Respiratory Influenza viral load as a marker of poor prognosis in patients with severe symptoms

•Respiratory samples collected from ICU hospitalized patients are heterogeneous.•The virus-over-cell ratio in respiratory samples allows some standardization of influenza virus load measurement.•Standardized respiratory viral load could help predicting poor outcome in patients with acute respiratory distress syndrome. The link between influenza virus (IV) viral load (VL) in respiratory samples and disease severity is not clearly established. This study was designed to assess IV-VL in respiratory samples from flu patients admitted to intensive care unit (ICU). Patients admitted to ICU for IV infection, as documented by RT-PCR, with respiratory failure were included in the study during 5 flu-seasons (2014–2018). Routine ICU management parameters were recorded. Real-time amplification Ct-values for IV and cell GAPDH gene were measured in each respiratory sample collected at ICU admission. Among 105 included patients, 59 (56.1%) presented an acute respiratory distress syndrome (ARDS). The overall mortality was 21%. IV-load assessed by amplification Ct-values and virus-over-cell ratio (expressed as log10) in each respiratory sample ranged from 20 to 40 and −5.2–3.7, respectively, and did not differ according to the type of sample and IV-A or -B type. Cell richness was higher in samples from ARDS patients compared to non-ARDS (p = 0.0003) but no difference was noted for IV Ct-values. In ARDS-patients, IV Ct-values (p = 0.020) and the virus-per-cell ratio (p = 0.038) were significantly higher in sample from patients who eventually died compared to those who survived. These 2 parameters remain independently associated with mortality with an odd-ratio of 1.21 and 2.19, respectively (p