Figure 2.

Urine data in the clinical diagnosis of patients with polyuria. The first step is to determine whether the basis of polyuria is a water diuresis or an osmotic diuresis. A urine osmolality (U_Osm) <250 mosm/kg H₂O suggests a water diuresis. This could be due to diabetes insipidus (DI) or primary polydipsia. The cause of the water diuresis can be determined by examining the change in U_Osm in response to a rise in plasma sodium concentration (P_Na) to >145 mmol/l and the administration of 1-desamino-8-d-arginine vasopressin (dDAVP). A U_Osm >300 mosm/kg H₂O suggests that the polyuria is due to an osmotic diuresis or a medullary interstitial disease impairing the process of concentrating the urine in the renal medulla. These 2 disorders can be separated by calculating the rate of excretion of osmoles. The cause of the osmotic diuresis can be determined by measuring the individual osmoles in the urine (e.g., glucose, urea, and sodium chloride [NaCl]). A large amount of mannitol is not commonly given; hence, it is not likely to be the sole cause of a large and sustained osmotic diuresis. ATN, acute tubular necrosis; ECFV, extracellular fluid volume.