A 32-year-old woman presents to the emergency department on day 3 of acute onset fever (39.5°C), cough, myalgia, and headache during the influenza season. She has no significant comorbidities. Which investigation is most appropriate to confirm influenza virus infection at this stage of illness?

Explanation

## Diagnostic Approach to Influenza ### Timing and Test Selection **Key Point:** RT-PCR (reverse transcription polymerase chain reaction) on nasopharyngeal swab is the gold standard for acute-phase influenza diagnosis, particularly within the first 3–5 days of symptom onset when viral load is highest. **High-Yield:** The patient is on day 3 of illness — this is the optimal window for viral detection. RT-PCR has: - Sensitivity: 95–98% - Specificity: >99% - Can differentiate influenza A from B and identify pandemic strains ### Why RT-PCR on Nasopharyngeal Swab? 1. **Specimen collection:** Nasopharyngeal swab (or aspirate) captures the highest viral concentration during acute infection. 2. **Molecular amplification:** RT-PCR amplifies viral RNA, making it exquisitely sensitive even when viral shedding is declining. 3. **Rapid turnaround:** Results available within 2–4 hours in most labs. 4. **Clinical utility:** Enables early antiviral therapy (oseltamivir) initiation, which is most effective if started within 48 hours of symptom onset. ### Comparison of Diagnostic Methods | Investigation | Timing | Sensitivity | Specificity | Clinical Use | |---|---|---|---|---| | **RT-PCR (nasopharyngeal)** | Days 1–5 (peak: 1–3) | 95–98% | >99% | **Acute diagnosis; gold standard** | | Rapid antigen test | Days 1–3 | 40–60% | 95–98% | Bedside screening; lower sensitivity | | Viral culture | Days 1–7 | 40–80% | 100% | Research; slow (3–10 days) | | HI antibody titre (acute + convalescent) | Paired sera 2–4 weeks apart | — | — | Retrospective/epidemiologic confirmation only | | Chest X-ray | Any time | — | — | Assess complications (pneumonia); not diagnostic | | Blood culture | Any time | — | — | Detects secondary bacterial infection; not for viral diagnosis | **Clinical Pearl:** A single acute-phase serum HI titre is not diagnostic — a ≥4-fold rise between acute and convalescent sera (collected 2–4 weeks apart) is required. This is too slow for acute management. **Mnemonic:** **RAPID** for acute viral diagnosis: - **R**T-PCR (molecular) - **A**ntigen detection (rapid but less sensitive) - **P**eak timing (days 1–3 for influenza) - **I**mmunofluorescence (alternative; less commonly used now) - **D**irect specimen (nasopharyngeal, not serum) [cite:Harrison 21e Ch 195] [cite:Park 26e Ch 8]