Septic arthritis (Staphylococcus aureus)

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How the diagnosis is reached

A single hot, swollen, agonising joint is septic arthritis until an aspirate says otherwise, and the reason for that reflex is speed: bacterial enzymes and the neutrophil response strip articular cartilage within days, so a joint left undrained over a weekend can be permanently destroyed. Everything in the background raises the risk rather than lowering it. Rheumatoid arthritis, diabetes and recent corticosteroids are three of the strongest predisposing factors, and the graze supplies a route in. The previous gout is the trap, because it invites the assumption that this is another attack, and the joint is the one part of the picture that cannot be diagnosed by history. The serum urate is normal here, but a normal or even low urate is common during an acute gout attack, so it never excludes gout and never confirms sepsis. The aspirate is what settles it. A synovial white cell count of 78,000 with 94% neutrophils is in the septic range, a low synovial glucose reflects bacterial consumption, and the absence of crystals on polarised light removes gout and pseudogout, the two conditions that most often masquerade as this. Gram-positive cocci in clusters name the organism as a staphylococcus, and Staphylococcus aureus is the commonest cause of native joint sepsis in adults. Positive blood cultures matter beyond the joint: they raise the question of a seeded valve, which is why the echocardiogram is done even with no murmur. The joint itself has to be drained, by arthroscopic washout or repeated large-bore aspiration, since antibiotics alone will not clear a closed collection of pus. Intravenous flucloxacillin covers methicillin-sensitive Staphylococcus aureus, vancomycin where MRSA is likely, for about two weeks intravenously and four more orally, the joint rested and then mobilised early to keep its function. The sequencing here was already right: taking the aspirate before the first dose is what preserved the organism and its sensitivities, which a single dose can erase for a six-week course.

Key clinical clues

• A single hot, swollen joint the patient will not let anyone move, with fever
• Rheumatoid arthritis, diabetes, recent steroids and a skin breach over the same joint
• A previous history of gout that makes the wrong diagnosis feel obvious
• Synovial white cells of 78,000 with 94% neutrophils and a low synovial glucose
• No crystals on polarised light, with Gram-positive cocci in clusters on the aspirate

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