A 65-year-old man with known COPD (FEV₁ 35% predicted) presents with acute worsening of dyspnea, increased sputum purulence, and fever for 3 days. Chest X-ray shows no focal consolidation. Which investigation is most appropriate to identify the causative organism and guide antibiotic therapy?

Explanation

## Investigation of COPD Exacerbation **Key Point:** Sputum culture and Gram stain are the investigations of choice for identifying the causative organism in acute exacerbation of COPD (AECOPD) with purulent sputum. ### Why Sputum Culture and Gram Stain? **High-Yield:** In AECOPD with purulent sputum: 1. **Sputum Gram stain** provides rapid morphological identification (Gram-negative rods, Gram-positive cocci, etc.) 2. **Sputum culture** identifies the organism and enables antibiotic susceptibility testing (AST) 3. **Non-invasive, readily available, cost-effective** — can be obtained at bedside 4. **Guides targeted therapy** — reduces unnecessary broad-spectrum antibiotics **Clinical Pearl:** The presence of **purulent sputum** (yellow/green) indicates bacterial infection and justifies culture. Sputum samples should be **expectorated** (not saliva) and processed within 2 hours. **Mnemonic: AECOPD + Purulent Sputum = Sputum Culture + Gram Stain** ### Common Organisms in AECOPD | Organism | Frequency | Gram Stain | Clinical Context | |---|---|---|---| | *Haemophilus influenzae* | 30–40% | Gram-negative coccobacillus | Most common; ampicillin resistance common | | *Streptococcus pneumoniae* | 20–30% | Gram-positive diplococci | Vaccine-preventable | | *Moraxella catarrhalis* | 10–15% | Gram-negative diplococcus | β-lactamase producer | | *Pseudomonas aeruginosa* | 5–10% | Gram-negative rod | Severe/hospitalized COPD; high mortality | | Viral (rhinovirus, influenza) | 25–50% | N/A | Often precedes bacterial superinfection | ### Role of Other Investigations **Warning:** Blood cultures are NOT routinely indicated in AECOPD unless there is clinical evidence of sepsis or bacteremia (hypotension, altered mental status). COPD exacerbation is a **lower respiratory tract infection**, not bacteremia. **Tip:** BAL is reserved for **immunocompromised patients** or when standard sputum cannot be obtained. It is invasive and unnecessary in routine AECOPD. **Procalcitonin** is a biomarker of bacterial infection but does NOT identify the organism and does not guide antibiotic selection — it is a prognostic/severity marker, not a diagnostic test. [cite:Harrison 21e Ch 254; GOLD 2023 Report]