> 3L Urine Output / 24 hours
This is an arbitrary definition, since the specific context is important; for example, is the polyuria appropriate physiologically or inappropriate? [Oster, J. et al. 1997]
Classify: Water, Solute, or Mixed Diuresis?
Contextualize: Appropriate or Inappropriate?
Rose BD, Post TW. 2001. (p. 772)
While there is no universal cutoff for urine osmolality in a water diuresis, the urine should be hyposmotic to the plasma. Urine Osm < 150 mOsm/L is suggestive of a pure water diuresis. Urine Osm between 150-300 mOsm/L may be seen in a mixed water/solute diuresis [Oster, J. et al. 1997]
Polydipsia
This is an appropriate diuresis. Evident by history.
Diabetes Insipidus
Inappropriate. Central (CDI) vs. Nephrogenic (NDI).
If thirst is intact, serum sodium may be normal [Skorecki, K et al. 2016. Chap. 16]
What are some common causes of CDI and NDI?
Bichet, D. 2021; Skorecki, K et al. 2016
CDI
Head Trauma
Encephalopathy
Pregnancy
EtOH
Numerous Meds
NDI
Lithium
Cisplatin
Foscarnet
Numerous Other Meds
HyperCa
Sickle Cell
How would you distinguish these two types of DI?
Urine osmolality typically > 300 mOsm/L unless mixed with a superimposed water diuresis. A total daily osmolar output of > 1000 mOsmol / day confirms solute diuresis [Bichet, D. 2021]
Total Daily Osmolar Output = 24 Hr Urine Volume x Urine Osm.
Urine studies will then identify the culprit principal electrolyte or non-electrolyte solute. Common examples below:
Does a solute diuresis lead to volume depletion or a free water deficit?
It depends on the solute: for example, unlike sodium, urea does not contribute to tonicity.
Therefore, a urea diuresis will result in loss of free water.
See electrolyte free water clearance in Concepts
Glucosuria
Commonly in DKA and HHS
Anticipated with SGLT2 Inhibitors
Urea Diuresis
Urea loading from protein-rich diet
Can occur in post-obstructive diuresis and ATN
Can occur in catabolic states with muscle breakdown
Any other cause of elevated BUN, such as GIB or rhabdomyolysis, may lead to urea diuresis
Sodium Diuresis
Usually intentional in the case of diuretics or appropriate after excessive salt load or IVF administration.
May also be a component of a post-obstructive diuresis
Why do some authorities believe a sodium diuresis is rarely inappropriate?
Many authorities believe sodium diuresis almost never causes an inappropriate polyuria becasue tubuloglomerular feedback in the kidney may protect against this. [Bichet, D.; Rose BD, Post TW. 2001]
Click Increase Na+ Flow to see how the tubuloglomerular feedback and the juxtaglomerular apparatus work in this context. Note that while only sodium is shown, chloride is also sensed in the macula densa.