Osmolal gap
Encyclopedia
Osmol gap in medical science is the difference between measured serum osmolality and calculated serum osmolarity.
(BUN), plus alcohol
in a person who has been drinking. As part of a laboratory blood test, a vial of blood is tested for the amount of these four ions and molecules that are present in the blood. From this measurement, the clinician can calculate the plasma osmolarity
of a patients blood. A second vial is also sent to the laboratory. This vial is put in an instrument that measures the freezing point depression of all the solutes in the plasma. This measurement gives the true plasma osmolality. The calculated osmolarity is then subtracted from the measured osmolality to provide the osmol gap, or the difference between these two values. If this gap falls within an acceptable range, then it is assumed that sodium, glucose, BUN and alcohol (if any) are indeed the major dissolved ions and molecules in the serum. If, however, the calculated gap is above an acceptable range, then it is an indication that there is something else dissolved in the serum that is producing an osmol gap, which can be a major clue in determining what is ailing the patient.
Calculated osmolality = 2 x [Na mmol/L] + [glucose mmol/L] + [urea mmol/L]
In non-SI laboratory units:
Calculated osmolality = 2 x [Na mmol/L] + [glucose mg/dL] / 18 + [urea mg/dL] / 2.8
(NB: divisor 18 respectively 2.8 to convert mg/dL into mmol/L)
A normal osmol gap is < 10 mOsm/kg .
Causes of an elevated osmol gap are numerous. Noted causes are
note that all these substances are of low molecular weight.
Theory
There are a variety of ions and molecules dissolved in the serum. The major constitutionals of clinical importance are sodium ions, glucose, and blood urea nitrogenBlood urea nitrogen
The blood urea nitrogen test is a measure of the amount of nitrogen in the blood in the form of urea, and a measurement of renal function. Urea is a by- product from metabolism of proteins by the liver and is removed from the blood by the kidneys.-Physiology:The liver produces urea in the urea...
(BUN), plus alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
in a person who has been drinking. As part of a laboratory blood test, a vial of blood is tested for the amount of these four ions and molecules that are present in the blood. From this measurement, the clinician can calculate the plasma osmolarity
Plasma osmolality
-Measured osmolality :Osmolality can be measured on an analytical instrument called an osmometer. It works on the method of depression of freezing point.Plasma osmolality is affected by changes in water content...
of a patients blood. A second vial is also sent to the laboratory. This vial is put in an instrument that measures the freezing point depression of all the solutes in the plasma. This measurement gives the true plasma osmolality. The calculated osmolarity is then subtracted from the measured osmolality to provide the osmol gap, or the difference between these two values. If this gap falls within an acceptable range, then it is assumed that sodium, glucose, BUN and alcohol (if any) are indeed the major dissolved ions and molecules in the serum. If, however, the calculated gap is above an acceptable range, then it is an indication that there is something else dissolved in the serum that is producing an osmol gap, which can be a major clue in determining what is ailing the patient.
Explanation of units
Since laboratories measure serum solutes in terms of freezing point depression, the reported units are properly units of osmolality. When a measure of serum solutes is calculated, it is often done in units of osmolarity. While it is possible to convert between osmolality and osmolarity, thereby deriving a more mathematically correct osmol gap calculation, in actual clinical practice this is not done. This is because the difference in absolute value of these two measurements that can be attributed to the difference in units will be negligible in a clinical setting. For this reason, the terms are often used interchangeably, though some object to equating the terms. Because the calculated osmol gap can therefore be a conflation of both terms (depending on how it is derived), neither term (osmolal gap nor osmolar gap) may be semantically correct. To avoid ambiguity, the terms "osmolal" and "osmolar" can be used when the units of molality or molarity are consistent throughout the calculation. When this is not the case, the term "osmol gap" can be used when units are mixed to provide a clinical estimate.Calculation
The osmol gap is typically calculated as:- OG = measured serum osmolality − calculated osmolality
Calculated osmolality = 2 x [Na mmol/L] + [glucose mmol/L] + [urea mmol/L]
In non-SI laboratory units:
Calculated osmolality = 2 x [Na mmol/L] + [glucose mg/dL] / 18 + [urea mg/dL] / 2.8
(NB: divisor 18 respectively 2.8 to convert mg/dL into mmol/L)
A normal osmol gap is < 10 mOsm/kg .
Causes
Osmol gaps are used as a screening tool to identify toxins.Causes of an elevated osmol gap are numerous. Noted causes are
- ethanolEthanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
intoxication - methanolMethanolMethanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
ingestion - ethylene glycolEthylene glycolEthylene glycol is an organic compound widely used as an automotive antifreeze and a precursor to polymers. In its pure form, it is an odorless, colorless, syrupy, sweet-tasting liquid...
ingestion - acetoneAcetoneAcetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...
ingestion - isopropyl alcoholIsopropyl alcoholIsopropyl alcohol is a common name for a chemical compound with the molecular formula C3H8O. It is a colorless, flammable chemical compound with a strong odor...
ingestion
External links
- Osmolar gap, fpnotebook.com
- Serum Osmolality, fpnotebook.com
- Acid-Base Physiology
- calculator at University of IowaUniversity of IowaThe University of Iowa is a public state-supported research university located in Iowa City, Iowa, United States. It is the oldest public university in the state. The university is organized into eleven colleges granting undergraduate, graduate, and professional degrees...