Hot Swollen Native Knee: Septic Arthritis Versus Gout
A 63-year-old man presents with a 2-day history of a swollen, hot right knee. Temperature is 38.7°C and heart rate 104 beats/min. He has well-controlled type 2 diabetes and a history of gout 5 years ago. He cannot weight-bear. The knee is held at 20° of flexion, with a large effusion and marked tenderness. Arthrocentesis yields turbid fluid: 52,000 WBC/µL with 94% neutrophils, no crystals on polarized microscopy, and a negative Gram stain. Peripheral WBC is 14.2 × 10⁹/L, CRP 210 mg/L, and creatinine 110 µmol/L. He is otherwise systemically well, with no hypotension, no intravenous drug use, and no recent instrumentation. The emergency physician wants to discharge him with oral antibiotics and rheumatology follow-up because there are no crystals and the Gram stain is negative.
1. How do you interpret the synovial fluid, and what is your management?
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The synovial white cell count of 52,000/µL with 94% neutrophils is in the septic range. The traditional threshold is above 50,000/µL with more than 90% neutrophils, and although no single value is perfectly sensitive or specific, a count in this range in a febrile patient with a hot, swollen knee must be treated as septic arthritis until proven otherwise. The negative Gram stain does not exclude infection: it is positive in only about half of culture-proven cases. Absence of crystals does not exclude gout either, because crystals can be missed, particularly if the specimen is examined late or the crystal burden is low.
Management: admit, take blood cultures, and aspirate again if there is any doubt, sending fluid for cell count, crystals, Gram stain, and culture. Start empirical intravenous antibiotics active against staphylococci and streptococci after cultures are taken, without a long delay. Arrange urgent arthroscopic or open washout within 24 hours, and repeat washout if the joint does not improve. Serial CRP and clinical review guide the duration of antibiotics.
2. The emergency physician argues that this is gout because the patient has a history of gout. How do you respond?
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A history of gout is a risk factor, not a diagnosis. Gout and septic arthritis can coexist, and an untreated septic joint can destroy cartilage within days. The peripheral white count and CRP are both raised, but neither separates the two reliably. The most useful discriminator is the synovial fluid: a count above 50,000/µL with neutrophil predominance strongly favors sepsis, and the absence of crystals on a single polarized examination does not exclude gout.
If the diagnosis is genuinely uncertain, the correct move is not to discharge but to repeat the aspiration, ask for a careful crystal search by an experienced examiner, and involve orthopaedics and rheumatology the same day. A trial of anti-inflammatory therapy with close review is acceptable only in a well patient with a modest cell count and a confident crystal diagnosis; here the count is high and the patient is systemically unwell.
The risk of a missed septic joint - irreversible cartilage loss, sepsis, and death - vastly outweighs the cost of a night in hospital. Document your reasoning and the discussion with the emergency physician.
3. What if the repeat aspirate shows negatively birefringent needle-shaped crystals and the cell count is 30,000/µL?
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That changes the probability substantially. Negatively birefringent needle-shaped crystals under polarized light are diagnostic of monosodium urate, and a count of 30,000/µL with a compatible clinical picture makes a gout flare the leading diagnosis. However, septic arthritis can coexist with gout, and a count of 30,000/µL is still above the typical non-inflammatory range.
The safe approach is to treat as gout but not to close the door on infection. Admit for anti-inflammatory therapy - an NSAID if renal function permits, colchicine, or corticosteroids - and observe the response over 12–24 hours. If fever persists, the CRP continues to rise, or the joint remains tense, repeat the aspiration and consider washout. Do not send the patient home from the emergency department on the same day.
Document the crystal finding explicitly, note the cell count and the clinical trajectory, and arrange rheumatology follow-up. Also review his urate-lowering therapy, since recurrent flares suggest inadequate control.