Urine Osmolality

What’s urine osmolality useful for?

Urine osmolality is useful for assessing the concentrating and diluting ability of the kidney.

“Urine Osmolality is an indirect functional assay of the ADH-renal axis. Urine Osmolality range: 50 mOsm/L (no ADH) to 1200 mOsm/L (maximum ADH).” Pocket Medicine Urine osmolality is a surrogate for ADH. A urine osmolality > 100 mOsm/L in a euvolemic patient without any other issues is suggestive of SIADH.

Normal Range of Urine Osmolality

Normal Range: 50-1200 mOsm/kg —- Ferri’s Practical Guide.

Urine Osmolality in a Hyponatremic Patient

In the context of hyponatremia, an osmolality of 100 mOsm/kg is a cut off for osmolality that is usually used.
In the context of hypotonic hyponatremia (“true” hyponatremia),
–Urinary osmolality > 100 mOsm/kg is considered inappropriately high. ADH which controls urine osmolality is not doing what it should be doing to raise the low serum osmolality. This would suggest SIADH and other conditions where ADH is also high like glucocorticoid deficiency, hypothyroidism, and drugs.
-Urine osmolality <100 mOsm/kg is considered appropriately low. ADH is working as it should. Conditions include 1) Primary Polydipsia and 2) Low Solute (Beer Potomania syndrome / “Tea and toast”
-Urine osmolality that varies. Reset osmostat.

Urine Osmolality in a Hypernatremic Patient

It should be >700 due to a maximum ADH effect. A urine osmolality of > 700 suggests extra-renal water loss while a urine osmolality of < 700 suggests a renal water loss.

More on Urine Osmolality, From Mayo Clinic Lab

The Following is from the Mayo Clinic Lab.

What is urine osmolality
“Osmolality is an index of the solute concentration. Urine osmolality is a measure of the concentration of osmotically active particles, principally sodium, chloride, potassium, and urea; glucose can contribute significantly to the osmolality when present in substantial amounts in urine. Urinary osmolality corresponds to urine specific gravity in nondisease states.

The ability of the kidney to maintain both tonicity and water balance of the extracellular fluid can be evaluated by measuring the osmolality of the urine either routinely or under artificial conditions. More information concerning the state of renal water handling or abnormalities of urine dilution or concentration can be obtained if urinary osmolality is compared to serum osmolality and if urine electrolyte studies are performed. Normally, the ratio of urine osmolality to serum osmolality is 1.0 to 3.0, reflecting a wide range of urine osmolality.

Reference Values
0-11 months: 50-750 mOsm/kg
≥ 12 months: 150-1,150 mOsm/kg

Interpretation
With a normal fluid intake and normal diet, a patient will produce a urine of about 500 to 850 mOsmol/kg water. Above the age of 20 years, there is an age-dependent decline in the upper reference range of approximately 5 mOsm/kg/year.

The normal kidney can concentrate a urine to 800 to 1,400 mOsmol/kg and with excess fluid intake, a minimal osmolality of 40 to 80 mOsmol/kg can be obtained.

With dehydration, the urine osmolality should be 3 to 4 times the plasma osmolality.” Mayo Clinic Lab

 

 

Reference

mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/9260