What is urinary osmolarity?
The urinary osmolarity is the concentration of osmotic active solutes in the urine. Being this a somewhat ambiguous concept, it will be explained through the most classic example: a mixture. Any liquid mixture is composed of a solvent, generally water as in the case of urine, and one or more solutes.
Even when they are «mixed» they are not «combined»; that is, none of the components of the mixture lose their own chemical characteristics. The same phenomenon occurs in urine. Its main component, water, serves as a solvent for a series of solutes or particles that leave the body through it.
Its concentration can be measured or calculated through a series of formulas or equipment. This concentration is known as urinary osmolarity. The difference with osmolality is that it is measured in the number of particles per kilo and not per liter, as is the case with osmolarity.
However, in urine, since it is basically water, the calculation is very similar unless there are pathological conditions that modify it dramatically.
What does it consist of?
The process by which urine is concentrated or diluted is very complex, requiring two independent renal systems to integrate properly: the creation of a solute gradient and antidiuretic hormone activity.
Urinary concentration and dilution
The creation of the solute osmolar gradient occurs in the loop of Henle and in the renal medulla. There the osmolarity of the urine increases from values similar to those of plasma (300 mOsm/kg) to levels close to 1200 mOsm/kg, all this thanks to the reabsorption of sodium and chloride in the thick portion of the ascending loop of Henle.
Subsequently, urine passes through the cortical and medullary collecting ducts, where water and urea are reabsorbed, thus helping to create the osmotic gradient.
Likewise, the thin part of the ascending loop of Henle contributes to the decrease in urinary osmolarity due to its permeability to chlorine, sodium, and to a lesser degree to urea.
As its name indicates, the antidiuretic hormone prevents or decreases the expulsion of urine to, under normal conditions, save water.
This hormone, also known as vasopressin, is then activated in situations of high plasma osmolarity (>300 mOsm/kg) to reabsorb water, which eventually dilutes the plasma, but concentrates the urine.
What is urinary osmolarity used for?
Urinary osmolarity is a laboratory study that is indicated to know the concentration of urine with greater precision than that obtained through urinary density, since it does not only measure solutes but the number of molecules per liter of urine.
It is indicated in many medical conditions, both acute and chronic, in which there may be kidney damage, hydroelectrolytic disorders, and metabolic compromise.
Consequences of increased urinary osmolarity
dehydration.
High protein intake.
Syndrome of inappropriate secretion of antidiuretic hormone.
Mellitus diabetes.
Chronic liver diseases.
adrenal insufficiency.
Heart failure.
Septic and hypovolemic shock.
Consequences of decreased urinary osmolarity
Acute kidney infections.
Diabetes insipidus.
Acute or chronic renal failure.
hyperhydration
Treatment with diuretics.
How is urinary osmolarity calculated?
first formula
The easiest method to calculate urine osmolarity is knowing the urine density and applying the following formula:
Urine osmolarity (mOsm/kg or L) = urine density – 1000 x 35
In this expression the value «1000» is the osmolarity of the water and the value «35» is a renal osmolar constant.
Unfortunately there are many factors that affect this result, such as the administration of certain antibiotics or the presence of protein and glucose in the urine.
second formula
To use this method, it is necessary to know the concentration of electrolytes and urea in urine because the elements with osmotic power in urine are sodium, potassium and the aforementioned urea.
Urinary osmolarity (mOsm/K or L) = (Na u + K u) x 2 + (Urea u/5.6)
In said expression:
Na u: urinary sodium.
K u: urinary potassium.
Urea u: Urinary urea.
Urine can be eliminated in different concentrations: isotonic, hypertonic and hypotonic. The terms isoosmolar, hyperosmolar, or hypoosmolar are not usually used for cacophonous reasons, but they refer to the same thing.
osmolar clearance
To determine the concentration of solutes, the osmolar clearance formula is used:
C osm = (Osm) urine x V min/Osm) blood
In this formula:
Cosm: osmolar clearance.
(Osm) urine: urinary osmolarity.
V min: minute volume of urine.
(Osm) blood: plasma osmolarity.
From this formula it can be deduced that:
If urine and plasma have the same osmolarity, they are discarded from the formula and the osmolar clearance would be equal to the urinary volume. This occurs in isotonic urine.
When urinary osmolarity is greater than plasmatic osmolarity, it is referred to as hypertonic or concentrated urine. This implies that osmolar clearance is greater than urinary flow.
If the urinary osmolarity is less than the plasmatic one, the urine is hypotonic or dilute and it is concluded that the osmolar clearance is less than the urinary flow.
Normal values
Depending on the conditions in which the urine samples are taken, the results may vary. These collection modifications are intentionally made for specific purposes.
Aqueous withdrawal test
The patient stops consuming liquids for at least 16 hours, consuming only dry food at dinner. The results oscillate between 870 and 1310 mOsm/Kg with an average value of 1090 mOsm/kg.
Exogenous administration of desmopressin
Desmopressin fulfills a role similar to vasopressin or antidiuretic hormone; that is, it reabsorbs water from the urine to the plasma, reducing the amount of urine excreted and, therefore, increasing its concentration.
The normal values obtained in this test are between 700 and 1300 mOsm/Kg, depending on the age and clinical conditions of the patient.
liquid overload test
Although the ability to dilute urine is of little clinical interest, it may be useful in diagnosing certain central disorders in the management of urinary osmolarity, such as central diabetes insipidus or syndrome of inappropriate secretion of antidiuretic hormone.
20 ml/kg of water is administered over a short time and then urine is collected over 3 hours. Typically, urine osmolarity drops to values of around 40 or 80 mOsm/kg in the absence of associated pathologies.
All these highly variable results only have value when they are studied by a specialist doctor, evaluated in laboratories and in the patient’s clinic.