Acute monoarthritis is one of the most diagnostically demanding presentations in UK emergency medicine — and for good reason. Missing septic arthritis risks joint destruction, systemic sepsis, and medicolegal catastrophe; over-treating gout with unnecessary joint aspiration and antibiotics carries its own harms. For trainees working through FRCEM SBA revision, this differential sits squarely within the RCEM curriculum’s musculoskeletal and infectious disease domains, and it generates clinical scenario questions that reliably discriminate between candidates who truly understand the pathophysiology and those who have simply memorised a list. This article works through the evidence base, ED decision-making framework, and exam technique you need to approach these questions with confidence.
FRCEM SBA revision: Key Points
- Never exclude septic arthritis on clinical grounds alone — even classical gout features can coexist with infection; joint aspiration is the investigation of choice.
- Synovial fluid white cell count >50,000 cells/mm³ strongly favours septic arthritis, but no single threshold is absolute; Gram stain is positive in only ~50–75% of cases.
- Staphylococcus aureus is the commonest causative organism in adults; Neisseria gonorrhoeae should be considered in sexually active younger adults.
- Gout is caused by monosodium urate crystal deposition; pseudogout involves calcium pyrophosphate dihydrate (CPPD) crystals — these are examinable distinctions with different microscopy findings.
- Do not start urate-lowering therapy (e.g. allopurinol) during an acute gout attack — it can precipitate or prolong the flare.
- The FRCEM SBA exam frequently tests the Kocher criteria (adapted for adults), the approach to synovial fluid interpretation, and the correct first-line antibiotic choice according to local/national guidance.
Clinical Context and Epidemiology
Acute monoarthritis accounts for a significant proportion of musculoskeletal attendances to UK emergency departments. Gout affects approximately 2.5% of the UK adult population and its prevalence is rising, driven by dietary change, obesity, diuretic use, and an ageing population. NICE guidance on gout highlights that acute gout is most common in middle-aged and older men, though post-menopausal women represent a growing cohort. Septic arthritis, by contrast, occurs in roughly 6–10 per 100,000 population annually in the UK, with mortality of up to 11% and significant morbidity including permanent joint damage in survivors if treatment is delayed. The first metatarsophalangeal joint (podagra) is the classical site for gout; septic arthritis most frequently affects the knee, hip, and shoulder in adults, though any joint can be involved.
The clinical overlap is substantial. Both conditions produce a hot, swollen, exquisitely tender joint. Both can present with fever and elevated inflammatory markers. Both can affect the knee in elderly patients on diuretics — precisely the scenario that FRCEM SBA clinical scenarios are designed to exploit. The fundamental principle underpinning ED management is that septic arthritis must be excluded before any other diagnosis is accepted.
Pathophysiology
Septic Arthritis
Septic arthritis most commonly arises from haematogenous seeding of the synovial membrane, which lacks a basement membrane and is therefore highly susceptible to bacteraemic inoculation. Direct inoculation (trauma, iatrogenic) and contiguous spread from osteomyelitis or adjacent soft tissue infection are less common mechanisms. Once bacteria gain access to the joint space, the synovial membrane mounts an intense inflammatory response. Cytokine release (IL-1β, TNF-α, IL-6) drives neutrophil influx. Proteolytic enzymes released by both bacteria and infiltrating leucocytes degrade articular cartilage, with irreversible damage potentially occurring within hours. This is the biological basis for the axiom that septic arthritis is a surgical emergency. Staphylococcus aureus — including methicillin-resistant strains — accounts for approximately 40–50% of adult cases. Streptococcal species account for a further 25–30%. Neisseria gonorrhoeae is the commonest cause in sexually active adults under 40 in populations with higher gonorrhoea prevalence, and carries a better prognosis with prompt treatment.
Crystal Arthropathy: Gout and Pseudogout
Gout results from the deposition of monosodium urate (MSU) monohydrate crystals in and around joints, a consequence of chronic hyperuricaemia (serum urate >360 µmol/L in women, >420 µmol/L in men is the conventional threshold, though NICE acknowledges individual variation). Acute attacks are triggered by rapid fluctuations in urate levels — hence the prohibition on starting allopurinol acutely. Crystal deposition activates the NLRP3 inflammasome within synovial macrophages, generating IL-1β and initiating a robust neutrophilic inflammatory cascade that is clinically indistinguishable from early septic arthritis on examination alone.
Pseudogout (calcium pyrophosphate crystal deposition, CPPD) involves a distinct crystal species: calcium pyrophosphate dihydrate. It tends to affect larger joints — classically the knee — in older patients, and is associated with metabolic conditions including hyperparathyroidism, haemochromatosis, and hypomagnesaemia. Under compensated polarised light microscopy, MSU crystals appear as strongly negative birefringent needle-shaped crystals, whilst CPPD crystals are weakly positive birefringent and rhomboid. These microscopic descriptions are direct fodder for FRCEM SBA exam questions — know them precisely.
ED Assessment and Management
History
A structured history should cover: time course (gout classically peaks within 24 hours; septic arthritis typically develops over days), prior episodes and known diagnosis, fever and systemic symptoms, recent intercurrent illness or procedure, sexual history and risk factors for gonococcal infection, immunosuppression (diabetes, rheumatoid arthritis, renal failure, corticosteroid use), skin breaks near the joint, and current medications (diuretics, ciclosporin, and low-dose aspirin raise urate; allopurinol and febuxostat lower it). However, history cannot safely differentiate the two diagnoses.
Examination
Record temperature, heart rate, and blood pressure. Document the joint(s) involved, presence of effusion, skin changes (overlying erythema, warmth, tophi), and range of movement. Tophi suggest chronic tophaceous gout but do not exclude concurrent infection. Examine skin for psoriatic plaques, pustular lesions of disseminated gonococcal infection, or wounds suggesting direct inoculation.
Investigations
Joint aspiration is the single most important investigation. It should be performed in any patient with an acutely inflamed single joint where septic arthritis cannot be confidently excluded. Send synovial fluid for:
- Microscopy, culture, and sensitivity (MC&S) — urgent Gram stain and culture
- White cell count and differential
- Crystal analysis under polarised light microscopy
- Glucose (compare to simultaneous serum glucose)
Interpretation of synovial fluid WCC: normal is <200 cells/mm³; non-inflammatory (e.g. osteoarthritis) is typically <2,000; inflammatory (including crystal arthropathy) 2,000–50,000; septic arthritis typically >50,000, though overlap is well documented. A WCC >100,000 cells/mm³ is highly specific for septic arthritis. Importantly, crystals in the fluid do not exclude co-existing infection — this is a classic exam trap and a genuine clinical danger.
Peripheral bloods: FBC, CRP, ESR, U&E, LFTs, blood cultures (before antibiotics), serum urate (note: urate may be falsely low during an acute attack due to the acute phase response — a normal or low urate does not exclude gout). Plain radiographs should be obtained to exclude fracture, gas in the joint (suggesting gas-forming infection), or chondrocalcinosis (supporting CPPD). Ultrasound may demonstrate effusion and guide aspiration but does not distinguish infection from crystal disease reliably.
Management Algorithm
Suspected septic arthritis: Joint aspiration (diagnostic and partially therapeutic), urgent orthopaedic referral, blood cultures, and prompt empirical antibiotic therapy. NICE guidance on septic arthritis recommends antibiotic choice guided by local formulary and microbiology advice; flucloxacillin IV is the standard empirical choice for most adults in the absence of penicillin allergy or specific risk factors; clindamycin or vancomycin for MRSA risk or penicillin allergy. Formal surgical washout (arthroscopic or open) may be required. Do not delay antibiotics for the sake of culture results if clinical deterioration occurs, but take cultures first wherever feasible.
Acute gout: Management according to NICE Clinical Knowledge Summaries on gout includes: NSAIDs (e.g. naproxen 500 mg twice daily) as first-line if not contraindicated, colchicine (500 mcg two to four times daily) as an alternative or addition, and short-course oral corticosteroids (prednisolone 30 mg daily for 5 days) when NSAIDs and colchicine are both contraindicated or poorly tolerated. Intra-articular corticosteroids are an option if only one or two joints are affected and infection has been excluded. Do not commence allopurinol or adjust existing urate-lowering therapy during an acute attack. Ensure adequate analgesia, rest, and hydration. Arrange GP follow-up for long-term urate-lowering therapy discussion.
Acute pseudogout (CPPD): Treatment mirrors that of acute gout — NSAIDs, colchicine, and corticosteroids. Investigate for underlying metabolic causes, particularly in younger patients or those with atypical presentations.
How the FRCEM SBA Exam Tests This Topic
Within the FRCEM SBA revision syllabus, acute monoarthritis sits within the musculoskeletal and infectious disease sections of the RCEM curriculum. Candidates should review the Royal College of Emergency Medicine curriculum to confirm exact domain mapping. Questions on this topic characteristically use one of the following stems:
- A synovial fluid result is given with a cell count and microscopy finding — which diagnosis is most likely, or what is the next step?
- A clinical vignette presents a patient with a hot knee and known gout — the question tests whether the candidate appropriately pursues joint aspiration to exclude infection.
- An antibiotic question specifying patient characteristics (age, penicillin allergy, sexual history) and asking for the correct empirical regimen.
- A microscopy-based question distinguishing MSU from CPPD crystal appearances under polarised light.
- A question about what not to do — commencing allopurinol acutely, or accepting a crystal-positive aspirate as definitive proof of a non-infective aetiology.
The FRCEM SBA exam format rewards candidates who can apply knowledge to nuanced clinical scenarios rather than recall isolated facts. Practice with high-quality FRCEM SBA practice questions is essential for developing this applied reasoning — pattern recognition built through repeated exposure to well-constructed vignettes is the proven mechanism by which candidates increase their score above the pass mark.
Revision Pearls: High-Yield Facts
- Crystals in synovial fluid do not exclude concurrent septic arthritis — always send for MC&S regardless of crystal findings.
- MSU crystals: needle-shaped, strongly negatively birefringent (yellow when parallel to the compensator axis). CPPD crystals: rhomboid, weakly positively birefringent (blue when parallel).
- Serum urate may be normal or low during an acute gout flare — a normal result does not exclude the diagnosis.
- Flucloxacillin IV is the first-line empirical antibiotic for septic arthritis in adults without penicillin allergy or MRSA risk factors in the UK.
- Kocher criteria (originally paediatric, adapted for adults): fever >38.5°C, non-weight bearing, ESR >40 mm/hr, WBC >12,000 — increasing numbers of criteria raise the probability of septic vs. transient synovitis, though ED ultrasound and aspiration remain definitive.
- Starting or stopping urate-lowering therapy during an acute gout attack can precipitate or prolong the flare — a classic exam distractor.
- Pseudogout is associated with haemochromatosis, hyperparathyroidism, hypomagnesaemia, and Wilson’s disease — these metabolic screens are worth requesting in younger patients.
- Blood cultures should be drawn before antibiotics in suspected septic arthritis, but antibiotics should not be delayed if the patient is systemically unwell — a recurring principle across sepsis SBA questions.
Common Pitfalls — Where Candidates Lose Marks
- Accepting a diagnosis of gout based on history alone without appreciating the need to exclude infection in a hot, swollen joint.
- Confusing the crystal microscopy findings for MSU and CPPD, or misremembering the birefringence direction.
- Selecting allopurinol commencement as the correct acute management — this is nearly always a distractor.
- Choosing an antibiotic without accounting for stated allergy or risk factors in the question stem (e.g. selecting flucloxacillin in a patient with MRSA bacteraemia history).
- Forgetting that a synovial WCC between 20,000 and 50,000 represents a diagnostic grey zone — this range does not confidently distinguish crystal arthropathy from early septic arthritis.
- Overlooking gonococcal arthritis in a young sexually active adult — this is a deliberate distractor in demographic-specific vignettes.
How EM Learning Centre Supports Your FRCEM Single Best Answer Revision
Mastering the differential between septic arthritis and gout exemplifies the calibre of clinical reasoning demanded by the FRCEM SBA exam — the ability to integrate pathophysiology, investigation interpretation, and management decisions under time pressure. Isolated reading is rarely sufficient. Structured, repeated exposure to well-written clinical scenarios, with detailed explanations anchored in UK guidelines, is what consistently moves candidates from borderline to confident pass.
The FRCEM Single Best Answer revision course at EM Learning Centre is built around exactly this principle. Questions are mapped to the RCEM curriculum, written to the standard of the actual exam, and accompanied by high-quality explanatory content — covering musculoskeletal emergencies, infectious disease, and the full breadth of the syllabus. Whether you are working through a structured timetable or targeting specific weak areas, the EM Learning Centre provides the depth and clinical rigour that FRCEM SBA online revision demands.
If you are also preparing for the MRCEM SBA, the platform supports both pathways, with content differentiated by level and curriculum mapping. Explore the MRCEM SBA revision resources alongside FRCEM preparation to build a comprehensive, evidence-based knowledge base that will serve you throughout your training.
References
- National Institute for Health and Care Excellence. Gout: diagnosis and management. NICE guideline NG219. 2022. NICE (nice.org.uk)
- NICE Clinical Knowledge Summaries. Gout. Last revised 2023. NICE Clinical Knowledge Summaries
- National Institute for Health and Care Excellence. Septic arthritis. NICE Clinical Knowledge Summary. NICE Clinical Knowledge Summaries
- Royal College of Emergency Medicine. Emergency Medicine Curriculum. RCEM, London. Royal College of Emergency Medicine
- Mathews CJ, Weston VC, Jones A, Field M, Coakley G. Bacterial septic arthritis in adults. The Lancet. 2010;375(9717):846–855. Referenced via The BMJ
- British National Formulary. Gout — treatment summary; antibacterials — use for septic arthritis. British National Formulary (BNF)
