A 32-year-old man presents with acute onset of severe right knee pain, swelling, and inability to bear weight for 2 days. He has fever (38.5°C) and a history of intravenous drug use. On examination, the knee is warm, erythematous, and shows marked effusion with severe pain on passive motion. Which is the investigation of choice to confirm the diagnosis of septic arthritis?

## Diagnosis of Septic Arthritis: Investigation of Choice ### Clinical Presentation The patient presents with classic features of acute septic arthritis: - Acute monoarticular arthritis (knee) - Fever and systemic toxicity - Severe pain on passive motion (cardinal sign) - Risk factor: intravenous drug use (Staphylococcus aureus common) ### Gold Standard Investigation **Key Point:** Synovial fluid analysis (arthrocentesis) is the gold standard and investigation of choice for confirming septic arthritis. It provides both diagnostic and therapeutic benefit. ### Synovial Fluid Findings in Septic Arthritis | Parameter | Septic Arthritis | Rheumatoid Arthritis | Gout | |-----------|------------------|----------------------|------| | **WBC count** | >50,000/µL (often >100,000) | 2,000–50,000/µL | 2,000–100,000/µL | | **Neutrophils** | >90% | 50–70% | 50–90% | | **Glucose** | <50% of serum (often <25) | <50% of serum | Normal | | **Protein** | >3 g/dL | >3 g/dL | Variable | | **Culture** | Positive in 50–70% | Negative | Negative | | **Gram stain** | Positive in 50–60% | Negative | Negative | | **Crystal type** | None | Rhomboid (RA) | Needle-shaped (MSU) | **High-Yield:** A synovial WBC count >50,000/µL with >90% neutrophils and positive culture is pathognomonic for septic arthritis. ### Why Arthrocentesis is Mandatory 1. **Diagnostic confirmation** — Culture is the only definitive test; Gram stain identifies organism in ~50–60% of cases 2. **Therapeutic benefit** — Joint aspiration reduces intra-articular pressure and removes inflammatory mediators 3. **Organism identification** — Guides antibiotic selection (e.g., S. aureus in IVDU) 4. **Sensitivity** — Culture sensitivity increases if fluid is sent in aerobic/anaerobic bottles or synovial fluid is inoculated directly into blood culture bottles **Clinical Pearl:** In septic arthritis, do NOT delay arthrocentesis for imaging. Early aspiration and empiric antibiotics (within 24 hours) reduce risk of permanent joint damage. ### Timing and Technique **Key Point:** Arthrocentesis should be performed urgently (same day) under aseptic conditions. Send fluid for: - Cell count and differential - Gram stain - Aerobic and anaerobic culture - Glucose and protein - Crystal analysis (to exclude gout/pseudogout) [cite:Harrison 21e Ch 330]