Positive fluid balance as an early biomarker for acute kidney injury: a prospective study in critically ill adult patients Maria Olinda Nogueira Ávila; Paulo Novis Rocha; Caio A. Perez; Tássia Nery Faustino; Paulo Benigno Pena Batista; Luis Yu; Dirce Maria T. Zanetta; Emmanuel A. Burdmann;