urine osmolality

Reference interval

• 50-1200 mosm/kg
• with average fluid intake: 300-900 mosm/kg
• after 12 hours of fluid restriction: > 850 mosm/kg
• 24 hour urine: 300-900 mosm/kg

Clinical significance

• urine osmolality
• > 300 mOsm/kg in a patient with polyuria suggests osmotic diuresis
  • < 100 mOsm kg H2O suggests appropriately suppressed ADH
    • primary polydipsia
    • decreased solute intake (anorexia)
  • > 100 mOsm kg H2O suggests ADH excess (SIADH)
  • elderly may not be able to dilute urine < 150 mOsm/kg H2O, thus
    • 150 mOsm kg H2O suggests decreased solute intake in the elderly
    • > 150 mOsm kg H2O may suggest ADH excess (SIADH) in the elderly

Decreases

• diabetes insipidus
• polydipsia

Methods

• see osmolality

Specimen

• stable at 4 degrees C for 24 hours

More general terms

• osmolality (Osm)
• urine chemistry test

More specific terms

• urine osmolal gap

Additional terms

• serum osmolality

Component of

• electrolyte/fluid management panel