What are the signs of dehydration in individuals, particularly the elderly, young children, and those with chronic illnesses like diabetes or kidney disease?

Signs of Dehydration

The most reliable signs of dehydration vary significantly by age and clinical context, with laboratory confirmation via serum osmolality >300 mOsm/kg being essential in older adults, while clinical assessment using specific physical signs is more appropriate in children and younger adults. 1, 2

In Older Adults (Elderly)

Laboratory Assessment is Essential

• Directly measured serum osmolality >300 mOsm/kg is the gold standard for diagnosing dehydration in older adults with Grade B recommendation and 94% expert consensus. 1, 2, 3
• If direct measurement unavailable, use calculated osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L), with action threshold >295 mmol/L. 1, 2, 3
• Elevated serum sodium concentration (>145 mEq/L) is a strong indicator, with dehydrated elderly patients averaging 146 mEq/L versus 134 mEq/L in non-dehydrated patients. 4
• Elevated BUN combined with other markers strengthens the diagnosis. 2

Limited Useful Physical Signs

• Dry axilla has moderate sensitivity (44%) but excellent specificity (89%) and is the most reliable physical sign in elderly patients. 5, 4
• Sunken eyes show good specificity (83%) but poor sensitivity. 4
• Missing drinks between meals (reported by patient or caregiver) has sensitivity 1.00 and specificity 0.77 in one study. 6
• Expressing fatigue has sensitivity 0.71 and specificity 0.75. 6
• Combining "missing drinks between meals" AND "expressing fatigue" improves specificity to 0.92 while maintaining sensitivity of 0.71. 6

Signs That Should NOT Be Used in Elderly

• Skin turgor, dry mouth, urine color, urine specific gravity, and weight change are unreliable with Grade A recommendation against use and 100% consensus. 1, 5, 6
• Bioelectrical impedance has not been shown to be diagnostically useful. 1, 2
• Tachycardia, orthostatic dysregulation, and feeling thirsty are inadequate for diagnosis. 5, 6

In Young Children and Infants

Mild Dehydration (3%-5% fluid deficit)

• Increased thirst 1
• Slightly dry mucous membranes 1

Moderate Dehydration (6%-9% fluid deficit)

• Loss of skin turgor 1
• Tenting of skin when pinched 1
• Dry mucous membranes 1

Severe Dehydration (≥10% fluid deficit)

• Severe lethargy or altered state of consciousness 1
• Prolonged skin tenting and skin retraction time (>2 seconds) 1
• Cool and poorly perfused extremities 1
• Decreased capillary refill (>2 seconds correlates with fluid deficit) 1
• Rapid, deep breathing (sign of acidosis) 1

Most Predictive Signs in Children

• Rapid deep breathing, prolonged skin retraction time, and decreased perfusion are more reliably predictive than sunken fontanelle or absence of tears. 1
• Capillary refill time shows good correlation with fluid deficit, though fever, ambient temperature, and age can affect accuracy. 1

In Patients with Diabetes

Critical Laboratory Findings

• Effective serum osmolality ≥320 mOsm/kg defines Hyperosmolar Hyperglycemic State (HHS), a life-threatening emergency. 2, 3
• Calculate effective osmolality: 2[measured Na (mEq/L)] + glucose (mg/dL)/18. 3
• Correct sodium for hyperglycemia: for each 100 mg/dL glucose above 100 mg/dL, add 1.6 mEq to sodium value. 2, 3
• Blood glucose ≥600 mg/dL with osmolality ≥320 mOsm/kg indicates HHS. 3

Clinical Presentation

• Diabetic patients are at higher risk for dehydration due to osmotic diuresis from hyperglycemia. 7
• Monitor for altered mental status, which worsens with increasing osmolality. 3

In Patients with Chronic Kidney Disease

Increased Vulnerability

• Children and adults with CKD have reduced kidney concentrating capacity, increasing dehydration risk. 8
• Baseline low fluid intake combined with poor concentrating ability creates particular vulnerability. 8
• Assessment is more challenging than in healthy populations due to altered baseline renal function. 8

Assessment Approach

• Use serum osmolality >300 mOsm/kg as primary indicator. 1, 2
• Monitor for signs of volume depletion more frequently during illness or heat exposure. 8
• Assess individual dehydration risk factors including medication use (diuretics), baseline fluid intake, and disease stage. 8

Volume Depletion from Blood Loss, Vomiting, or Diarrhea

After Blood Loss

• Large postural pulse change (≥30 beats per minute from lying to standing) is 97% sensitive and 98% specific when blood loss ≥630 mL. 1
• Severe postural dizziness resulting in inability to stand 1
• Note: These findings are from younger adults not taking beta-blockers; sensitivity may vary in elderly. 1
• Decreased venous filling (empty veins) and low blood pressure suggest hypovolemia. 1

After Vomiting or Diarrhea

• Presence of at least 4 of these 7 signs indicates moderate to severe volume depletion: 1
  • Confusion
  • Non-fluent speech
  • Extremity weakness
  • Dry mucous membranes
  • Dry tongue
  • Furrowed tongue
  • Sunken eyes

Critical Pitfalls to Avoid

• Never rely on clinical signs alone in elderly patients without laboratory confirmation, as signs are subtle and unreliable with high false-negative rates. 1, 5, 6
• Always screen older adults for dehydration when contacting healthcare, with clinical condition changes, or when malnourished. 1, 2
• Ensure glucose and urea are within normal ranges when interpreting osmolality, as abnormalities independently elevate values. 2, 3
• In diabetic patients, failure to correct sodium for hyperglycemia can mask true hyponatremia. 2, 3
• Dehydration is associated with increased mortality and doubled risk of 4-year disability in older adults, making early detection critical. 1, 2, 7