A respiratory sample collected from a ventilated patient with suspected influenza is tested by two methods: immunofluorescence (DFA) returns negative, while RT-PCR for influenza A/B returns positive for influenza A (H1N1)pdm09. What does this discordance most likely reflect?

• A False positive RT-PCR due to contamination; repeat DFA is required
• B DFA is more specific and the RT-PCR result should be discarded
• C Lower sensitivity of DFA compared to RT-PCR; molecular testing is the gold standard ✓
• D The virus is present but non-viable; clinical significance is uncertain

Explanation

RT-PCR (NAAT) for influenza has sensitivity of 86–100% and is the gold standard for influenza diagnosis, particularly in ICU patients where viral shedding may be low. Rapid antigen tests and direct fluorescent antibody (DFA) assays have sensitivity of only 50–70% compared to RT-PCR, especially in adult immunocompromised patients or late in illness. WHO recommends RT-PCR as confirmatory standard. The discordance in this case reflects the well-documented inferior sensitivity of DFA, not RT-PCR false positivity. Treatment with oseltamivir should be initiated based on the PCR result without waiting for DFA confirmation.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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