A 62-year-old woman with known COPD (FEV₁ 35% predicted) presents with acute worsening of dyspnea, increased sputum purulence, and fever for 2 days. Chest examination reveals crackles at the right base. Which investigation is most appropriate to identify the causative organism and guide antimicrobial therapy?

Explanation

## Investigation of Choice for Identifying Causative Organism in COPD Exacerbation **Key Point:** Sputum culture with Gram stain is the most appropriate investigation to identify the causative pathogen and guide targeted antimicrobial therapy in acute COPD exacerbation with suspected bacterial infection. ### Rationale for Sputum Culture In COPD exacerbation with purulent sputum, bacterial infection is the likely trigger. Sputum culture directly identifies the organism and allows antimicrobial susceptibility testing (AST), enabling targeted therapy rather than empirical broad-spectrum antibiotics. **High-Yield:** Common bacterial pathogens in COPD exacerbation: - *Streptococcus pneumoniae* (most common) - *Haemophilus influenzae* - *Moraxella catarrhalis* - *Pseudomonas aeruginosa* (in severe disease, FEV₁ <25%) ### Gram Stain Interpretation **Mnemonic: GRAM STAIN INTERPRETATION IN RESPIRATORY SAMPLES** - **G**ram-positive cocci in clusters → *Staphylococcus aureus* - **G**ram-positive cocci in pairs/chains → *Streptococcus pneumoniae* - **G**ram-negative coccobacilli → *Haemophilus influenzae* - **G**ram-negative diplococci → *Neisseria meningitidis* - **G**ram-negative rods → *Enterobacteriaceae*, *Pseudomonas aeruginosa* ### Comparison of Investigations | Investigation | Diagnostic Utility | Limitations | |---|---|---| | **Sputum culture + Gram stain** | Identifies organism; guides targeted therapy | Requires adequate sample (>25 WBC, <10 epithelial cells per LPF); 48–72 hrs for results | | Blood culture | Useful if bacteremia suspected (sepsis) | Low sensitivity in COPD exacerbation; not routine | | Chest X-ray/CT | Excludes pneumonia, pneumothorax, empyema | Does not identify organism; imaging findings non-specific | | Procalcitonin/CRP | Markers of inflammation | Non-specific; do not guide organism identification or therapy selection | **Clinical Pearl:** A good-quality sputum sample is essential—the patient should produce sputum from the lower respiratory tract (not saliva). Inadequate samples lead to contamination and misidentification. ### Pathological Context COPD exacerbations are triggered by: 1. **Bacterial infection** (50% of cases)—sputum culture indicated 2. **Viral infection** (25%)—culture negative; viral PCR may help 3. **Environmental triggers** (pollution, allergens)—no organism identified In this case, purulent sputum and fever suggest bacterial infection, making sputum culture the key investigation. **Tip:** Empirical therapy (e.g., amoxicillin-clavulanate or fluoroquinolone) may be started while awaiting culture results in severe exacerbations. Culture results allow de-escalation or switch to targeted therapy within 48–72 hours.