A 58-year-old man with known osteoarthritis of the right hip presents with acute worsening of pain and new-onset fever (38.5°C). Physical examination shows warmth and swelling over the hip joint. Plain radiographs show typical OA changes. Which investigation is most appropriate to rule out superimposed septic arthritis?

## Investigation of Choice: Septic Arthritis vs. Osteoarthritis **Key Point:** Synovial fluid aspiration with complete analysis (cell count, differential, Gram stain, culture, and crystal microscopy) is the gold standard to differentiate septic arthritis from inflammatory or degenerative joint disease. ### Clinical Scenario Acute fever, warmth, and swelling superimposed on chronic OA raise concern for **septic arthritis**, which is a medical emergency. Imaging alone cannot exclude infection; synovial fluid analysis is diagnostic. ### Synovial Fluid Analysis in Septic Arthritis | Parameter | Septic Arthritis | OA | Gout | RA | |-----------|------------------|-----|------|----| | **WBC count** | >50,000 (often >100,000) | <2,000 | 2,000–100,000 | 2,000–50,000 | | **PMN %** | >90% | <50% | >90% | >50% | | **Gram stain** | Positive in 50–70% | Negative | Negative | Negative | | **Culture** | Positive (gold standard) | Negative | Negative | Negative | | **Crystals** | Absent | Absent | Needle-shaped, intracellular | Absent | **High-Yield:** WBC >50,000 with >90% PMNs is highly suggestive of septic arthritis. Culture is definitive but takes 48–72 hours; empiric antibiotics must be started immediately after aspiration. ### Procedure Essentials 1. **Sterile aspiration** (ultrasound-guided for deep joints like hip). 2. **Send for:** Cell count, differential, Gram stain, aerobic/anaerobic culture, fungal culture (if immunocompromised), crystal microscopy. 3. **Do NOT delay antibiotics** while awaiting culture results if septic arthritis is clinically suspected. **Clinical Pearl:** Fever + acute joint pain + warmth = septic arthritis until proven otherwise. Do not rely on imaging or serology alone. **Mnemonic: SEPTIC JOINT FLUID — WBC >50K, PMN >90%, Culture +** ### Why Other Investigations Are Insufficient - **ESR/CRP only:** Elevated in both septic and inflammatory arthritis; non-specific and cannot exclude infection. - **Imaging (X-ray, CT):** Shows structural changes but cannot identify infection; may be normal early in septic arthritis. [cite:Harrison 21e Ch 297; Robbins 10e Ch 26]