Platelet IFITM3 is increased in patients with H1N1/A influenza infection and correlates with influenza-related mortality. Whole blood samples were drawn from patients with confirmed H1N1/A influenza infection or healthy matched control subjects. Purified platelets were isolated as described in “Methods.” (A) Scatter plot with significantly enriched (red) and repressed (blue) platelet transcripts identified by using RNA-seq (adjusted P < .05). IFITM3 is indicated (enlarged dot). (B) Representative Integrative Genomics Viewer browser images of IFITM3 mRNA expression in a patient with H1N1/A influenza (bottom) or a healthy control subject (top). Representative of n = 3 patients with H1N1/A influenza and n = 3 healthy control subjects. (C) Plasma levels of IFNα in healthy control subjects and patients with H1N1/A influenza. (D) Validation with quantitative RT-PCR for IFITM3 mRNA was performed in platelets isolated from patients with H1N1/A influenza (n = 31) or healthy matched control subjects (n = 24). (E) Immunoblot (top) and densitometric quantification (bottom) of IFITM3 and β-actin expression in platelets from patients with H1N1/A influenza (n = 31) or healthy matched control subjects (n = 24). (F) In a subset of patients with H1N1/A influenza (n = 10), IFITM3 protein in platelets was measured longitudinally on ICU days 1, 4, and 8. Twenty-eight-day infection-related mortality was prospectively captured. Patients with H1N1/A influenza were divided into those who survived their influenza illness (survivors, n = 5) and those who died of their influenza illness before hospital discharge (nonsurvivors, n = 5). The dotted line on the bottom represents the average IFITM3 protein expression in platelets from healthy matched control subjects (n = 14).