Which differentiating prerenal azotemia with ATN features favouring pre-renal: (A) Urine osmolality > 500 mosmol/kg, (B) Sodium spot excretion < 10 ml/L, (C) Plasma transferrin/Ig ratio, (D) none
Answer is A. The normal 24-hour urine osmolality is, on average, 500-800 mOsmol/kg of water.
After 12-14 hours of fluid intake restriction, the urine osmolality should exceed 850 mOsmol/kg of water