serum osmolality

Reference interval

• 275-295 mosm/kg, age <= 60 years (adults & children)
• 280-301 mosm/kg, age >= 61 years (elderly)

Clinical significance

• physiology
  • thirst is normally triggered by serum osmolality of > 290 mosm/kg
  • the organum vasculosum lamina terminalis is a chemoreceptor zone in the brain that senses osmolality
  • ADH is released from the hypothalamus when the plasma osmolality is > 280 mosm/kg
    • plasma ADH levels zero for osmolality < 280 mosm/kg
    • plasma ADH levels linearly related to osmolality for values > 280 mosm/kg
• pathology
  • an osmolal gap > 10 mOsm/L may be secondary to various toxins (see osmolal gap)
  • normal osmolal gap, however does not rule the presence of a toxin

Increases

• dehydration
• hypersomolar nonketotic diabetic coma
• diabetic ketoacidosis
• diabetes insipidus
• tube feeding

Decreases

• adrenal insufficiency
• hypopituitarism
• water intoxication (polydipsia)
• postoperative states
• SIADH

Methods

• see osmolality

Notes

• calculated serum osmolality: , (2 Serum Sodium [mEq/L]) + Plasma Glucose (mg/dL)/18 + , Blood Urea Nitrogen (mg/dL)/2.8 + Ethanol (mg/dL)/4.6 if present

More general terms

• osmolality (Osm)

Additional terms

• serum osmolal gap
• urine osmolality

Component of

• electrolyte/fluid management panel
• coma panel