A 35-year-old man with suspected Mycoplasma pneumoniae respiratory infection is prescribed azithromycin. His symptoms persist after 5 days of therapy. To confirm M. pneumoniae infection and assess for macrolide resistance, which investigation is most appropriate?

## Diagnosis of Mycoplasma pneumoniae Infection ### Gold Standard Investigations **Key Point:** Serology (IgM/IgG antibodies) or PCR-based detection of M. pneumoniae DNA from respiratory secretions is the investigation of choice for confirming M. pneumoniae infection. ### Why Serology and PCR Are Preferred **High-Yield:** M. pneumoniae cannot be cultured on routine media (blood agar, MacConkey) because it lacks a cell wall and requires specialized PPLO (Pleuropneumonia-Like Organisms) media. **Diagnostic Methods:** | Method | Timing | Sensitivity | Use | |---|---|---|---| | IgM antibodies | Appears day 3–7 of illness; peaks week 2–3 | ~90% | Acute infection; early diagnosis | | IgG antibodies | Appears week 2–3; persists months–years | ~95% | Confirms past or recent infection | | PCR (respiratory secretions) | Positive from symptom onset; highest sensitivity early | ~95% | Most sensitive; detects organism directly; can assess resistance genes | | Culture on PPLO media | Slow; takes 2–3 weeks | ~50% | Not practical for acute diagnosis | ### Why Persistent Symptoms Suggest Resistance **Clinical Pearl:** If M. pneumoniae symptoms persist after 5 days of azithromycin, macrolide resistance should be suspected. PCR-based testing can identify resistance-conferring mutations (e.g., 23S rRNA methylation genes) directly from respiratory secretions. ### Why Other Options Are Incorrect **Warning:** The following are NOT diagnostic for M. pneumoniae: - **Chest X-ray:** Shows pneumonic pattern (often atypical/interstitial) but is non-specific; many pathogens cause similar infiltrates - **Sputum Gram stain/culture on blood agar:** M. pneumoniae is Gram-negative but does NOT grow on routine bacterial media; it requires PPLO agar and takes weeks - **ESR/CRP:** Non-specific markers of inflammation; elevated in many infections and do not identify the organism **Mnemonic:** **CAMP** — Cannot be cultured on routine media; Atypical pneumonia; Macrolide-resistant strains emerging; PCR/serology for diagnosis. [cite:Harrison 21e Ch 181]