Oral Anticoagulant Apixaban for Treatment of Venous Thromboembolism

Inclusion Criteria: * Children and adolescents with a newly diagnosed primary VTE. * Treatment with apixaban has to begin within 72 hours of the diagnosis of VTE. * Patients who have received heparin at the beginning of their diagnosis and are willing to switch to apixaban, are allowed to do so within 72 hours of their treatment. Patients who are switched to apixaban are not allowed to receive any other systemic anticoagulant therapy. * VTE confirmed by diagnostic imaging. * Children with body weight of ≥ 40 kg and are able to tolerate medication by mouth or administered via an NG or GT tube. * Adequate liver function: AST and/or ALT \<5 times ULN, and/or Direct bilirubin \<2 times ULN * Platelet count of at least ≥ 30,000/ul. * Adequate renal function: \>30% of GFR for age * Female subjects of childbearing potential must not be nursing or pregnant with negative urine pregnancy test within 72 hours prior to dosing with study medication. * Signed written informed consent Exclusion Criteria: * Current or recent (within 3 months) apixaban administration. * Patient is unable to tolerate medications by oral route or by n/g administration, peptic ulcer disease, intestinal obstruction, toxic megacolon, typhlitis, or short gut syndrome. * History of primary bleeding disorder and first degree family history of bleeding disorder. * Active bleeding or high risk of bleeding at the time of study entry. * History of significant head injury and/or any history of intracranial hemorrhage. * Conditions predispose to intracranial bleed; e.g. brain tumor, brain metastasis. * Uncontrolled Grade 3 or 4 severe hypertension. * History of allergy to apixaban or factor Xa inhibitors.