Admit: platelet count <20K or significant mucous membrane bleeding
Discharge: platelet count >20K AND asymptomatic OR only minor petechiae
Immune thrombocytopenic purpura
Marrow infiltration 
Thrombotic thrombocytopenic purpura
Viral infections 
Disseminated intravascular coagulation
Marrow suppression 
Heparin-Induced Thrombocytopenia 
Congenital thrombocytopenia