 Critical illness and the presence of a central venous catheter CVC are the most important risk factors for deep venous thrombosis DVT in children. Catheter-associated thrombosis CADVT is highly prevalent and associated with poor outcomes in critically ill children. Yet, based on underpowered pediatric trials, prophylaxis against CADVT is not recommended in children. The recommendation to provide prophylaxis against thrombosis in critically ill adults should not be applied to children because the hemostatic system and co-morbidities vastly differ between age groups. Pivotal trials are urgently needed to determine whether prophylaxis can prevent CADVT and its complications in critically ill children. However, the timing and extent of reduction in thrombin generation, the biochemical goal of prophylaxis, needed to prevent CADVT in children are unclear. The goal of this application is to explore the efficacy of early prophylaxis against CADVT in critically ill children. Aim 1 is to obtain preliminary evidence on the effect of early prophylaxis on the incidence of CADVT in critically ill children. Based on the natural history of CADVT, we hypothesize that among critically ill children, prophylaxis administered 24 hours after catheter insertion decreases the incidence of ultrasound-diagnosed CADVT compared with no prophylaxis. In this phase 2a trial, children admitted to the intensive care unit with a newly inserted central venous catheter will receive enoxaparin adjusted according to anti-Xa activity, a control group will not receive enoxaparin adjusted according to anti-Xa activity. Enoxaparin has become the standard pediatric anticoagulant for prophylaxis despite the absence of conclusive data. We will use Bayesian approach to determine whether further trials are warranted. Aim 2 is to evaluate the effect of an anti-Xa activity-directed prophylactic strategy on thrombin generation in critically ill children. We hypothesize that among critically ill children, standard prophylactic dose of enoxaparin adjusted by anti-Xa activity reduces thrombin generation to 700 nM.min, as measured by ETP. In non-critically ill adults, prophylactic dose of enoxaparin proven to prevent DVT reduces ETP to 700 nM.min.Endogenous thrombin potential is the best available measure of thrombin generation. We will measure endogenous thrombin potential and anti-Xa activity at multiple time points then examine their relationship in all children enrolled in the phase 2a trial. The proposed research challenges the current paradigm on prophylaxis against CADVT in children. High quality evidence is needed to prevent CADVT and its complications in this vulnerable population. 