II. Definitions

  1. Osmolality
    1. Total solute concentration in a fluid compartment
  2. Tonicity
    1. Concentration of osmoles in a compartment that generate osmotic pressure gradient
    2. Tonicity impacts water traversal from one compartment to another
    3. Only solutes that cannot move freely between compartments generate an osmotic gradient
      1. Blood Urea Nitrogen (BUN) does not affect tonicity, but does influence osmolality
      2. BUN crosses easily between compartments and does not generate an osmotic gradient
    4. Extracellular Fluid Tonicity is based on only a few effective osmoles (of which Sodium is key)
      1. Sodium concentration (primary)
      2. Glucose (severe Hyperglycemia in Diabetes Mellitus)
      3. Mannitol or Sorbitol (used to decrease Intracranial Pressure in closed Head Trauma)

III. Interpretation: Normal

  1. Serum Osmolality: 280-300 mOsm/(kg water)

IV. Calculation: Calculated Serum Osmolality

  1. Serum Osms = 2*Na + Glu/18 + BUN/2.8 + Et/4.6 + Man/18
  2. Annotation
    1. Where Na is Serum Sodium
    2. Where Glu is Serum Glucose
    3. Where BUN is Blood Urea Nitrogen
    4. Where Et is Blood Alcohol Level
    5. Where Man is Mannitol

VI. Causes: Decreased Serum Osmolality

  1. SIADH
  2. Hyponatremia
  3. Overhydration

Calculation (FPnotebook.com: DrBits) Open in New Window

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Related Studies

Ontology: Serum osmolality measurement (C0202151)

Concepts Laboratory Procedure (T059)
SnomedCT 166719000, 144008001, 44628004
English Serum osmolality, serum osmolality, serum osmolality (lab test), Serum osmolality measurement, serum osmolality test, Serum osmolality (& level), Serum osmolality (& level) (procedure), Serum Osmolality Test, Osmolality measurement, serum, Osmolality measurement, serum (procedure)
Dutch serumosmolariteit
French Osmolalité sérique
German Osmolalitaet des Serums
Italian Osmolalità sierica
Portuguese Osmolalidade sérica
Spanish Osmolalidad del suero, medición de osmolalidad en suero (procedimiento), medición de osmolalidad en suero
Japanese 血清浸透圧, ケッセイシントウアツ
Czech Osmolalita séra
Hungarian Szérum ozmolaritás

Ontology: Blood osmolarity (C0596196)

Definition (CSP) measures moles per liter of blood; increases with dehydration and decreases with overhydration; normal osmolarity is contributed to mostly by sodium, chloride, potassium, urea and glucose, as well as other ions and substances in the blood.
Concepts Laboratory Procedure (T059)
Italian Osmolarità ematica
Japanese 血液浸透圧, ケツエキシントウアツ
Czech Osmolarita krve
English Blood osmolarity, blood osmolarity, osmolarity blood
Hungarian Vér ozmolaritás
French Osmolarité sanguine
Portuguese Osmolaridade do sangue
Spanish Osmolaridad de la sangre
Dutch bloedosmolariteit
German Osmolaritaet des Blutes

Ontology: tonicity disorder (C0679318)

Concepts Pathologic Function (T046)
English tonicity disorder