A 52-year-old woman with rheumatoid arthritis on methotrexate and adalimumab presents with acute onset of left shoulder pain, fever (39.2°C), and inability to move the shoulder for 2 days. She denies recent trauma or injection. On examination, the shoulder is warm and swollen with severe pain on all passive movements. Synovial fluid obtained by arthrocentesis shows: WBC 62,000/μL (92% neutrophils), glucose 22 mg/dL (serum 105 mg/dL), protein 5.8 g/dL, Gram stain shows Gram-positive cocci in clusters, and culture is pending. Which organism is most likely and what is the most appropriate antibiotic regimen?

Explanation

## Organism Identification **Key Point:** Gram-positive cocci in clusters = **Staphylococcus aureus** (the most common cause of septic arthritis in adults, accounting for ~40–50% of cases). **Mnemonic:** **CLUSTERS = Staph; CHAINS = Strep** - Staph aureus: grape-like clusters - Strep pyogenes: long chains ## Risk Factors for Septic Arthritis in This Patient | Risk Factor | Relevance | |---|---| | Rheumatoid arthritis | Impaired joint integrity, chronic inflammation | | Biologic DMARDs (adalimumab) | Immunosuppression → increased infection risk | | Methotrexate | Further immunosuppression | | Age >50 | Increased susceptibility | **High-Yield:** Patients on TNF-α inhibitors have a 2–3-fold increased risk of septic arthritis, particularly with atypical organisms (TB, fungal). However, Staph aureus remains most common. ## Antibiotic Selection for Staph aureus ```mermaid flowchart TD A[Gram+ Cocci in Clusters]:::outcome --> B[Presumed Staph aureus]:::outcome B --> C{MRSA Risk Factors?}:::decision C -->|No/Low Risk| D[Nafcillin or Cefazolin]:::action C -->|High Risk| E[Vancomycin or Linezolid]:::action D --> F[Await Culture + Susceptibilities]:::action E --> F F --> G{Susceptibilities Known?}:::decision G -->|MSSA| H[Switch to Nafcillin/Cefazolin]:::action G -->|MRSA| I[Continue Vancomycin/Linezolid]:::action ``` **Clinical Pearl:** In this patient with no recent hospitalization or healthcare exposure, MRSA risk is low. Nafcillin (preferred) or cefazolin (alternative) are appropriate empirical choices. Vancomycin is reserved for MRSA or β-lactam allergy. ## Dosing Regimens | Drug | Dose | Frequency | Notes | |---|---|---|---| | **Nafcillin** | 2 g | IV Q4H | Preferred for MSSA; better CNS penetration than cephalosporins | | **Cefazolin** | 2 g | IV Q8H | Alternative; good bone/joint penetration | | **Vancomycin** | 15–20 mg/kg | IV Q8–12H | For MRSA or β-lactam allergy; target trough 15–20 μg/mL | **Warning:** Do NOT use penicillin G alone for Staph aureus—resistance is common (penicillinase production). Penicillin G is appropriate only for Strep pyogenes. ## Duration and Monitoring - **IV therapy:** 2–4 weeks (depending on organism, joint, and clinical response) - **Surgical drainage:** Urgent arthroscopic washout required - **Follow-up cultures:** Repeat arthrocentesis at 24–48 hours to confirm sterilization [cite:Campbell's Operative Orthopaedics 13e Ch 42; Harrison 21e Ch 124]