Assessment of Dehydration

Serum osmolality is the gold standard for assessing dehydration, with a threshold of >300 mOsm/kg indicating dehydration in older adults. 1

Diagnostic Approach

Directly measured serum osmolality >300 mOsm/kg should be used to identify low-intake dehydration in older adults 1
When direct measurement is not available, calculated serum osmolarity using the equation: osmolarity = 1.86 (Na+ + K+) + 1.15 glucose + urea + 14 (all measured in mmol/L) with an action threshold of >295 mmol/L can be used as a screening tool 1

Simple clinical signs and tests commonly used to assess dehydration should NOT be used as they lack diagnostic accuracy, particularly in older adults: 1
- Skin turgor
- Mouth dryness
- Weight change
- Urine color or specific gravity
Bioelectrical impedance should NOT be used to assess hydration status in older adults as it has not been shown to be diagnostically useful 1

All older persons should be considered at risk of low-intake dehydration 1
Assessment of fluid intake by care staff is often highly inaccurate, with studies showing poor correlation (r = 0.122) between staff-recorded and actual fluid intake 1
Older persons and their informal carers may use appropriate tools to assess fluid intake, but should also request serum osmolality assessment periodically 1

For non-geriatric populations, physical examination findings may be more reliable, with the following progression of signs: 2
- Mild dehydration (3-5% fluid deficit): increased thirst and slightly dry mucous membranes
- Moderate dehydration (6-9% fluid deficit): loss of skin turgor and dry mucous membranes
- Severe dehydration (≥10% fluid deficit): severe lethargy or altered consciousness and prolonged skin tenting

Two main types of dehydration should be recognized: 3, 4
- Water loss dehydration (hyperosmolar) - due to either increased sodium or glucose
- Salt and water loss dehydration (hyponatremia)

The terms "dehydration" and "hypovolemia" are often incorrectly used interchangeably 3
In older adults, renal function is often poor, making renal parameters unreliable indicators of dehydration 1
Clinical judgment alone is highly fallible in assessing dehydration in older adults 1
Assessment of fluid intake by care staff is frequently inaccurate, often underestimating actual intake by approximately 700 ml/day 1
Saliva osmolality may have potential as a less invasive marker for both water-loss and water-and-solute-loss dehydration in older individuals, showing better diagnostic accuracy than physical signs and urine markers 5

For older adults with measured serum osmolality >300 mOsm/kg who appear unwell, subcutaneous or intravenous fluids should be offered in parallel with encouraging oral fluid intake 1
In mild dehydration, older persons should be encouraged to drink more fluid according to preference (tea, coffee, fruit juice, water, etc.) 1
Oral rehydration therapy and sports drinks are NOT indicated for low-intake dehydration 1