MetabolicNephrology

Urine Osmolality

Normal Range
500–850 mOsm/kg (Random), 300–900 mOsm/kg (24h)
Sample Type
Urine (Random or 24h)
Fasting
No
Unit
mOsm/kg

Function

Urine Osmolality is a precise measurement of the concentration of dissolved particles in the urine, such as sodium, potassium, chloride, urea, and glucose. Unlike specific gravity, which measures the weight of particles, osmolality measures the number of particles. It is the most accurate way to assess the kidney's ability to concentrate or dilute urine in response to the body's hydration status and the influence of Antidiuretic Hormone (ADH).

Why it is Ordered

This test is a cornerstone in evaluating fluid and electrolyte imbalances. It is ordered to:

  • Evaluate Hyponatremia: Determining if low blood sodium is due to water excess or salt loss.
  • Diagnose Diabetes Insipidus: Differentiating between central (lack of ADH) and nephrogenic (kidney resistance to ADH) causes of excessive urination.
  • Assess Kidney Function: Checking for tubular damage or acute kidney injury.
  • Investigate SIADH: Syndrome of Inappropriate Antidiuretic Hormone secretion, where the body retains too much water.

Associated Conditions

  • SIADH: Characterized by inappropriately concentrated urine despite low serum osmolality.
  • Diabetes Insipidus: Characterized by very dilute urine despite high serum osmolality.
  • Dehydration: Leads to high urine osmolality as the kidneys conserve water.
  • Acute Tubular Necrosis: Often results in a 'fixed' osmolality close to that of plasma (~300 mOsm/kg).

Check Your Result

Check Your Result

Interactive
mOsm/kg

Enter your lab result to see where you stand compared to the standard reference range.

Why Context Matters

Urine osmolality is highly sensitive to recent fluid intake and diet. A single random sample can be misleading if the patient recently consumed a large amount of water, caffeine, or alcohol (a diuretic). Furthermore, the presence of glucose (in diabetes) or radiopaque contrast dyes from recent imaging can artificially elevate osmolality. A second opinion ensures that the urine results are carefully correlated with simultaneous blood osmolality and sodium levels.

Lab ranges are statistical averages, not biological laws. "Normal" for a 20-year-old male isn't normal for a 60-year-old female.

Clinical References

Source-of-truth databases and clinical guidelines for Urine Osmolality:

Research & Guidelines

Related Indicators

Lipid Profile

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Urine Bicarbonate

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Albumin

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DR

Medically Reviewed by Dr. Binoy Babu, MBBS

Board Certified Doctor • 10+ Years Clinical Experience

Dr. Babu is a practicing physician dedicated to empowering patients with clear, actionable medical information. He founded 2opi to bridge the gap between complex lab reports and patient understanding, ensuring everyone has access to a reliable second opinion.

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