Serum Osmolality/Osmolarity

id

91

full_title

Serum Osmolality/Osmolarity

short_title

Serum Osmolality

med_description

Calculates expected serum osmolarity, for comparison to measured osmolality to detect unmeasured compounds in the serum.

description

The Serum Osmolality/Osmolarity calculates expected serum osmolarity, for comparison to measured osmolarity to detect unmeasured compounds in the serum.

keywords

Serum Osmolality/Osmolarity, serum os, serum osmolar, serum osmolar gap, serum osmol, serum mdcalc, serum osmlar calc, serum osmolarity, serum osmolality, serum osmolal, serum osmol gap, serum osmolality calc, serum osmolality calc, bun osmolar, bun osmolal, BUN osmolar, BUN osmolal, na osmolarity, Na osmolarity, Glucose osmolarity, glucose omolar, etoh osmolarity, etoh osmolar, sodium osmolar, sodium osmolarity, ethanol osmolar, ethanol osmolarity, osmol gap, osmol serum, osmol blood, osmol/osmol, osmolarity mw, osmolarity molecular weight, osmolarity mol, osmolarity calc, osmolality calc, osmolality weight, osmolarity mol, osmolarity mw, serum compounds, serum compounds calc, serum compounds, osmolarity, osmolality, osmolarity/osmolality

complaint

[ "AMS", "Intoxication/Overdose", "Vomiting", "Weakness" ]

formula

Traditional equation:

Serum osmolarity, US units = (2 × Na + (BUN / 2.8) + (glucose / 18) + (ethanol / 4.6)

Purssell et al equation:

Serum osmolarity, US units = (2 × Na + (BUN / 2.8) + (glucose / 18) + (ethanol / 3.7)

Note: the range displayed in the calculator uses the equations above.

Osmolar gap equation:

Osm gap = measured serum osm - calculated serum osm

Note: the osmolar gap displayed in the calculator uses the Pursell et al. equation.

evidence

measurements

[ { "name": "Osmoles (Osm)", "unit": "osm", "error_min": "200", "error_max": "700", "warn_min": "270", "warn_max": "350", "conversion": "1", "normal_max_si": "295", "normal_max_us": "295", "normal_min_si": "275", "normal_min_us": "275", "units_si": "mmol/kg", "units_us": "mOsm/kg" }, { "name": "Glucose", "unit": "glu", "error_min": "1", "error_max": "1700", "warn_min": "50", "warn_max": "1000", "conversion": "0.0555", "normal_max_si": "5.8", "normal_max_us": "105", "normal_min_si": "3.9", "normal_min_us": "70", "units_si": "mmol/L", "units_us": "mg/dL" }, { "name": "Sodium", "unit": "na", "error_min": "100", "error_max": "200", "warn_min": "0", "warn_max": "170", "conversion": "1", "normal_max_si": "145", "normal_max_us": "145", "normal_min_si": "136", "normal_min_us": "136", "units_si": "mmol/L", "units_us": "mEq/L" }, { "name": "BUN", "unit": "bun", "error_min": "1", "error_max": "500", "warn_min": "2", "warn_max": "300", "conversion": "0.357", "normal_max_si": "7.1", "normal_max_us": "20", "normal_min_si": "2.9", "normal_min_us": "8", "units_si": "mmol/L", "units_us": "mg/dL" }, { "name": "Ethanol (ETOH)", "unit": "etoh", "error_min": "-0.01", "error_max": "700", "warn_min": "0", "warn_max": "400", "conversion": "0.217", "normal_max_si": "17.36", "normal_max_us": "80", "normal_min_si": "0", "normal_min_us": "0", "units_si": "mmol/L", "units_us": "mg/dL" } ]

information

If values are in SI units, these are already molar (mmol/L for example), so these do not need to be divided by their molecular weight.

refrences

{ "Clinical Practice Guidelines": [], "Manufacturer Website": [], "Original/Primary Reference": [ { "href": "https://www.sciencedirect.com/science/article/pii/S0196064401166667", "text": "Purssell RA, Pudek M, Brubacher J, Abu-Laban RB.Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap.Ann Emerg Med 2001;38 (6): 653–9.doi:10.1067/mem.2001.119455. PMID 11719745." } ], "Other References": [ { "href": "https://www.ncbi.nlm.nih.gov/pubmed/4751712", "text": "Glasser L, et. al. Serum Osmolality and its applicability to drug overdose. Am. J. Clin. Path. 1973; 60:695." } ], "Outcomes": [], "Validation": [ { "href": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4135495/", "text": "Siervo M, Bunn D, Prado CM, Hooper L. Accuracy of prediction equations for serum osmolarity in frail older people with and without diabetes. The American Journal of Clinical Nutrition 2014;100(3):867-876. doi:10.3945/ajcn.114.086769." } ], "Validations": [] }

pearls

At extremes of serum glucose, such as in hyperosmolar hyperglycemic state (HHS), osmolar gap is skewed, and clinical judgment should be used to make decisions regarding correction.

Notes on osmolality versus osmolarity:

• Osmolality and osmolarity are often used interchangeably in medical practice.

• Both relate to solution concentration, but:

  • Osmolality is defined as the number of solute particles per kilogram of solvent (in this case, body fluid).

  • Osmolarity is defined as the number of solute particles per liter of solution.

  • Osmolarity is affected by temperature and pressure (due to volume changes), while osmolality is not (mass remains constant).

  • Serum osmolality is what is measured by an osmometer.

• In dilute solutions like blood plasma, 1 kg of water is approximately equal to 1 L.

• Therefore, the numerical values of osmolality and osmolarity are nearly identical, making the terms functionally interchangeable in clinical contexts.

Notes on osmolar versus osmolal gap:

• Osmolar gap is the difference between the measured osmolality and the calculated osmolarity.
  
• Osmolal gap is the difference between measured and calculated osmolality.
  
• Since solute concentrations (e.g., sodium, glucose, BUN) are typically measured in mEq or mg (or mmol) per dL or L (i.e., volume), we typically calculate the expected osmolarity.
• Thus, in clinical practice, osmolar gap is the standard term used.

usecase

An osmolar/osmolal gap (difference between measured and expected) >10 can be caused by toxic alcohols (e.g., ethanol, methanol, ethylene glycol, isopropyl alcohol*), mannitol, and lorazepam infusions (which contain propylene glycol).

*Isopropyl alcohol causes a large osmolar gap but does not cause an anion gap metabolic acidosis.

reasons

next_advice

Methanol and ethylene glycol ingestions are treated by blocking alcohol dehydrogenase with fomepizol or ethanol, and often sodium bicarbonate if severely acidotic.

next_actions

next_management

diseases

[ "Acid-Base Disturbance", "Ingestion/Overdose" ]

instructions

published

2022-04-21T20:29:47.947Z

purpose

[ "Formula" ]

search_terms

[ "toxicology", "gap", "anion gap", "ag", "osmolar gap" ]

seo

{ "keywords_en": "Serum Osmolality/Osmolarity, serum os, serum osmolar, serum osmolar gap, serum osmol, serum mdcalc, serum osmlar calc, serum osmolarity, serum osmolality, serum osmolal, serum osmol gap, serum osmolality calc, serum osmolality calc, bun osmolar, bun osmolal, BUN osmolar, BUN osmolal, na osmolarity, Na osmolarity, Glucose osmolarity, glucose omolar, etoh osmolarity, etoh osmolar, sodium osmolar, sodium osmolarity, ethanol osmolar, ethanol osmolarity, osmol gap, osmol serum, osmol blood, osmol/osmol, osmolarity mw, osmolarity molecular weight, osmolarity mol, osmolarity calc, osmolality calc, osmolality weight, osmolarity mol, osmolarity mw, serum compounds, serum compounds calc, serum compounds, osmolarity, osmolality, osmolarity/osmolality", "meta_description_en": "The Serum Osmolality/Osmolarity calculates expected serum osmolarity, for comparison to measured osmolarity to detect unmeasured compounds in the serum." }

specialty

[ "Critical Care", "Emergency Medicine", "Endocrinology", "Hospitalist Medicine", "Internal Medicine", "Nephrology", "Toxicology" ]

departments

[ "Endocrine and Metabolic", "Renal" ]

tags

[]

version_number

1

versions

[]

related

[ { "calcId": 2045, "short_title_en": "Urine Anion Gap", "slug": "urine-anion-gap" }, { "calcId": 117, "short_title_en": "Winters' Formula", "slug": "winters-formula-metabolic-acidosis-compensation" }, { "calcId": 2157, "short_title_en": "GAP Index for IPF", "slug": "gap-index-idiopathic-pulmonary-fibrosis-ipf-mortality" } ]

ismed

false

section

[ "whenToUseViewed", "pearlsPitfallsViewed", "nextStepsViewed", "evidenceViewed" ]

cleaned_departments

[ "endocrine", "urology" ]

cleaned_use

[ "An osmolar/osmolal gap (difference between measured and expected) >10 can be caused by toxic alcohols (e.g., ethanol, methanol, ethylene glycol, isopropyl alcohol*), mannitol, and lorazepam infusions (which contain propylene glycol)." ]

pub

true