How to assess dehydration status?

How to Check Dehydration Status

Measure serum or plasma osmolality directly—it is the only validated method for assessing dehydration, with a threshold >300 mOsm/kg indicating dehydration. 1, 2

Gold Standard Assessment

Primary Method: Direct Serum Osmolality Measurement

• Serum osmolality >300 mOsm/kg definitively indicates low-intake dehydration and requires immediate intervention 1, 2, 3
• This threshold was established by comparing osmolality in hydrated versus dehydrated individuals and is associated with increased mortality and doubled risk of 4-year disability 2
• The European Society for Clinical Nutrition and Metabolism (ESPEN) gives this a Grade B recommendation with 94% consensus 2
• Before interpreting elevated osmolality as dehydration, verify that serum glucose and urea are within normal ranges, as these contribute to total osmolality 1

Alternative: Calculated Serum Osmolarity

• When direct measurement is unavailable, use the formula: Osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L) 1, 2, 3
• Apply an action threshold of >295 mmol/L when using calculated osmolarity 1, 2
• This is a second-line screening tool with Grade B recommendation 2

Methods That Should NOT Be Used

Clinical Signs (Grade A Recommendation Against Use)

The following have been proven unreliable through Cochrane systematic review and should be completely avoided: 1, 3

• Skin turgor (poor sensitivity 0-44%) 1, 4
• Mouth dryness 1, 3
• Weight change 1, 3
• Sunken eyes 4
• Capillary refill time 4
• Tachycardia 4

Laboratory Tests to Avoid

• Urine color - inadequate diagnostic accuracy (area under curve 0.49-0.57) 2, 4
• Urine specific gravity - not diagnostically useful 2, 4
• Urine osmolality - unreliable in older adults 2
• Bioelectrical impedance - Grade A recommendation against use with 100% consensus 1, 2, 3

Special Considerations for Older Adults

Why Older Adults Require Different Assessment

• All older persons should be considered at risk for low-intake dehydration 3
• Renal function is often impaired, making renal parameters unreliable 3
• Clinical judgment alone is highly fallible in this population 3

Monitoring Approach

• Older persons and their caregivers should request periodic serum osmolality assessment from healthcare providers 1, 3
• Fluid intake assessment by care staff is notoriously inaccurate, with correlation as low as r = 0.122 between recorded and actual intake 1, 3
• Staff assessments typically underestimate intake by approximately 700 ml/day 1, 3

Clinical Algorithm

Step 1: Obtain direct serum osmolality measurement 1, 2

Step 2: Interpret results:

• >300 mOsm/kg = Dehydrated, requires intervention 1, 2, 3
• 275-300 mOsm/kg = Normal range 5
• <275 mOsm/kg = Hypo-osmolar (possible overhydration) 2

Step 3: If patient appears unwell with osmolality >300 mOsm/kg, initiate subcutaneous or intravenous fluids alongside oral intake 1, 3

Step 4: If patient appears 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, 3

Critical Pitfalls to Avoid

• Do not rely on clinical examination alone—a multidisciplinary consensus found no universally accepted definition for dehydration and that signs are subtle and unreliable 5
• Do not use "dehydration" and "hypovolemia" interchangeably—these represent different physiologic states 5
• Do not trust fluid intake records in institutional settings without verification 1, 3
• Do not use oral rehydration therapy or sports drinks for low-intake dehydration—these are indicated only for volume depletion from diarrhea/vomiting 1, 3