## Distinguishing SARS-CoV-2 from Influenza A ### Key Clinical Discriminators **Key Point:** Anosmia (loss of smell) and ageusia (loss of taste) are hallmark features of SARS-CoV-2 infection that occur early in the disease course and are relatively uncommon in seasonal influenza A. ### Comparison Table | Feature | SARS-CoV-2 | Influenza A | | --- | --- | --- | | **Anosmia/Ageusia** | Common (30–68% of cases), early onset | Rare (<5%) | | **Fever & Myalgia** | Present but non-specific | Present, prominent | | **Cough & Dyspnea** | Common but non-specific | Common but non-specific | | **Elevated Ferritin/D-dimer** | Marker of severity in COVID-19 | Not typical | | **Onset** | Gradual (2–7 days) | Abrupt (6–24 hours) | ### Pathophysiology of Anosmia in COVID-19 1. SARS-CoV-2 binds ACE2 receptors on olfactory epithelial cells and sustentacular cells 2. Direct viral invasion or neuroinflammation damages olfactory neurons 3. Occurs early, often before systemic symptoms peak 4. Recovers over 2–4 weeks in most cases **High-Yield:** Anosmia/ageusia in SARS-CoV-2 is so characteristic that WHO and CDC recognize it as a screening criterion. Influenza A causes systemic symptoms but does NOT typically cause loss of smell or taste. **Clinical Pearl:** A patient presenting with fever, cough, AND sudden loss of smell should raise strong suspicion for SARS-CoV-2 over influenza, even if other symptoms overlap. ### Why Ferritin/D-dimer Are NOT Discriminators While elevated ferritin and D-dimer are markers of **severity** in COVID-19 (associated with cytokine storm and thromboinflammation), they are not present in *all* SARS-CoV-2 cases and are not specific to COVID-19—severe influenza can also elevate these markers. [cite:Harrison 21e Ch 297]
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