- Estimated GFR <60 ml/min/1.7m² with significant prerenal physiology as judged by prior documented volume mediated changes in renal function, a fractional excretion of urea <35%, or a fractional excretion of sodium <1%. For GFR 30-60: must have serum sodium </= 135 mEq/L OR large home diuretic dose (total daily loop diuretic dose >/= 120 mg/d in furosemide equivalents OR concomitant thiazide use). For GFR <30: no additional criteria needed.