A 58-year-old woman with rheumatoid arthritis on methotrexate and prednisone presents with acute onset of right hip pain, fever (39.2°C), and inability to ambulate. She denies recent trauma or instrumentation. Hip aspiration yields turbid fluid with WBC 62,000/mm³ (88% neutrophils), glucose 32 mg/dL (serum glucose 110 mg/dL), protein 5.8 g/dL, and negative Gram stain and culture (preliminary). Blood cultures are pending. What is the most likely causative organism, and what is the recommended empiric antibiotic regimen?

Explanation

## Septic Arthritis in Immunocompromised Hosts: Organism & Empiric Therapy ### Clinical Context A 58-year-old woman on immunosuppressive therapy (methotrexate + prednisone) with acute septic arthritis of the hip. Synovial fluid is diagnostic (WBC 62,000, low glucose, negative Gram stain does NOT exclude bacterial infection). ### Most Likely Organism: Staphylococcus aureus **Key Point:** In non-gonococcal, non-meningococcal septic arthritis, *Staphylococcus aureus* is the most common cause overall, accounting for 40–50% of cases. This is true even in immunocompromised patients without prosthetic joints. **High-Yield:** Causative organisms of septic arthritis vary by risk factor: | Risk Factor | Most Common Organism | Second Most Common | |---|---|---| | **No risk factors (community)** | *S. aureus* (MSSA) | *Streptococcus* spp. | | **IVDU** | *S. aureus* (MRSA) | Gram-negative rods | | **Prosthetic joint** | *S. aureus* (MRSA) | *Coagulase-negative Staph* | | **Immunocompromised (no prosthetic)** | *S. aureus* | *Streptococcus*, Gram-negatives | | **Sexually active, urethritis** | *N. gonorrhoeae* | — | | **Meningitis + arthritis** | *N. meningitidis*, *S. pneumoniae* | — | ### Empiric Antibiotic Regimen **Mnemonic: VANCE** — Vancomycin + Ceftriaxone for empiric septic arthritis: - **Vancomycin** 15–20 mg/kg IV Q8–12H (target trough 15–20 μg/mL) - Covers MRSA and other resistant Gram-positive cocci - Penetrates synovial fluid well - **Ceftriaxone** 2 g IV Q12H - Covers *Streptococcus* spp., some Gram-negatives - Excellent synovial penetration - Covers *N. gonorrhoeae* if present **Rationale:** This combination covers the most likely organisms (*S. aureus*, including MRSA) and common alternatives (*Streptococcus*, Gram-negatives) while awaiting culture and susceptibility results. ### Why Gram Stain Negativity Does NOT Exclude Infection **Clinical Pearl:** A negative Gram stain has ~50% sensitivity for bacterial septic arthritis. Do NOT withhold antibiotics based on negative Gram stain if synovial fluid analysis is otherwise diagnostic (elevated WBC, low glucose, positive culture or clinical suspicion). ### Adjustment Based on Culture Results ```mermaid flowchart TD A[Culture Result + Susceptibility]:::outcome --> B{Organism ID?}:::decision B -->|MSSA| C[Cefazolin 2g IV Q8H<br/>or Nafcillin 2g IV Q4H]:::action B -->|MRSA| D[Continue Vancomycin]:::action B -->|Streptococcus| E[Ceftriaxone alone<br/>or Penicillin G]:::action B -->|Gram-negative| F[Ceftazidime + Gentamicin<br/>or Fluoroquinolone]:::action C --> G[Typical Duration: 2–4 weeks IV]:::action D --> G E --> G F --> G ``` ### Duration of Therapy - **Native joint:** 2–4 weeks IV antibiotics (minimum 2 weeks) - **Prosthetic joint:** 4–6 weeks IV (or longer if hardware retained) - Transition to oral antibiotics only after clinical improvement and negative repeat cultures **High-Yield:** Immunocompromised patients may require longer courses and closer monitoring for relapse. [cite:Harrison 21e Ch 328; Rockwood & Green's Fractures in Adults 10e Ch 58]