Monitoring Dehydration
The gold standard for assessing dehydration is serum osmolality, with a threshold of >300 mOsm/kg indicating dehydration in older adults. 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.
Clinical Assessment of Dehydration
Physical Examination Signs
- Simple clinical signs commonly used to assess dehydration (skin turgor, mouth dryness, weight change) should NOT be used as the sole indicators of hydration status in older adults as they lack diagnostic accuracy 1, 2
- For children and adults, the following signs may help determine the degree of dehydration:
- Rapid, deep breathing, prolonged skin retraction time, and decreased perfusion are more reliable indicators of dehydration than sunken fontanelle or absence of tears 3
- A combination of at least four of the following seven signs increases likelihood of moderate to severe dehydration: confusion, non-fluent speech, weakness in limbs, dry mucous membranes, dry tongue, furrowed tongue, and sunken eyes 4
Laboratory Assessment
- Serum osmolality is the most accurate method for diagnosing dehydration 1, 5
- When serum osmolality cannot be measured directly, blood urea nitrogen to creatinine ratio >20 may indicate water-and-solute-loss dehydration 2
- Bioelectrical impedance should NOT be used to assess hydration status as it has not been shown to be diagnostically useful, particularly in older adults 6, 1
Emerging Assessment Methods
- Saliva osmolality has shown moderate diagnostic accuracy (sensitivity 70-78%, specificity 68-72%) for detecting both water-loss and water-and-solute-loss dehydration 2
- Saliva flow rate, urine color, and urine specific gravity have poor diagnostic accuracy for dehydration in older adults 2
Monitoring Protocols
General Population
- Hydration status should be assessed through a combination of:
- Physical examination findings (as outlined above)
- Laboratory values (serum osmolality when available)
- Clinical history 7
- Fluid balance monitoring is recommended to identify dehydration and concurrent conditions 6
Older Adults
- All older adults should be considered at risk of low-intake dehydration 1, 4
- Assessment of fluid intake by care staff is often highly inaccurate, with studies showing poor correlation between staff-recorded and actual fluid intake 6
- Older persons and their informal carers may use appropriate tools to assess fluid intake, but should also request serum osmolality assessment periodically 6, 1
Patients with Diarrhea
- Monitor for signs of dehydration including:
- Continued passing of many stools
- Excessive thirst
- Sunken eyes
- Fever 6
- Reassess hydration status after 3-4 hours of rehydration therapy and continue treatment according to the degree of dehydration 6
Management Based on Assessment
- For mild dehydration: Encourage increased oral fluid intake according to patient preference (tea, coffee, fruit juice, water) 6, 3
- For moderate dehydration: Administer oral rehydration solution (ORS) at appropriate doses based on age 3
- For severe dehydration or older adults with measured serum osmolality >300 mOsm/kg who appear unwell: Provide subcutaneous or intravenous fluids in parallel with encouraging oral fluid intake 6, 3
Pitfalls and Caveats
- Clinical judgment alone is highly fallible in assessing dehydration in older adults 1
- In older adults, renal function is often poor, making renal parameters unreliable indicators of dehydration 1
- Assessment of fluid intake by care staff frequently underestimates actual intake by approximately 700 ml/day 6
- Oral rehydration therapy and sports drinks are NOT indicated for low-intake dehydration in older adults 6
- Soft drinks are not recommended for rehydration due to their high osmolality 6
- Anti-diarrheal agents are contraindicated for the treatment of diarrheal disease 6