How do you assess hydration status in a patient?

How to Check Hydration Status of a Patient

The gold standard for assessing hydration status is directly measured serum or plasma osmolality, with a threshold >300 mOsm/kg indicating dehydration. 1

Primary Assessment Method: Serum Osmolality

• Directly measured serum osmolality using freezing point depression is the reference standard for diagnosing low-intake dehydration. 1, 2
• A value >300 mOsm/kg definitively classifies a patient as dehydrated, based on rigorous validation studies comparing hydrated versus dehydrated states. 1
• Before interpreting elevated osmolality as dehydration, verify that serum glucose and urea are within normal range—if abnormal, these should be corrected first. 1
• In low-intake dehydration, osmolality may be elevated even when individual components (sodium, potassium, urea, glucose) remain within normal ranges due to general fluid concentration causing small rises across all components. 1

Alternative When Direct Osmolality Unavailable

If direct osmolality measurement is not available, use the calculated osmolarity equation with a threshold >295 mmol/L as a screening tool. 1

• The validated equation is: Osmolarity = 1.86 (Na+ + K+) + 1.15 glucose + urea + 14 (all measured in mmol/L). 1
• This equation has 85% sensitivity and 59% specificity for identifying dehydration in older adults and has been validated across diverse populations including those with diabetes, poor renal function, and varying care settings. 1
• A calculated osmolarity >295 mmol/L should trigger immediate advice and support with drinking, followed by direct osmolality measurement within a few days to confirm diagnosis and guide intensive intervention. 1

Methods That Should NOT Be Used

Do not rely on traditional clinical signs and tests—they are unreliable for assessing hydration status, particularly in older adults. 1, 3

The following methods have been proven inadequate through systematic review and should be avoided:

• Skin turgor testing 1, 3
• Mouth dryness 1
• Weight change 1
• Urine color 1
• Urine specific gravity 1
• Bioelectrical impedance 1
• Tachycardia 3
• Orthostatic vital signs 3
• Sunken eyes 3

A Cochrane systematic review definitively established that none of these clinical signs or simple tests are consistently useful for indicating hydration status in older adults. 1

Supplementary Assessment Tools with Limited Value

While the following have some diagnostic value, they should supplement—not replace—osmolality measurement:

• Inferior vena cava ultrasonography shows high diagnostic value for hyperosmolar dehydration. 3
• Axillary dryness has demonstrated diagnostic utility. 3
• History of not drinking between meals (from patient or informant) provides useful information. 3
• Blood pressure assessment and clinical symptoms in the eye (tear production, palpitating pressure) and mouth (thirst, mucous wetness) can provide supplementary information but should not be used alone. 4

Clinical Algorithm for Hydration Assessment

1. Order serum osmolality (direct measurement preferred) or calculate osmolarity using the validated equation if direct measurement unavailable. 1

2. Interpret results:

   • Osmolality >300 mOsm/kg = dehydrated 1
   • Calculated osmolarity >295 mmol/L = likely dehydrated, requires confirmation 1
   • Osmolality ≤280 mOsm/kg = hypo-osmolar 2

3. Verify glucose and urea are normal before attributing elevated osmolality to dehydration. 1

4. Reassess regularly until corrected, then monitor periodically with ongoing support for adequate fluid intake. 1

Important Caveats

• Distinguish dehydration from volume depletion: Extracellular water loss from diarrhea, vomiting, or renal sodium loss presents with normal or low plasma osmolality, not elevated osmolality. 1
• The terms "dehydration" and "hypovolemia" are incorrectly used interchangeably—they represent different physiological states. 2
• In older adults, renal function is often impaired, making renal parameters unreliable for detecting low-intake dehydration. 1
• Clinical judgment alone is highly fallible in older adults for assessing hydration status. 1
• Fluid intake assessment by staff or caregivers is notoriously inaccurate, with studies showing very low correlation (r = 0.122) between staff assessments and actual intake. 1