Urine Osmolality Test Procedure
For an adult patient with no significant medical history, collect a random untimed urine sample (preferably first morning void) in a clean container, send it to the laboratory within 3 hours at room temperature, and request direct osmolality measurement by freezing-point depression osmometry. 1, 2
Sample Collection
- Obtain a random untimed urine specimen rather than a timed collection, as this is the preferred method for most clinical applications 3
- First morning void is optimal but a random specimen is acceptable for most diagnostic purposes 3, 1
- Use a clean, dry container without preservatives 4
- Collect at least 5-10 mL of urine for adequate testing 4
Sample Handling and Storage
- Process within 3 hours of collection if possible, though urine osmolality remains extremely stable at room temperature for up to 36 hours with mean changes <1 mosmol/kg 2
- Room temperature storage is preferred for urine specimens, as refrigeration (4-8°C) only maintains stability for 4 days compared to 5 days at room temperature 5
- Do NOT refrigerate unless delays beyond 12 hours are anticipated 2
- Samples can be frozen at -21°C for >14 days if longer storage is required 5
Laboratory Measurement
- Direct measurement of osmolality by freezing-point depression osmometry is the gold standard and should always be requested over calculated values 3, 1, 6
- The laboratory will measure the concentration of all dissolved particles in the urine sample 4, 5
- Results are reported in mOsm/kg H₂O 3, 6
Clinical Interpretation Context
When interpreting results, the clinician must simultaneously obtain:
- Serum osmolality (or calculate using: Osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14, all in mmol/L) 1, 6
- Serum sodium, potassium, glucose, and urea to rule out other causes of altered osmolality 1, 6
- Clinical context including hydration status and symptoms 1, 7
Key Diagnostic Thresholds
- Urine osmolality <200-300 mOsm/kg with elevated serum osmolality is pathognomonic for diabetes insipidus 1, 7
- Urine osmolality >500 mOsm/kg with serum osmolality <275 mOsm/kg suggests SIADH 7
- Normal urine osmolality ranges widely (50-1200 mOsm/kg) depending on hydration status 4
Critical Pitfalls to Avoid
- Do NOT use urine specific gravity, urine color, or reagent strips as substitutes for osmolality measurement—these have inadequate diagnostic accuracy 6, 8
- Do NOT rely on clinical signs like skin turgor or mouth dryness to assess hydration status, especially in older adults 1, 6, 7
- Be aware that certain substances affect measurements: large molecules like radiographic contrast or mannitol will falsely elevate specific gravity relative to osmolality, while ethanol in urine can obscure true osmolality 8, 9
- Ensure glucose and urea are normalized before interpreting osmolality results, as abnormalities in these affect interpretation 1, 6
Special Considerations
- For patients with suspected diabetes insipidus, proceed directly to genetic testing (90% are X-linked AVPR2 variants) rather than relying solely on water deprivation tests 1, 7
- In older adults or those at risk of dehydration, screen with serum osmolality first (threshold >300 mOsm/kg indicates dehydration) before ordering urine studies 3, 1, 6