of microbiology.
- If clinical suspicion is high it is imperative to start treatment for septic arthritis, even in the absence of fever. Antibiotic choice is guided by local policy and knowledge of sensitivities, and there is considerable discussion in the literature about the best antibiotics to use.
- 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.
Further reading and references
Gout; NICE CKS, February 2018 (UK access only)
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.
Carlin E, Flew S; Sexually acquired reactive arthritis. Clin Med (Lond). 2016 Apr16(2):193-6. doi: 10.7861/clinmedicine.16-2-193.
Lyme disease; NICE Guidance (April 2018)
Rheumatoid arthritis; NICE CKS, January 2019 (UK access only)
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.
Douglas RJ; Aspiration and injection of the knee joint: approach portal. Knee Surg Relat Res. 2014 Mar26(1):1-6. doi: 10.5792/ksrr.2014.26.1.1. Epub 2014 Feb 27.
Lombardi M, Cardenas AC; Hemarthrosis. StatPearls Publishing 2019-2019 Feb 1.