What is the normal range of urine osmolarity?

Normal Range of Urine Osmolarity

Normal urine osmolality typically ranges from 300-900 mOsm/kg, with the kidney's ability to concentrate urine up to 1200 mOsm/kg during dehydration. 1

Physiological Context

The kidney's concentrating ability reflects its response to hydration status and antidiuretic hormone (vasopressin) activity. Understanding normal ranges requires recognizing that urine osmolality varies considerably based on fluid intake and physiological demands 1:

• Normal range: 300-900 mOsm/kg represents typical daily variation in healthy individuals 1
• Maximum concentration: up to 1200 mOsm/kg occurs during states of dehydration when the kidney maximally concentrates urine 1
• Minimum concentration: can approach 50-100 mOsm/kg in states of water loading, though this is not part of the "normal" range but rather represents the kidney's diluting capacity

Clinical Interpretation Framework

Abnormally Low Urine Osmolality

• <200 mOsm/kg is inappropriately dilute and suggests diabetes insipidus (central or nephrogenic) when combined with elevated serum sodium or osmolality 2
• This represents a pathological inability to concentrate urine despite physiological triggers to do so 3

Target Range for Adequate Hydration

• ≤500 mOsm/kg is considered a desirable target from a long-term health perspective, ensuring sufficient urinary output to reduce renal health risks and circulating vasopressin 4
• This threshold distinguishes between minimal hydration for euhydration versus optimal hydration for long-term health 4

Abnormally High Urine Osmolality

• >500 mOsm/kg in SIADH (syndrome of inappropriate antidiuretic hormone) combined with low serum osmolality (<275 mOsm/kg) and hyponatremia confirms the diagnosis 2
• Values consistently above this range may indicate inadequate fluid intake or excessive fluid losses 1

Relationship to Serum Osmolality

Serum osmolality is the gold standard for assessing hydration status, not urine osmolality. 5, 1

• Normal serum osmolality: 275-295 mOsm/kg 1
• Dehydration threshold: >300 mOsm/kg indicates low-intake dehydration 5, 1
• Overhydration: <275 mOsm/kg suggests excess water or SIADH 1

The critical diagnostic principle is that urine osmolality must be interpreted in context with serum osmolality—the relationship between these values reveals whether the kidney is responding appropriately to the body's hydration status 3, 2.

Practical Measurement Considerations

When direct osmolality measurement is unavailable, surrogate markers can be used 1:

• Urine specific gravity ≥1.013 detects urine osmolality >500 mOsm/kg with very high accuracy 4
• Calculated osmolarity formula: 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L), with action threshold >295 mmol/L for serum 5, 1

Common Pitfalls to Avoid

• Do not rely on urine color, specific gravity alone, or clinical signs (skin turgor, mouth dryness) to assess hydration status, especially in older adults—these are unreliable 5, 2
• Always check serum glucose and urea when interpreting elevated osmolality, as these can independently elevate values and confound interpretation 5, 3
• In older adults, renal function is often impaired, making urine concentration parameters less reliable indicators of hydration status 5, 1