Septic Arthritis
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Direct answer
A hot, swollen, exquisitely painful joint with fever is septic arthritis until an aspirate proves otherwise, and the aspirate comes before the first antibiotic dose: cell count, Gram stain, culture and crystals, plus blood cultures. Staphylococcus aureus is the commonest organism overall, streptococci follow, and Neisseria gonorrhoeae heads the list in sexually active young adults, whose tenosynovitis and pustular lesions complete a classic triad. A synovial white cell count above 50,000 per microlitre with over 90 percent neutrophils points to infection, but finding crystals never excludes it, since gout and infection coexist. Empiric therapy is vancomycin plus a third-generation cephalosporin, adjusted to cultures, with the joint drained because undrained pus destroys cartilage within days.
What you must remember
- Organism map: Staphylococcus aureus commonest overall; streptococci next; gonococcus in young adults; Gram-negatives with elderly, immunosuppressed or genitourinary sources; Salmonella and tuberculosis in the Indian chronic-monoarthritis differential.
- Synovial fluid signature: WBC above 50,000 per microlitre (often above 100,000), neutrophils above 90 percent; Gram stain positive in roughly half to three-quarters, culture positive in the large majority of non-gonococcal cases.
- The crystal trap: urate or pyrophosphate crystals and infection coexist — a polarised microscope finding never cancels the culture.
- Gonococcal pattern: tenosynovitis, dermatitis and migratory polyarthritis; diagnosis by genital and joint NAAT; treated with ceftriaxone, often without surgical drainage.
- Empiric antibiotics: vancomycin plus ceftriaxone (or another third-generation cephalosporin) after cultures are drawn, narrowed once organisms and susceptibilities return.
- Source control: needle aspiration repeated daily, or arthroscopic washout for large joints, thick pus or no improvement — the dictum that the sun should not set on undrained septic arthritis.
- Duration: about two weeks intravenous then two to four weeks oral, tracked clinically with CRP; longer for staphylococcal or prosthetic disease.
- After S. aureus bacteraemia: search the skin portal, and echocardiograph — the joint may be the embolic tip of endocarditis.
How to work through a hot knee
A 58-year-old diabetic man presents with a swollen, warm right knee of two days and fever 38.6 degrees. Step one: aspirate before any antibiotic — 20 mL of turbid fluid goes for cell count, Gram stain, culture and crystals, with two blood culture sets drawn in parallel. Step two: the result of 90,000 WBC per microlitre with 95 percent neutrophils and Gram-positive cocci in clusters converts the suspicion into certainty; vancomycin starts now. Step three: drainage the same day — aspiration to dryness, repeated daily; if fluid reaccumulates or the patient fails to improve in 48-72 hours, arthroscopic washout. Step four: cultures grow methicillin-sensitive Staphylococcus aureus, so therapy narrows to cloxacillin-class or cefazolin for the standard course, watching CRP fall. Step five: ask why this man — foot intertrigo, a recent boil, poorly controlled glycaemia — and complete the look-out with an echocardiogram if bacteraemia was documented.
Now the counter-case that earns marks: a 24-year-old woman with tender wrists and ankles, pustules on her fingers, and a knee that is warm but yields only 30,000 WBC with negative Gram stain. That is disseminated gonococcal disease — NAAT of genital, rectal and pharyngeal sites plus blood cultures — treated with ceftriaxone, and surgical drainage is rarely required.
Where students slip
The recurring examination error is stopping at the crystal: monosodium urate needles on polarised microscopy feel like a diagnosis, yet the neutrophil count and culture decide management, and steroid injected into an unrecognised septic joint converts a treatable infection into rapid chondrolysis — never inject an unexplained hot joint. The second slip is antibiotic-first behaviour: oral antibiotics taken at home before aspiration sterilise the culture and bury the diagnosis while the joint continues to melt. In the Indian context, add the chronic imitator: a "cold" monoarthritis of hip or knee with evening fever and high ESR in a young patient is tuberculosis until synovial biopsy and NAAT say otherwise — weeks of empirical anti-inflammatory drugs for a tuberculous knee are among the commonest referrals to orthopaedic OPDs. Finally, remember the iatrogenic category: unqualified intra-articular injections should be asked about directly.
Frequently asked questions
What synovial fluid count suggests septic arthritis?
More than 50,000 white cells per microlitre with over 90 percent neutrophils strongly suggests infection, though lower counts occur with gonococcal, mycobacterial and partially treated disease.
Which is the commonest organism in adult native-joint septic arthritis?
Staphylococcus aureus, with streptococci second and gonococcus leading in young sexually active adults.
Does finding crystals exclude infection?
No — gout and septic arthritis coexist; every hot joint needs cell count, Gram stain and culture regardless of crystals seen.
How is a septic joint drained?
By repeated needle aspiration to dryness or arthroscopic washout for loculated pus or failing joints — antibiotics alone cannot sterilise an undrained effusion.
What defines disseminated gonococcal arthritis?
The triad of migratory polyarthralgia, tenosynovitis and pustular skin lesions, confirmed by NAAT and treated with ceftriaxone.