During an influenza outbreak in a hospital, a 68-year-old man with COPD presents with fever, cough, and dyspnea on day 3 of illness. Rapid antigen test is positive for influenza A. The infection control team needs to identify the strain and assess for antiviral resistance. Which investigation is most appropriate for this purpose?

Explanation

## Investigation of Choice for Strain Identification and Antiviral Resistance Assessment ### Why Viral Culture + Sequencing + Susceptibility Testing **Key Point:** When strain identification and antiviral resistance testing are required (as in outbreak investigations and surveillance), viral culture followed by nucleotide sequencing and neuraminidase inhibitor (NAI) susceptibility testing is the gold standard [cite:Park 26e Ch 8]. **High-Yield:** Viral culture allows isolation of the virus for molecular characterization, phylogenetic analysis, and functional antiviral resistance testing — essential for outbreak management and public health surveillance. ### Clinical Context: Outbreak Investigation **Clinical Pearl:** In hospital outbreaks, strain identification (H1N1, H3N2, or pandemic strains) and detection of antiviral-resistant variants (e.g., oseltamivir-resistant neuraminidase mutants) guide infection control measures, treatment decisions, and prophylaxis strategies. ### Diagnostic Methods for Strain Identification and Resistance | Investigation | Purpose | Turnaround | Clinical Use | |---|---|---|---| | **Viral Culture + Sequencing** | Strain ID, phylogenetics, resistance mutations | 5–14 days | Gold standard for outbreak investigation; identifies specific strain and resistance patterns | | **Hemagglutination Inhibition (HI)** | Serological strain differentiation | 2–3 days | Useful for serology but does NOT assess antiviral resistance; limited for acute outbreak management | | **Immunofluorescence** | Rapid detection only | 2–4 hours | Confirms influenza but does NOT identify strain or resistance | | **Complement Fixation Test (CFT)** | Serological confirmation | 2–3 days | Detects antibodies; does NOT identify strain or assess resistance | **Mnemonic:** **SCAN** for outbreak investigation — **S**equencing (strain ID), **C**ulture (isolate virus), **A**ntiviral resistance testing (NAI susceptibility), **N**euraminidase mutations (detect resistance). ### Why Other Options Are Inadequate 1. **Hemagglutination inhibition (HI) assay:** HI is a serological method that can differentiate influenza A subtypes (H1, H3) and strains, but it does NOT assess antiviral resistance. It is slower than molecular methods and does not provide information on neuraminidase inhibitor susceptibility. 2. **Immunofluorescence microscopy:** This rapid technique confirms influenza virus presence but does NOT identify the specific strain (H1N1 vs. H3N2) or detect antiviral resistance mutations. It is a detection method, not a characterization method. 3. **Complement fixation test (CFT):** CFT is a serological test that detects antibodies to influenza but provides no information on strain identity or antiviral resistance. It is neither rapid nor useful for outbreak investigation. **Key Point:** Viral culture is essential because it produces infectious virus that can be sequenced (for strain identification and phylogenetic analysis) and tested for functional antiviral resistance (neuraminidase inhibitor susceptibility assays). This is the only method that comprehensively addresses both outbreak epidemiology and treatment resistance.