Why are older adults at risk for dehydration?

Older Adults and Dehydration Risk: Impaired Response to Serum Osmolality

Older adults are at risk for dehydration primarily due to impaired response to serum osmolality, which blunts both thirst sensation and kidney concentration ability. 1

Physiological Changes Contributing to Dehydration Risk

• Aging blunts two key protective physiological responses to inadequate fluid intake: thirst sensation and primary urine concentration by the kidney 1
• Older adults have reduced total body water, providing a smaller fluid reserve compared to younger adults 1
• Many older adults use medications such as diuretics and laxatives which increase fluid losses 1
• Renal function is often poor in older adults, so renal parameters no longer accurately signal low-intake dehydration 1

Non-Physiological Factors Increasing Dehydration Risk

• Memory problems may cause older adults to forget to drink and forget that they haven't drunk (not being prompted by thirst) 1
• Many older adults voluntarily reduce fluid intake due to:
  • Continence concerns and fear of incontinence 1
  • Issues about getting to the toilet in time 1
  • Loss of social drinking routines as social isolation increases 1
• Physical access to drinks can be challenging for older adults 1
• Swallowing problems and dysphagia can limit fluid intake 1

Clinical Significance of Dehydration in Older Adults

• Low-intake dehydration is common in older adults, especially those who are frail and vulnerable 1
• High-quality cohort studies consistently show that older adults with raised serum osmolality (>300 mOsm/kg) have:
  • Increased risk of mortality 1
  • Doubled risk of 4-year disability 1
• Dehydrated older adults are 6 times more likely to die in hospital than those who are euhydrated 2

Assessment of Dehydration

• Directly measured serum or plasma osmolality is the primary indicator of hydration status 1
• An action threshold of >300 mOsm/kg should be used to identify low-intake dehydration 1
• When direct measurement is unavailable, calculated osmolarity can be used:
  • Formula: osmolarity = 1.86 × (Na+ + K+) + 1.15 × glucose + urea + 14 (all in mmol/L)
  • Action threshold: >295 mmol/L 1
• Simple clinical signs (skin turgor, mouth dryness, urine color) should NOT be used to assess hydration status in older adults 1
• Bioelectrical impedance should NOT be used to assess hydration status 1

Prevention and Management

• All older persons should be considered at risk of low-intake dehydration and encouraged to consume adequate amounts of fluids 1
• Regular screening for dehydration is recommended when older adults:
  • Contact the healthcare system
  • Experience unexpected clinical changes
  • Are malnourished or at risk of malnutrition 1
• Strategies to increase fluid intake include:
  • Identifying and overcoming individual barriers to drinking 3
  • Addressing institutional barriers in care settings 3
  • Encouraging small amounts of fluid throughout the day rather than large amounts at once 4

Common Pitfalls

• Relying solely on clinical signs without laboratory confirmation can lead to misdiagnosis 5
• Failing to recognize dehydration due to the absence of typical signs in older adults 5
• Not considering dehydration as a potential cause of mental status changes in older adults 2
• Assuming older adults will drink when thirsty, when their thirst sensation is actually impaired 1