Objective: To determine whether increased antithrombin loss is present in children with chylothorax after cardiac surgery. Methods: Plasma and pleural effusion samples of children with chylous and non-chylous pleural effusion were assayed for antithrombin activity. Results: Ten children with chylothorax and five children with non-chylous pleural effusion were investigated. There was statistically significant increase in mean antithrombin activity in chylous samples (32.2 ± 11.4%) compared to non-chylous samples (14.4 ± 13.9%), and significant decrease in plasma of children with chylothorax (44.6 ± 15.4%) compared to children with non-chylous pleural effusion (69.9 ± 22.4%). Seven of 10 children with chylous and none of the children without chylous developed thrombosis (p < 0.007). Conclusions: Increased loss of antithrombin is present in children with chylothorax, potentially predisposing these children to an increased risk of thrombosis. Repeated antithrombin substitution should be considered in critically ill children with chylothora
