Water diuresis and osmotic diuresis

Both water diuresis and osmotic diuresis can lead to hypernatremia!  How do we differentiate?

 

Water diuresis and osmotic diuresis , both present with hypernatremia and polyuria!

In water diuresis, the osmolar excretion per day is less than 1000 mOsm/day (calculated from urine osmolar concentration and urine volume) and in osmotic diuresis, the osmolar excretion per day is>1000   mOsm/day.

Water diuresis occurs in Diabetes insipidus

Osmotic diuresis occurs in Parenteral nutrition with heavy protein intake(10 grams protein yield 50 mOsm of urea) and  Diabetes mellitus(glucose is the osmole in urine that drags water with it)

It is nice to understand this concept. However in clinical situation the numbers don’t matter so much and the free water loss  is usually  from a combination of several factors!

Like,  Critically ill intubated patient recovering from ATN on TPN for nutrition. Here you have no access to free water, water diuresis from recovering ATN(some degree of renal concentrating defect/nephrogenic DI) and osmotic diuresis from TPN!

Nevertheless, it helps us identify the major contributor !

Yuvaraj

 

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