Assessing Dehydration with Urine Osmolality
Urine osmolality should NOT be used to determine if a patient is dehydrated—serum osmolality is the gold standard instead. 1
Why Urine Osmolality Fails as a Dehydration Marker
The ESPEN guidelines explicitly state that urine osmolality, urine specific gravity, and urine color shall NOT be used to assess hydration status in adults, particularly older adults, with a Grade A recommendation. 1 This strong recommendation is based on:
- A Cochrane systematic review demonstrating that urinary measures lack diagnostic accuracy for detecting water-loss dehydration when compared against the reference standard of serum osmolality. 1
- The largest diagnostic accuracy study to date (313 participants) confirmed that urine osmolality, urine specific gravity, and urine color have sensitivity and specificity too low to be clinically useful (below the 70% threshold). 2
- Renal function decline in older adults means renal parameters no longer accurately signal low-intake dehydration, making urine-based tests unreliable. 1
The Correct Approach: Serum/Plasma Osmolality
Directly measured serum or plasma osmolality >300 mOsm/kg indicates dehydration and should trigger immediate intervention. 1 This threshold was rigorously developed by comparing hydrated versus dehydrated younger adults and has been validated in cohort studies showing associations with increased mortality and doubled risk of 4-year disability. 1
When Direct Measurement Is Unavailable
If laboratory osmolality measurement is not accessible, use the calculated osmolarity equation (Grade B recommendation): 1
Osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L)
- Action threshold: >295 mmol/L indicates dehydration and warrants intervention. 1
- This equation has 85% sensitivity and 59% specificity for detecting low-intake dehydration in older adults. 1
Critical Interpretation Caveat
Before interpreting elevated serum osmolality as dehydration, verify that serum glucose and urea are within normal range, as these can independently elevate osmolality. 1 In true low-intake dehydration, osmolality rises due to small increases across all components (sodium, potassium, glucose, urea) that remain within normal ranges individually but collectively signal fluid concentration. 1
Understanding the Physiological Disconnect
In dehydration, you would expect:
- Serum osmolality: elevated (>300 mOsm/kg) reflecting concentrated blood. 1
- Urine osmolality: also elevated as kidneys concentrate urine to conserve water. 3, 4
However, this expected relationship breaks down in older adults and those with renal impairment, making urine osmolality an unreliable marker. 1 Additionally, in diabetes insipidus, the dissociation is dramatic—plasma osmolality >300 mOsm/kg while urine osmolality remains inappropriately diluted <200 mOsm/kg. 5
Other Tests to Avoid
Do NOT use these commonly recommended but invalid tests: 1
- Skin turgor
- Mouth dryness
- Weight change
- Urine color
- Urine specific gravity
- Bioelectrical impedance (Grade A recommendation against use with 100% consensus) 1
Clinical Algorithm for Dehydration Assessment
- Measure serum or plasma osmolality directly (preferred method). 1
- If >300 mOsm/kg: Patient is dehydrated—initiate treatment. 1
- If direct measurement unavailable: Calculate osmolarity using the equation above. 1
- If calculated osmolarity >295 mmol/L: Provide hydration support and recheck with direct measurement within days. 1
- Verify glucose and urea are normal before attributing elevated osmolality to dehydration. 1
Treatment Based on Severity
- For patients appearing well with osmolality >300 mOsm/kg: Encourage increased oral fluid intake with preferred beverages (tea, coffee, juice, water—NOT oral rehydration solutions or sports drinks). 1
- For patients appearing unwell with osmolality >300 mOsm/kg: Administer subcutaneous or intravenous hypotonic fluids in parallel with encouraging oral intake (Grade A recommendation). 1