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Serum osmolal gap: Wikis

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Pathophysiology sample values
BMP/ELECTROLYTES:
Na+=140 Cl=100 BUN=20 /
Glu=150
K+=4 CO2=22 PCr=1.0 \
ARTERIAL BLOOD GAS:
HCO3-=24 paCO2=40 paO2=95 pH=7.40
ALVEOLAR GAS:
pACO2=36 pAO2=105 A-a g=10
OTHER:
Ca=9.5 PO4=1 Mg2+=2.0
CK=55 BE=−0.36 AG=16
SERUM OSMOLARITY/RENAL:
PMO = 300 PCO=295 POG=5 BUN:Cr=20
URINALYSIS:
UNa+=80 UCl=100 UAG=5 FENa=0.95
UK+=25 USG=1.01 UCr=60 UO=800
PROTEIN/GI/LIVER FUNCTION TESTS:
LDH=100 TP=7.6 AST=25 TBIL=0.7
ALP=71 Alb=4.0 ALT=40 BC=0.5
AST/ALT=0.6 BU=0.2
AF alb=3.0 SAAG=1.0 SOG=60
CSF:
CSF alb=30 CSF glu=60 CSF/S alb=7.5 CSF/S glu=0.4

Osmol gap (or "osmolal gap", or "osmolality gap", [1] or "osmole gap",[2]) is the difference between measured serum osmolality and calculated serum osmolarity.

Contents

Theory

There are a variety of ions and molecules dissolved in the serum. The major constitutionals of clinical importance are sodium ions, glucose, blood urea nitrogen (BUN) and alcohol if one 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 a machine 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[3], 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.[4] 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.[4]

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 more common 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 .[5]

Causes

Osmol gaps are used as a screening tool to identify toxins.[6]

Causes of an elevated osmol gap are numerous. Noted causes are

note that all these substances are of low molecular weight.

See also

References

External links

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