What are the normal ranges for urine and serum osmolality?

Normal Urine and Serum Osmolality Ranges

The normal range for serum osmolality is 275-300 mOsm/kg, while normal urine osmolality ranges from 50-1200 mOsm/kg depending on hydration status. 1, 2

Serum Osmolality

• Normal serum osmolality ranges from 275-300 mOsm/kg 1, 2
• Serum osmolality >300 mOsm/kg indicates low-intake dehydration requiring intervention 3, 2
• Serum osmolality <275 mOsm/kg indicates hyposmolality, which may suggest overhydration or inappropriate ADH secretion 4
• Serum osmolality is the sum of concentrations of osmotically active components, primarily sodium, chloride, bicarbonate, potassium, glucose, and urea 1, 2

Measurement of Serum Osmolality

• Direct measurement of serum osmolality is the reference standard for assessing hydration status 3, 2
• When direct measurement is unavailable, calculated osmolarity can be used with the following equation:
  • Osmolarity = 1.86 × (Na+ + K+) + 1.15 × glucose + urea + 14 (all measured in mmol/L) 3, 2
  • An action threshold of >295 mmol/L should be used when using the calculated osmolality formula 3, 2
• The equation by Worthley et al. is recommended as the simplest and best formula to calculate plasma osmolality:
  • Posm = 2[Na+] + glucose (mg/dL)/18 + BUN (mg/dL)/2.8 5

Urine Osmolality

• Normal urine osmolality ranges from 50-1200 mOsm/kg, varying widely based on hydration status 6, 7
• Urine osmolality is a more accurate measure of urine concentration than urine specific gravity 6
• Urine specific gravity (USG) can be used to estimate urine osmolality when direct measurement is unavailable 6, 7
• The correlation between USG and urine osmolality is approximately 0.75 6
• For estimation of urine osmolality from specific gravity:
  • Multiply the last two digits of the urine density by 33.5 for clean samples 7
  • For samples with proteinuria and/or glycosuria, multiply by 32 with adjusted urine density 7

Clinical Significance

• Serum osmolality >300 mOsm/kg indicates dehydration, which is associated with increased risk of mortality and disability, particularly in older adults 3, 1
• Interpretation of raised serum osmolality depends on checking that serum glucose and urea are within normal ranges 3
• Simple clinical signs like skin turgor, mouth dryness, or urine color should NOT be used to assess hydration status, especially in older adults 3, 4
• Bioelectrical impedance should NOT be used to assess hydration status 3, 4

Pitfalls to Avoid

• Do not confuse low-intake dehydration (hyperosmolality) with volume depletion, as they require different fluid management approaches 2
• Pathological urines (containing protein, glucose, ketones, etc.) have significantly poorer correlation between USG and osmolality than "clean" urines 6
• In pathological urines, direct measurement of urine osmolality should be used rather than estimation from specific gravity 6
• Assessment of fluid intake is often highly inaccurate, particularly in older adults in residential care settings 2, 4