A 58-year-old man with known COPD (FEV₁ 35% predicted) presents with acute worsening of dyspnea, increased sputum purulence, and fever over 3 days. Chest X-ray shows no new infiltrates. To identify the causative organism and guide antibiotic therapy, which investigation is most appropriate?

## Investigation of Choice for COPD Exacerbation with Suspected Infection **Key Point:** Sputum culture and Gram stain are the investigations of choice for identifying the causative organism in COPD exacerbations with purulent sputum, enabling targeted antimicrobial therapy. ### Diagnostic Approach to COPD Exacerbation ```mermaid flowchart TD A[COPD exacerbation suspected]:::outcome --> B{Purulent sputum present?}:::decision B -->|Yes| C[Sputum culture + Gram stain]:::action B -->|No| D[Clinical diagnosis, empiric therapy]:::action C --> E[Organism identification]:::outcome E --> F[Targeted antibiotic therapy]:::action D --> G[Broad-spectrum antibiotics]:::action ``` ### Common Organisms in COPD Exacerbation | Organism | Frequency | Gram Stain | Clinical Note | |----------|-----------|-----------|---------------| | *Haemophilus influenzae* | 25–40% | Gram-negative coccobacillus | Most common; pleomorphic | | *Streptococcus pneumoniae* | 10–20% | Gram-positive diplococci | Alpha-hemolytic | | *Moraxella catarrhalis* | 5–15% | Gram-negative diplococcus | Oxidase-positive | | *Pseudomonas aeruginosa* | 5–10% | Gram-negative rod | In advanced COPD (FEV₁ < 25%) | | Respiratory viruses | 20–30% | N/A (culture negative) | Rhinovirus, influenza, RSV | **High-Yield:** In COPD exacerbations with purulent sputum, sputum culture is the single best investigation to identify the organism and guide antibiotic selection [cite:Harrison 21e Ch 254]. ### Gram Stain Interpretation in Sputum **Quality Assessment:** - **Acceptable sample:** ≥ 25 WBCs and < 10 squamous epithelial cells per low-power field - **Poor sample:** ≥ 10 epithelial cells (contamination with saliva) **Clinical Pearl:** A good-quality sputum sample showing gram-negative coccobacilli (pleomorphic) with no growth on chocolate agar is highly suggestive of *Haemophilus influenzae*, the most common organism in COPD exacerbations. **Warning:** Do not rely on blood cultures alone in COPD exacerbation — bacteremia is uncommon (< 5% of cases). Sputum culture is far more likely to yield the causative organism. ### Why Sputum Culture is Superior 1. **Direct sampling:** Obtains organisms from the site of infection (lower respiratory tract) 2. **Sensitivity:** Higher yield than blood culture in non-bacteremic exacerbations 3. **Organism identification:** Allows species and susceptibility testing 4. **Non-invasive:** Simple, safe, and cost-effective 5. **Guides therapy:** Enables de-escalation from broad-spectrum to targeted antibiotics **Mnemonic:** **Purulent Sputum = Sputum Culture** — when COPD exacerbation has purulent sputum, send sputum culture first.