Respiratory syncytial virus-positive bronchiolitis in hospitalized infants is associated with thrombocytosis

Background: Secondary thrombocytosis is associated with a variety of clinical conditions, one of which is lower respiratory tract infection. However, reports on thrombocytosis induced by viral infections are scarce. Objectives: To assess the rate of thrombocytosis (platelet count > 500 × 109/L) in hospitalized infants with bronchiolitis and to investigate its potential role as an early marker of respiratory syncytial virus infection. Methods: Clinical data on 469 infants aged ≤ 4 months who were hospitalized for bronchiolitis were collected prospectively and compared between RSV-positive and RSVnegative infants. Results: The rate of thrombocytosis was significantly higher in RSV-positive than RSV-negative infants (41.3% vs. 29.2%, P = 0.031). The odds ratio of an infant with bronchiolitis and thrombocytosis to have a positive RSV infection compared to an infant with bronchiolitis and a normal platelet count was 1.7 (P = 0.023, 95% confidence interval 1.07-2.72). There was no significant difference in mean platelet count between the two groups. Conclusions: RSV-positive bronchiolitis in hospitalized young infants is associated with thrombocytosis.