Plasma Osmolarity: What is it? Differences, Terms, Ranges and Measurements

In steady state, our total body water content and salt content remain constant.

An increase or decrease in water and salt consumption parallels an equivalent change in kidney water and salt excretion.

The homeostasis is achieved through the glomerular filtration process to produce a plasma ultrafiltrate. The tubules then process this ultrafiltrate so that the final urine flow rate and solute excretion meet the homeostatic needs of the body.

Osmolality and osmolarity are measurements of the solute concentration in a solution. In practice, there is a negligible difference between the absolute values ​​of the different measurements. For this reason, both terms are used interchangeably, although they refer to different units of measurement.

Plasma osmolality measures the body’s electrolyte-water balance. There are several methods to arrive at this amount through measurement or calculation.

Osmolality and osmolarity are measurements that are technically different, but functionally the same for normal use.

While osmolality (with an “ℓ”) is a measure of the osmoles (Osm) of solute per kilogram of solvent (osmol / kg or Osm / kg), osmolarity (with an “r”) is defined as the number osmoles of solute per liter (L) of solution (osmol / L or Osm / L).

Osmolality is an estimate of the plasma osmolar concentration and is proportional to the number of particles per kilogram of solvent, this is what is used when the values ​​are measured by a laboratory. As such, larger numbers indicate a higher concentration of solutes in the plasma.

The doubling of the sodium counts for the negative ions associated with sodium and the exclusion of potassium approximately allows for incomplete dissociation of the sodium chloride.

The term osmolarity has largely been replaced by osmolality, even when analyzing calculated values.

Measured osmolality (MO)

Osmolality can be measured on an analytical instrument called an osmometer. It works on the freezing point depression method.

Osmolalidad versus osmolaridad

Osmolarity is affected by changes in water content, as well as temperature and pressure. In contrast, osmolality is independent of temperature and pressure.

For a given solution, the osmolarity is slightly less than the osmolality, because the total weight of the solvent (the divisor used for osmolality) excludes the weight of any solutes, while the total volume of the solution (used for osmolarity) includes the solute content.

Otherwise, one liter of plasma would be equivalent to one kilogram of plasma, and plasma osmolality and plasma osmolality would be the same.

However, at low concentrations (below about 500 mM), the mass of the solute is negligible compared to the mass of the solvent, and the osmolarity and osmolality are very similar.

Technically, the terms can be compared as follows:


  • Clinical laboratories.
  • Header calculations.


  • Osmometer (freezing point depression osmometer, or vapor pressure depression osmometer).
  • Derived from laboratory data that was measured in solutions.

Appropriate term

  • Osmolalidad.

Therefore, the bedside calculations are actually in units of osmolarity, whereas the laboratory measurements will provide readings in units of osmolality.

In practice, there is an almost negligible difference between the absolute values ​​of the different measurements. For this reason, both terms are used interchangeably, although they refer to different units of measurement.

Ranges of plasma osmolarity


The normal reference range for human plasma osmolality is approximately 275-295 / 299 mosm / kg (mmol / kg) milli-osmoles per kilogram.

Serum or plasma osmolality is a measure of the different solutes in plasma. Among other applications, serum osmolality is indicated to assess the etiology of hyponatremia and can be used to detect alcohol intoxication by osmolal gap.

However, the reference range varies significantly and depends on the laboratory performing the test.


The plasma osmolarity of some reptiles, especially those in a freshwater aquatic environment, may be lower than that of mammals (eg, <260 mOsm / L) under favorable conditions.

Consequently, osmotically balanced solutions for mammals (such as 0.9% normal saline) are likely to be mildly hypertonic for such animals.

Many arid species of reptiles and hibernating uricotheic species allow significant elevations in plasma osmolarity (eg,> 400 mOsm / L) that could be fatal to some mammals.

Clinical relevance of osmolarity

As cell membranes in general are freely permeable to water, the osmolality of extracellular fluid (ECF) is approximately equal to that of intracellular fluid (ICF). Therefore, plasma osmolality is a guide for intracellular osmolality.

This is important as it shows that changes in extracellular fluid osmolality have a large effect on intracellular fluid osmolality, changes that can cause problems with normal cell volume and function.

If the extracellular fluid became too hypotonic, the water would easily fill the surrounding cells, increasing their volume and potentially flattening them ( cytolysis ).

Many poisons, drugs, and diseases affect the balance between intracellular fluid and extracellular fluid, affecting individual cells and homeostasis as a whole.

The osmolality of the blood increases with dehydration and decreases with overhydration. In normal people, the increased osmolality in the blood will stimulate the secretion of antidiuretic hormone (ADH).

This will result in more water reabsorption, more concentrated urine, and less concentrated blood plasma. Low serum osmolality will suppress antidiuretic hormone release, resulting in decreased water reabsorption and more concentrated plasma.

Increased osmolarity occurs frequently after illness due to chronic neurotoxic diseases such as Lyme disease. The elevation may be associated with mortality from stroke.

Calculated Osmolarity (CO)

In medical laboratory reports, this amount is often listed as “Osmo, Calc” or “Osmo (Calc).” According to the SI international unit, use the following equation:

Calculated osmolarity = 2 Na + Glucose + Urea (all in mmol / L).

To calculate plasma osmolality use the following equation (typical in the USA):

= 2 [Na +] + [Glucose] / 18 + [BUN] /2.8 where [Glucose] and [BUN] are measured in mg / dL.

If the patient has ingested ethanol, the ethanol level should be included in the calculated osmolality:

= 2 [Na +] + [Glucosa] / 18 + [BUN] /2.8 + [Etanol] /3.7

Based on the molecular weight of ethanol, the divisor should be 4.6, but empirical data shows that ethanol does not behave as an ideal osmole.

Space osmolar (OG)

The osmolar space is the difference between the measured osmolality and the calculated osmolarity. The difference in units is attributed to the difference in the way blood solutes are measured in the laboratory compared to the way they are calculated.

The laboratory value measures the freezing point depression, properly called osmolality, while the calculated value is given in units of osmolarity.

Although these values ​​are presented in different units, when there is a small amount of solute compared to the total volume of solution, the absolute values ​​of osmolality versus osmolarity are very close.

This often creates confusion as to which units you are referring to. For practical purposes, the units are considered interchangeable. The resulting “osmolar gap” can be considered osmolar or osmolal, since both units have been used in its derivation.

The measured osmolality is abbreviated “OM”, the calculated osmolarity is abbreviated “OC” and the osmolality gap is abbreviated “BO”.

Clinically, the osmolar space is used to detect the presence of an osmotically active particle that is not normally found in plasma, usually a toxic alcohol such as ethanol, methanol, or isopropyl alcohol.


Plasma Osmolality – Usually ordered to investigate hyponatremia. The osmotic gap can also be requested if the presence of osmotically active agents such as mannitol and glycine (a chemical used in surgical irrigation fluids) is suspected.

Urinary Osmolality – Often ordered in conjunction with plasma osmolality to help with diagnosis.

Stool osmolality : This can help evaluate chronic diarrhea that does not appear to be due to a bacterial or parasitic infection, meaning the stool may contain osmotically active substances (such as laxatives). The osmotic space of the stool can also be calculated.

Osmolality of urine

Urinary osmolality is a measure of urine concentration, with high values ​​indicating concentrated urine and small values ​​indicating dilute urine. The consumption of water (including the water contained in food) affects the osmolality of the urine.

In healthy humans with restricted fluid intake, urinary osmolality should be greater than 800 mOsm / kg, while a 24-hour urinary osmolality should average between 500 and 800 mOsm / kg.

Urinary osmolality in humans can range from about 50 to 1200 mOsm / kg, depending on whether the person has recently drunk a large amount of water (the lowest amount) or has been without water for a long time (the highest amount). .

Plasma osmolality with typical fluid intake often averages approximately 290 mOsm / kg H2O in humans.

In other animals

Some mammals are capable of higher osmolality than humans. This includes rats (approximately 3,000 mOsm / kg H2O), hamsters and mice (approximately 4,000 mOsm / kg H2O), and chinchillas (approximately 7,600 mOs / kg H2O).