A 58-year-old woman with COPD GOLD Stage 2 (FEV₁ 65% predicted) presents with acute exacerbation characterized by increased dyspnea, purulent sputum, and fever. She has no prior sputum culture. Which investigation is most appropriate to guide antimicrobial therapy in this acute exacerbation?

Explanation

## Microbial Diagnosis in COPD Exacerbation **Key Point:** Sputum culture and sensitivity is the most appropriate investigation to identify the causative pathogen and guide targeted antimicrobial therapy in acute COPD exacerbation with purulent sputum. ### Rationale for Sputum Culture **High-Yield:** In COPD exacerbations with purulent sputum (green or yellow), sputum culture identifies: - **Common pathogens:** *Haemophilus influenzae*, *Streptococcus pneumoniae*, *Moraxella catarrhalis*, *Pseudomonas aeruginosa* (in advanced disease) - **Antimicrobial resistance patterns:** Essential for tailoring therapy, especially in recurrent exacerbations - **Severity markers:** Gram-negative organisms or *Pseudomonas* indicate more severe disease requiring broader coverage ### Timing and Clinical Impact 1. **Sputum should be collected before antibiotics are started** to maximize culture yield 2. **Empiric therapy** is initiated based on COPD severity and local resistance patterns 3. **Culture results** allow de-escalation or escalation of therapy after 48–72 hours **Clinical Pearl:** Patients with COPD GOLD 3–4 or frequent exacerbations (≥2/year) are at higher risk for *Pseudomonas* colonization and should have sputum cultured regularly to guide prophylactic or empiric therapy. ### Comparison of Investigations | Investigation | Purpose in COPD Exacerbation | Diagnostic Value | |---|---|---| | Sputum culture & sensitivity | Identify pathogen and resistance; guide targeted therapy | **Gold standard for microbial diagnosis** | | Chest X-ray | Exclude pneumonia, pneumothorax, pulmonary edema | Supportive; not diagnostic for infection | | Procalcitonin | Marker of bacterial infection; guides antibiotic initiation | Adjunctive; does not identify organism | | CBC with differential | Assesses systemic inflammation; elevated WBC suggests infection | Non-specific; does not guide therapy | **Mnemonic: PURULENT = Pathogen Urgently Requires Culture Examination Leading to Effective Narrow-spectrum Therapy** [cite:Harrison 21e Ch 297; GOLD 2023]