Urine osmolarity is very variable, and depends on the pathology of the patient. Urine osmolarity allows the etiological search for a hydroelectrolyte disorder.
Formula: Osm = (Natremia + Serum K) x 2 + Blood Sugar + Uremia.
In biochemistry, osmolality is a measure of the number of osmoles of solute per kilogram of solvent.
Not to be confused with osmolarity, which is a measure of the number of osmoles of solute per liter of solution.
If the solvent is water, these measurements are almost equivalent for dilute solutions, since the density of distilled water is 0.9976 kg/l at 23°C.
Physiological osmolality is maintained within narrow limits. A minimal variation, of the order of a few %, leads to the initiation of the main regulatory factors: secretion of ADH, triggering of thirst, renal regulation by concentration-dilution.
- Isotonic hyponatremia (normal osmolality): decrease in the volume of plasma water during hyperprotidemia or hyperlipidemia.
- Hypertonic hyponatraemia (increased osmolality): glucose overload in diabetics, alcohol overload, contrast product overload, mannitol or glycerol overload. Extracellular hyperosmolality leads to water outflow from cells, dilution hyponatremia and a decrease in cell volume.
- Hypotonic hyponatremia (decreased osmolality and natremia): loss of extracellular fluid, reduction in sodium capital by urinary or digestive losses, reduction in the capacity for dilution of urine by the kidney, water retention by renal insufficiency, with inappropriate secretion of ADH during cancer, meningitis, trauma or taking medication (barbiturates, NSAIDs, carbamazepine, sulfonamides, opiates, etc.).
- Hypertonic hypernatremia (increased osmolality and natremia): intracellular dehydration and reduction in cell volume in the event of altered sensation of thirst (elderly people, children, unconscious subjects).
In children, the renal concentration power of sodium is different. An infant has a sodium (or chlorine) clearance close to 20% of that of an adult.
Diuresis and urinary osmolality vary widely depending on fluid intake and renal concentrating power.