## Most Common Organism in Septic Arthritis **Key Point:** Staphylococcus aureus is the most common causative organism of septic arthritis across all age groups and patient populations, accounting for approximately 40–50% of all cases. ### Epidemiology by Risk Group | Risk Group | Most Common Organism | Second Most Common | |---|---|---| | IV drug users | S. aureus (including MRSA) | Gram-negative bacilli, Candida | | Healthy adults | S. aureus | Streptococcus spp. | | Sexually active young adults | S. aureus | N. gonorrhoeae | | Neonates (<3 months) | S. aureus, Group B Strep | Gram-negative bacilli | | Immunocompromised | S. aureus | Fungal, atypical organisms | **High-Yield:** In IV drug users, S. aureus (including methicillin-resistant strains) is the predominant pathogen due to: - Skin colonization and poor hygiene at injection sites - Direct inoculation during needle insertion - Propensity for hematogenous spread to joints - Higher risk of right-sided endocarditis with septic emboli to joints ### Clinical Clue Recognition **Clinical Pearl:** The Gram stain finding of "cocci in clusters" is pathognomonic for S. aureus. This morphology, combined with the clinical context (IV drug use, acute monoarticular arthritis), makes S. aureus the diagnosis. **Mnemonic:** **SACK** — S. aureus is the **S**ingle most common cause; **A**cross all ages; **C**lusters on Gram stain; **K**eys to early diagnosis and empiric coverage. ### Why Empiric Coverage Matters Empiric therapy for suspected septic arthritis should cover S. aureus with: - Cefazolin or nafcillin (if methicillin-sensitive) - Vancomycin or linezolid (if MRSA suspected or confirmed) - In IV drug users: always consider MRSA coverage [cite:Harrison 21e Ch 330]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.