- What does it consist of?
- Urinary concentration and dilution
- What is it for?
- Consequences of increased urinary osmolarity
- Consequences of decreased urinary osmolarity
- How is it calculated?
- First formula
- Second formula
- Osmolar clearance
- Normal values
- Aqueous deprivation test
- Exogenous administration of desmopressin
- Liquid overload test
- References
The urinary osmolality is the concentration of active osmotic solutes in urine. This being 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"; in other words, 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 kilogram and not per liter, as occurs in osmolarity.
However, in urine, as it is basically water, the calculation is very similar unless there are pathological conditions that change them dramatically.
What does it consist of?
The process by which urine is concentrated or diluted is very complex, requiring two independent renal systems to be properly integrated: the creation of a solute gradient and the activity of antidiuretic hormone.
Urinary concentration and dilution
Creation of the solute osmolar gradient occurs in the loop of Henle and in the renal medulla. There, the osmolarity of 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 chlorine in the thick portion of the ascending loop of Henle.
Subsequently, the urine passes through the cortical and medullary collecting tubules, where water and urea are reabsorbed, thus helping to create the osmotic gradients.
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 extent, urea.
As its name implies, antidiuretic hormone prevents or reduces 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 that finally dilutes the plasma but concentrates the urine.
What is it 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 measures not only the 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, water and electrolyte disorders and metabolic compromise.
Consequences of increased urinary osmolarity
- Dehydration.
- High protein intake.
- Syndrome of inappropriate antidiuretic hormone secretion.
- Mellitus diabetes.
- Chronic liver disease.
- Adrenal insufficiency.
- Heart failure.
- Septic and hypovolemic shock.
Consequences of decreased urinary osmolarity
- Acute kidney infections.
- Diabetes insipidus.
- Acute or chronic kidney failure.
- Hyperhydration.
- Treatment with diuretics.
How is it calculated?
First formula
The simplest method to calculate urinary osmolarity is knowing the urinary density and applying the following formula:
Urinary osmolarity (mOsm / kg or L) = urinary 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 proteins 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 already mentioned urea.
Urinary osmolarity (mOsm / K or L) = (Na u + K u) x 2 + (Urea u / 5.6)
In this 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 cacophonic 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:
C osm: osmolar clearance.
(Osm) urine: urinary osmolarity.
V min: minute volume of urine.
(Osm) blood: plasma osmolarity.
From this formula it can be deduced that:
- In the event that urine and plasma have the same osmolarity, these 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 plasma osmolarity, we speak of hypertonic or concentrated urine. This implies that osmolar clearance is greater than urine flow.
- If urinary osmolarity is less than plasma, urine is hypotonic or dilute and it is concluded that osmolar clearance is less than urinary flow.
Normal values
Depending on the conditions in which the urine samples are collected, the results may vary. These pickup modifications are made intentionally for specific purposes.
Aqueous deprivation test
The patient stops consuming liquids for at least 16 hours, consuming only dry food at dinner. The results range 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 into 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 the urine is not of much 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 antidiuretic hormone secretion.
20 ml / kg of water is administered in a short time and then urine is collected for 3 hours. Typically, the osmolarity of urine drops to values around 40 or 80 mOsm / kg in the absence of associated pathologies.
All these variable results are only valuable when they are studied by a specialist doctor, evaluated in laboratories and in the patient's clinic.
References
- Wilczynski, Cory (2014). Urine Osmolality. Drugs & Diseases. Laboratory Medicine, retrieved from: emedicine.medscape.com
- Rodríguez - Soriano, Juan and Vallo - Boado, Alfredo (2003). Renal function and its study. Pediatric Nephrology, Second Edition, Elsevier Science, Chapter 3, 27-65.
- Koeppen, Bruce and Stanton, Bruce (2013). Regulation of Body Fluid Osmolality: Regulation of Water Balance. Renal Physiology, Fifth Edition, Chapter 5, 73-92.
- Godoy, Daniel et al. (2013). Practical approach to the diagnosis and treatment of polyuric states in patients with acute brain injury. Chilean Medical Journal, 141: 616-625.
- Wikipedia (last edition 2018). Urine osmolality. Recovered from: en.wikipedia.org
- Holm, Gretchen and Wu, Brian (2016). Urine Osmolality test. Recovered from: healthline.com