## Diagnosis of Mycoplasma pneumoniae Infection ### Why Serology (IgM ELISA) is the Investigation of Choice **Key Point:** Mycoplasma pneumoniae cannot be cultured on routine bacterial media (blood agar, chocolate agar) because it lacks a cell wall. Culture requires specialized PPLO (Pleuropneumonia-Like Organisms) media and takes 2–3 weeks, making it impractical for acute clinical decision-making. **High-Yield:** Serum IgM antibodies against M. pneumoniae appear within 1 week of symptom onset and are the gold standard for rapid diagnosis in acute infection. IgM positivity confirms active/recent infection. ### Diagnostic Methods for Mycoplasma pneumoniae | Investigation | Sensitivity | Specificity | Timing | Clinical Use | |---|---|---|---|---| | **Serum IgM ELISA** | 80–90% | 95%+ | 1 week from onset | **Gold standard for acute diagnosis** | | **Serum IgG** | High | High | Appears after 2–3 weeks | Indicates past infection, not acute | | **PCR (sputum/throat)** | 90%+ | 95%+ | Early (days 1–3) | Highly specific but not widely available | | **Culture (PPLO media)** | 50–60% | 100% | 2–3 weeks | Too slow for clinical use | | **Gram stain** | Very low | N/A | Immediate | M. pneumoniae is Gram-negative but not visible on routine staining | **Clinical Pearl:** Cold agglutinins (heterophile antibodies) are positive in ~50% of M. pneumoniae infections and appear by week 1–2, but they are non-specific and can be seen in other viral infections; serum IgM is more specific. ### Why Azithromycin Works Macrolides (azithromycin, clarithromycin) are the drugs of choice for M. pneumoniae because they inhibit bacterial protein synthesis and achieve high intracellular concentrations in respiratory epithelium [cite:KD Tripathi 8e Ch 52]. **Mnemonic: CAMP** — Chlamydia, Atypicals (Mycoplasma, Legionella), Mycobacteria — organisms treated with macrolides rather than beta-lactams.
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