A 68-year-old woman with rheumatoid arthritis on methotrexate and prednisolone presents with acute onset of left hip pain and fever (39.2°C) for 3 days. She denies recent trauma or surgery. Hip examination shows severe pain on internal rotation and flexion, with guarding. Synovial fluid obtained by ultrasound-guided aspiration shows: WBC 72,000/μL (88% neutrophils), glucose 18 mg/dL (serum 95 mg/dL), protein 5.8 g/dL, Gram stain negative, and culture pending. Inflammatory markers are elevated (CRP 12 mg/dL, ESR 68 mm/hr). What is the most likely organism?

Explanation

## Clinical Context & Risk Factors **Key Point:** This patient has multiple risk factors for septic arthritis: advanced age, immunosuppression (methotrexate + prednisolone), and pre-existing joint disease (RA). ## Synovial Fluid Analysis: Septic Pattern Confirmed | Parameter | Value | Interpretation | |-----------|-------|----------------| | **WBC** | 72,000/μL | Highly suggestive of bacterial infection (>50,000) | | **Neutrophils** | 88% | Predominant PMN response | | **Glucose** | 18 mg/dL (serum 95) | Only 19% of serum glucose — classic for septic arthritis | | **Protein** | 5.8 g/dL | Elevated; reflects acute inflammation | | **Gram stain** | Negative | Does not exclude infection; organism load may be low | ## Why *Staphylococcus aureus*? **High-Yield:** *S. aureus* is the **most common cause of septic arthritis** in adults across all age groups and risk profiles, accounting for ~40–50% of cases. ### Epidemiologic Pattern: | Risk Group | Most Common Organism | Comment | |------------|----------------------|---------| | **Elderly, immunocompromised** | *S. aureus* | Default pathogen; MRSA risk if healthcare exposure | | **Young, sexually active** | *N. gonorrhoeae* | Migratory polyarthritis; urethritis/cervicitis history | | **IV drug users** | *S. aureus* (including MRSA) | Often affects right-sided joints (tricuspid) | | **Prosthetic joint** | *S. epidermidis*, *S. aureus* | Biofilm-forming organisms | | **TB endemic region** | *M. tuberculosis* | Insidious onset; synovial TB often culture-negative | **Clinical Pearl:** In this 68-year-old with RA on immunosuppression, *S. aureus* is the default assumption until proven otherwise. The negative Gram stain does NOT exclude *S. aureus* — organism load may be low, or the smear quality suboptimal. ## Why Not Other Options? ### *Streptococcus pyogenes* (Group A Strep) - Causes acute septic arthritis but is **less common** than *S. aureus* in elderly patients - More typical in younger individuals with skin/soft tissue infection - Would not explain the immunocompromised host predilection ### *Neisseria gonorrhoeae* - Causes septic arthritis primarily in **young, sexually active** patients - Often presents with migratory polyarthritis + urogenital symptoms - Age 68 and no sexual history mentioned make this unlikely - Typically causes lower synovial WBC counts (20,000–50,000) ### *Mycobacterium tuberculosis* - Causes **chronic** septic arthritis (weeks to months), not acute (3 days) - Synovial fluid typically shows lymphocytic predominance, not neutrophilic - Culture is slow (weeks); Gram stain is negative (acid-fast stain needed) - More common in TB-endemic regions; less typical in India's urban elderly **Mnemonic:** **SAFE** organisms in septic arthritis: - **S** = *Staphylococcus aureus* (most common, all ages) - **A** = *Aerobic Gram-negatives* (elderly, urinary source) - **F** = *Fungal* (immunocompromised, indwelling catheters) - **E** = *Enterococci* (healthcare-associated) ## Management Implications **Warning:** Gram-negative cultures do NOT exclude *S. aureus*. Empiric therapy must cover *S. aureus* (including MRSA risk given age and immunosuppression) with vancomycin + ceftriaxone pending culture results. [cite:Harrison 21e Ch 330; Robbins 10e Ch 26]