A 38-year-old man from Delhi presents to the emergency department with a 3-day history of severe right knee pain, swelling, and inability to bear weight. He has a fever of 38.9°C and reports a small puncture wound to the knee sustained while gardening 5 days ago. On examination, the knee is warm, erythematous, and grossly swollen with severe pain on passive movement. Synovial fluid analysis shows WBC 85,000/μL (95% neutrophils), protein 6.2 g/dL, glucose 18 mg/dL (serum glucose 110 mg/dL), and negative Gram stain. Blood cultures are pending. What is the most appropriate next step in management?

Explanation

## Diagnosis and Clinical Reasoning **Key Point:** This patient has septic arthritis with a clear history of inoculation injury, systemic toxicity (fever), and classic synovial fluid findings (WBC >50,000/μL, low glucose, high protein). The negative Gram stain does NOT exclude bacterial infection—culture is the gold standard. ## Synovial Fluid Interpretation | Parameter | Septic Arthritis | Rheumatoid | Gout | |---|---|---|---| | WBC (cells/μL) | >50,000 (often >100,000) | 2,000–50,000 | 2,000–100,000 | | % Neutrophils | >90% | 50–70% | >90% | | Glucose (mg/dL) | <40 (often <25) | <40 | Variable | | Culture | Positive | Negative | Negative | | Crystals | Absent | Negative birefringent | Needle-shaped monosodium urate | **High-Yield:** A synovial glucose **<50% of serum glucose** (here: 18 vs 110) is highly specific for septic arthritis. ## Management Algorithm ```mermaid flowchart TD A[Suspected Septic Arthritis]:::outcome --> B[Synovial Fluid Analysis]:::action B --> C{WBC >50k + Glucose low + Neutrophils >90%?}:::decision C -->|Yes| D[Empirical Antibiotics STAT]:::urgent D --> E[IV Ceftriaxone + Gentamicin]:::action E --> F[Arthroscopic/Needle Washout]:::action F --> G[Await Culture Results]:::action G --> H[De-escalate if Susceptibilities Known]:::action C -->|No| I[Consider Alternative Diagnosis]:::outcome ``` ## Treatment Principles **Clinical Pearl:** Septic arthritis is a surgical emergency. Delay in washout increases risk of permanent cartilage damage and joint destruction. 1. **Empirical antibiotics** (after blood cultures, NOT delayed for culture): - **Ceftriaxone 2 g IV Q12H + Gentamicin 5–7 mg/kg IV Q24H** (covers Staph, Strep, Gram-negatives) - Adjust based on Gram stain and culture 2. **Urgent surgical drainage:** - Arthroscopic washout preferred (allows inspection, synovectomy if needed) - Needle aspiration alone is inadequate for large joints 3. **Duration:** 2–4 weeks IV antibiotics depending on organism and response **Warning:** Do NOT wait for culture results to start antibiotics—mortality and morbidity increase significantly with delayed treatment. Negative Gram stain does NOT exclude infection (sensitivity ~50%). [cite:Campbell's Operative Orthopaedics 13e Ch 42]