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

Dehydration diagnosis requires laboratory confirmation with serum osmolality >300 mOsm/kg as the gold standard, particularly in elderly patients, as clinical signs alone are unreliable and should never be used in isolation. 1

Laboratory Assessment: The Gold Standard

Serum osmolality measurement is the only reliable method for diagnosing dehydration across all populations:

• Directly measured serum osmolality >300 mOsm/kg definitively diagnoses dehydration with Grade B recommendation and 94% expert consensus from the American College of Nutrition 1
• If direct measurement is 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
• Elevated serum sodium >145 mEq/L strongly indicates dehydration in elderly patients 1
• Serum osmolality >300 mOsm/kg is associated with increased mortality and doubled risk of 4-year disability in older adults 2

Clinical Signs by Population

Elderly Patients: Critical Pitfalls

The American Geriatrics Society explicitly recommends NEVER relying on clinical signs alone in elderly patients without laboratory confirmation, as signs are subtle and unreliable with high false-negative rates. 1

Signs with limited utility (poor sensitivity, moderate specificity):

• Dry axilla shows 44% sensitivity but 89% specificity 3
• Sunken eyes demonstrates 83% specificity but poor sensitivity 3
• Delayed capillary refill time shows 83% specificity 3

Signs that are completely unreliable and should NOT be used:

• Skin turgor testing, dry mouth, feeling thirsty, tachycardia, orthostatic dysregulation, and dark urine have inadequate diagnostic value 4, 5
• Urine specific gravity, urine color, and urine osmolality have Grade A recommendation AGAINST use per ESPEN guidelines 2

The only clinical signs with any predictive value in elderly:

• Missing drinks between meals: sensitivity 1.00, specificity 0.77 in one study 5
• Expressing fatigue: sensitivity 0.71, specificity 0.75 5
• Combining both signs (missing drinks AND expressing fatigue) improves specificity to 0.92 5

Young Children and Infants

The CDC recommends monitoring for these progressive signs:

Mild to moderate dehydration:

• Increased thirst 1
• Slightly dry mucous membranes 1
• Loss of skin turgor with tenting when pinched 1

Severe dehydration (requires immediate intervention):

• Severe lethargy or altered consciousness 1
• Prolonged skin tenting and retraction time >2 seconds 1
• Cool and poorly perfused extremities 1

Patients with Diabetes

The American Diabetes Association defines Hyperosmolar Hyperglycemic State (HHS) as a life-threatening emergency:

• Effective serum osmolality ≥320 mOsm/kg using formula: 2[measured Na (mEq/L)] + glucose (mg/dL)/18 2
• Blood glucose ≥600 mg/dL 2
• Critical: Correct sodium for hyperglycemia—for each 100 mg/dL glucose above 100 mg/dL, add 1.6 mEq to sodium value 2
• Failure to correct sodium masks true hyponatremia 1

Patients with Kidney Disease

Children with chronic kidney disease have inherent vulnerability to dehydration due to reduced concentrating capacity 6

In diabetes insipidus specifically:

• Plasma osmolality elevated >300 mOsm/kg while urine osmolality remains inappropriately diluted <200 mOsm/kg 7
• This dissociation (urine osmolality less than plasma osmolality) is the diagnostic hallmark 7

Volume Depletion Assessment (After Blood Loss or Vomiting/Diarrhea)

The American College of Surgeons recommends monitoring:

• Large postural pulse change ≥30 beats per minute from lying to standing 1
• Severe postural dizziness resulting in inability to stand 1
• Presence of at least 4 of these 7 signs indicates moderate to severe volume depletion: confusion, non-fluent speech, extremity weakness, dry mucous membranes, dry tongue, furrowed tongue, and sunken eyes 1

Clinical Algorithm for Assessment

Step 1: Obtain serum osmolality (direct measurement preferred) or calculate using formula if unavailable 1, 2

Step 2: Interpret results:

• 300 mOsm/kg = dehydration confirmed, requires immediate intervention 1, 2

• 295-300 mOsm/kg = impending dehydration 2
• <275 mOsm/kg = hyposmolality, investigate for SIADH or overhydration 2

Step 3: In diabetic patients, calculate effective osmolality and correct sodium for hyperglycemia 1, 2

Step 4: Ensure serum glucose and urea are within normal ranges before interpreting osmolality 2, 7

Step 5: In elderly patients, if laboratory testing unavailable, only consider dehydration likely if patient both misses drinks between meals AND expresses fatigue 5

Key Pitfalls to Avoid

• Never diagnose dehydration in elderly based on clinical examination alone—this approach has unacceptably high false-negative rates 1, 4
• Do not use bioelectrical impedance—Grade A recommendation against use with 100% consensus 2
• Early detection is critical as dehydration associates with increased mortality 1, 8
• In vulnerable populations during warm weather, failure to increase water intake significantly increases mortality risk 8