## Investigation of Choice for Septic Arthritis **Key Point:** Synovial fluid analysis (arthrocentesis) is the gold standard and definitive investigation for diagnosing septic arthritis. It provides both diagnostic confirmation and therapeutic benefit (decompression). ### Why Synovial Fluid Analysis is Diagnostic | Parameter | Finding in Septic Arthritis | |-----------|-----------------------------| | **WBC count** | >50,000/μL (often >100,000/μL) | | **Neutrophil predominance** | >90% | | **Gram stain** | Positive in 50–60% of cases | | **Culture** | Gold standard; identifies organism | | **Glucose** | <40 mg/dL (lower than serum) | | **Protein** | >3 g/dL | | **Appearance** | Turbid, purulent | **High-Yield:** The combination of: 1. Markedly elevated WBC (>50,000/μL) 2. Neutrophil predominance (>90%) 3. Positive culture 4. Low synovial glucose relative to serum glucose ...confirms septic arthritis with >95% certainty. ### Clinical Pearl **Key Point:** Arthrocentesis should be performed immediately in any patient with suspected septic arthritis — do not delay for imaging. Synovial fluid culture is essential for organism identification and antibiotic susceptibility testing, guiding targeted therapy. **Warning:** Never delay arthrocentesis waiting for imaging or blood culture results. Early joint aspiration is both diagnostic and therapeutic (decompression reduces cartilage damage). ### Timing and Technique **Key Point:** Arthrocentesis is indicated in any monoarticular arthritis with: - Fever - Acute onset - Joint effusion - Restricted mobility Send fluid for: - Cell count and differential - Gram stain - Aerobic and anaerobic culture - Glucose and protein - Crystal analysis (to exclude gout/pseudogout mimicking infection) 
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