Osmol gap

Last updated
Serum osmotic gap
Synonyms osmolal gap, osmolality gap, osmolar gap [1] osmole gap [2]
LOINC 33264-3

In clinical chemistry, the osmol gap is the difference between measured blood serum osmolality and calculated serum osmolality.

Contents

Calculation

The osmol gap is typically calculated with the following formula (all values in mmol/L):

In non-SI laboratory units: Calculated osmolality = 2 x [Na mmol/L] + [glucose mg/dL] / 18 + [BUN mg/dL] / 2.8 + [ethanol/3.7] [3] (note: the values 18 and 2.8 convert mg/dL into mmol/L; the molecular weight of ethanol is 46, but empiric data shows that it does not act as an ideal osmole in solution and the appropriate divisor is 3.7 [4] )

A normal osmol gap is < 10 mOsm/kg . [5]

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, [6] 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. [7] 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. [7]

Causes

Osmol gaps are used as a screening tool to identify toxins. [8]

Causes of an elevated osmol gap are numerous. Generally there are 4 main causes:

All four are osmotically active substances found in humans. Accordingly, intoxications as listed below are reasons for an increased osmolar gap.

Alcohols

Sugars

Lipids

Proteins

Theory

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 p a CO2 = 40 p a O2 = 95 pH = 7.40
ALVEOLAR GAS:
p A CO2 = 36 p A O2 = 105 A-a g = 10
OTHER:
Ca = 9.5 Mg2+ = 2.0 PO4 = 1
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 = 0.6

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 nitrogen (BUN), plus ethyl 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 osmolality of a patient's 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 osmolality 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,(<10) then it is assumed that sodium, glucose, BUN 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.

See also

Related Research Articles

<span class="mw-page-title-main">Dehydration</span> Deficit of total body water

In physiology, dehydration is a lack of total body water that disrupts metabolic processes. It occurs when free water loss exceeds free water intake. This is usually due to excessive sweating, disease, or a lack of access to water. Mild dehydration can also be caused by immersion diuresis, which may increase risk of decompression sickness in divers.

Hyponatremia or hyponatraemia is a low concentration of sodium in the blood. It is generally defined as a sodium concentration of less than 135 mmol/L (135 mEq/L), with severe hyponatremia being below 120 mEq/L. Symptoms can be absent, mild or severe. Mild symptoms include a decreased ability to think, headaches, nausea, and poor balance. Severe symptoms include confusion, seizures, and coma; death can ensue.

Reference ranges for blood tests are sets of values used by a health professional to interpret a set of medical test results from blood samples. Reference ranges for blood tests are studied within the field of clinical chemistry, the area of pathology that is generally concerned with analysis of bodily fluids.

<span class="mw-page-title-main">Blood sugar level</span> Concentration of glucose present in the blood (Glycaemia)

The blood sugar level, blood sugar concentration, blood glucose level, or glycemia is the measure of glucose concentrated in the blood. The body tightly regulates blood glucose levels as a part of metabolic homeostasis.

<span class="mw-page-title-main">Uremia</span> Excess urea in the blood due to kidney dysfunction

Uremia is the condition of having high levels of urea in the blood. Urea is one of the primary components of urine. It can be defined as an excess in the blood of amino acid and protein metabolism end products, such as urea and creatinine, which would normally be excreted in the urine. Uremic syndrome can be defined as the terminal clinical manifestation of kidney failure. It is the signs, symptoms and results from laboratory tests which result from inadequate excretory, regulatory, and endocrine function of the kidneys. Both uremia and uremic syndrome have been used interchangeably to denote a very high plasma urea concentration that is the result of renal failure. The former denotation will be used for the rest of the article.

<span class="mw-page-title-main">Renal physiology</span> Study of the physiology of the kidney

Renal physiology is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodium, potassium, and other electrolytes; clearance of toxins; absorption of glucose, amino acids, and other small molecules; regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D.

The syndrome of inappropriate antidiuretic hormone secretion (SIADH), also known as the syndrome of inappropriate antidiuresis (SIAD), is characterized by a physiologically inappropriate release of antidiuretic hormone (ADH) either from the posterior pituitary gland, or an abnormal non-pituitary source. Unsuppressed ADH causes a physiologically inappropriate increase in solute-free water being reabsorbed by the tubules of the kidney to the venous circulation leading to hypotonic hyponatremia.

<span class="mw-page-title-main">Metabolic acidosis</span> Imbalance in the bodys acid-base equilibrium

Metabolic acidosis is a serious electrolyte disorder characterized by an imbalance in the body's acid-base balance. Metabolic acidosis has three main root causes: increased acid production, loss of bicarbonate, and a reduced ability of the kidneys to excrete excess acids. Metabolic acidosis can lead to acidemia, which is defined as arterial blood pH that is lower than 7.35. Acidemia and acidosis are not mutually exclusive – pH and hydrogen ion concentrations also depend on the coexistence of other acid-base disorders; therefore, pH levels in people with metabolic acidosis can range from low to high.

<span class="mw-page-title-main">Saline (medicine)</span> Saline water for medical purposes

Saline is a mixture of sodium chloride (salt) and water. It has a number of uses in medicine including cleaning wounds, removal and storage of contact lenses, and help with dry eyes. By injection into a vein, it is used to treat hypovolemia such as that from gastroenteritis and diabetic ketoacidosis. Large amounts may result in fluid overload, swelling, acidosis, and high blood sodium. In those with long-standing low blood sodium, excessive use may result in osmotic demyelination syndrome.

In medicine, Kt/V is a number used to quantify hemodialysis and peritoneal dialysis treatment adequacy.

The anion gap is a value calculated from the results of multiple individual medical lab tests. It may be reported with the results of an electrolyte panel, which is often performed as part of a comprehensive metabolic panel.

<span class="mw-page-title-main">Osmotic concentration</span> Molarity of osmotically active particles

Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution. The osmolarity of a solution is usually expressed as Osm/L, in the same way that the molarity of a solution is expressed as "M". Whereas molarity measures the number of moles of solute per unit volume of solution, osmolarity measures the number of osmoles of solute particles per unit volume of solution. This value allows the measurement of the osmotic pressure of a solution and the determination of how the solvent will diffuse across a semipermeable membrane (osmosis) separating two solutions of different osmotic concentration.

Plasma osmolality measures the body's electrolyte–water balance. There are several methods for arriving at this quantity through measurement or calculation.

Hyperosmolar hyperglycemic state (HHS), also known as hyperosmolar non-ketotic state (HONK), is a complication of diabetes mellitus in which high blood sugar results in high osmolarity without significant ketoacidosis. Symptoms include signs of dehydration, weakness, leg cramps, vision problems, and an altered level of consciousness. Onset is typically over days to weeks. Complications may include seizures, disseminated intravascular coagulopathy, mesenteric artery occlusion, or rhabdomyolysis.

In medicine, the urea-to-creatinine ratio (UCR), known in the United States as BUN-to-creatinine ratio, is the ratio of the blood levels of urea (BUN) (mmol/L) and creatinine (Cr) (μmol/L). BUN only reflects the nitrogen content of urea and urea measurement reflects the whole of the molecule, urea is just over twice BUN. In the United States, both quantities are given in mg/dL The ratio may be used to determine the cause of acute kidney injury or dehydration.

Ethylene glycol poisoning is poisoning caused by drinking ethylene glycol. Early symptoms include intoxication, vomiting and abdominal pain. Later symptoms may include a decreased level of consciousness, headache, and seizures. Long term outcomes may include kidney failure and brain damage. Toxicity and death may occur after drinking even in a small amount as ethylene glycol is more toxic than other diols.

<span class="mw-page-title-main">High anion gap metabolic acidosis</span> Medical condition

High anion gap metabolic acidosis is a form of metabolic acidosis characterized by a high anion gap. Metabolic acidosis occurs when the body produces too much acid, or when the kidneys are not removing enough acid from the body. Several types of metabolic acidosis occur, grouped by their influence on the anion gap.

In clinical chemistry, the urine anion gap is calculated using measured ions found in the urine. It is used to aid in the differential diagnosis of metabolic acidosis.

Stool osmotic gap is a measurement of the difference in solute types between serum and feces, used to distinguish among different causes of diarrhea.

<span class="mw-page-title-main">Salicylate poisoning</span> Medical condition

Salicylate poisoning, also known as aspirin poisoning, is the acute or chronic poisoning with a salicylate such as aspirin. The classic symptoms are ringing in the ears, nausea, abdominal pain, and a fast breathing rate. Early on, these may be subtle, while larger doses may result in fever. Complications can include swelling of the brain or lungs, seizures, low blood sugar, or cardiac arrest.

References

  1. ↑ "Osmolality Gap - Calculation and Interpretation". Archived from the original on 2009-08-04. Retrieved 2009-03-04.
  2. ↑ Lynd LD, Richardson KJ, Purssell RA, et al. (2008). "An evaluation of the osmole gap as a screening test for toxic alcohol poisoning". BMC Emerg Med. 8: 5. doi: 10.1186/1471-227X-8-5 . PMC   2390580 . PMID   18442409.
  3. ↑ Plasma osmolality
  4. ↑ Purssell RA, Pudek M, Brubacher J, Abu-Laban RB (December 2001). "Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap". Ann Emerg Med. 38 (6): 653–9. doi:10.1067/mem.2001.119455. PMID   11719745.
  5. ↑ Kapur G, Valentini RP, Imam AA, Jain A, Mattoo TK (June 2007). "Serum osmolal gap in patients with idiopathic nephrotic syndrome and severe edema". Pediatrics. 119 (6): e1404–7. doi:10.1542/peds.2006-2554. PMID   17485452.
  6. ↑ Converting between osmolality and osmolarity.
  7. 1 2 Erstad BL (September 2003). "Osmolality and osmolarity: narrowing the terminology gap". Pharmacotherapy. 23 (9): 1085–6. doi: 10.1592/phco.23.10.1085.32751 . PMID   14524639.
  8. ↑ Krahn J, Khajuria A (April 2006). "Osmolality gaps: diagnostic accuracy and long-term variability". Clin. Chem. 52 (4): 737–9. doi: 10.1373/clinchem.2005.057695 . PMID   16455871.
  9. ↑ Ammar KA, Heckerling PS (January 1996). "Ethylene glycol poisoning with a normal anion gap caused by concurrent ethanol ingestion: importance of the osmolal gap". Am. J. Kidney Dis. 27 (1): 130–3. doi:10.1016/S0272-6386(96)90040-2. PMID   8546127.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.