What are the guidelines for classifying dehydration based on severity of fluid loss and clinical symptoms?

Classification of Dehydration

Dehydration should be classified into two distinct pathophysiological types—low-intake dehydration (water deficit) and volume depletion (fluid and electrolyte loss)—as these require fundamentally different diagnostic approaches and treatments. 1, 2

Two Primary Types of Dehydration

Low-Intake Dehydration (Water Deficit)

• Characterized by elevated serum osmolality >300 mOsm/kg, which is the gold standard diagnostic criterion 1, 3
• Results from inadequate fluid intake without proportional electrolyte loss 2
• Presents with raised plasma osmolality as the key physiological marker 1
• Serum osmolality >295 mmol/l should trigger screening in older adults 1
• Often occurs without any single electrolyte being outside normal range, but all components rise slightly within normal limits 1

Volume Depletion (Extracellular Fluid Loss)

• Presents with normal or low serum osmolality and results from loss of both water and electrolytes 1, 2
• Occurs following vomiting, diarrhea, bleeding, or renal sodium loss 1
• Requires different assessment tools than low-intake dehydration 1, 2

Severity Classification by Fluid Deficit Percentage

Mild dehydration: 3-5% fluid deficit

• Increased thirst 4
• Slightly dry mucous membranes 4

Moderate dehydration: 6-9% fluid deficit

• Loss of skin turgor 4
• Dry mucous membranes 4

Severe dehydration: ≥10% fluid deficit

• Severe lethargy or altered consciousness 4
• Prolonged skin tenting 4

Clinical Assessment for Volume Depletion

Following Blood Loss

Use postural pulse change ≥30 beats per minute from lying to standing, which is 97% sensitive and 98% specific when blood loss exceeds 630 mL 1, 4

• Severe postural dizziness resulting in inability to stand is an alternative indicator 1
• Note that postural hypotension has limited additional predictive value 1
• Caution: Sensitivity and specificity may be reduced in older adults taking beta-blockers 1, 4

Following Vomiting or Diarrhea

A patient with ≥4 of the following 7 signs has moderate to severe volume depletion: 1, 2

• Confusion 1, 4
• Non-fluent speech 1, 4
• Extremity weakness 1, 4
• Dry mucous membranes 1, 4
• Dry tongue 1, 4
• Furrowed tongue 1, 4
• Sunken eyes 1, 4

Additional supporting signs include: 1, 4

• Decreased venous filling (empty veins) 1, 4
• Low blood pressure 1, 4
• Delayed capillary refill time 4

Diagnostic Algorithm

Step 1: Determine Dehydration Type

• If inadequate fluid intake without excessive losses: Measure serum osmolality directly 1, 3

  • Osmolality >300 mOsm/kg confirms low-intake dehydration 1, 3
  • Ensure glucose and urea are within normal range before interpreting 1

• If vomiting, diarrhea, or bleeding: Assess for volume depletion using clinical signs 1

  • Serum osmolality will be normal or low 1, 2

Step 2: Assess Severity

• For low-intake dehydration: Use serum osmolality thresholds (>300 mOsm/kg = dehydrated) 1, 3
• For volume depletion: Apply percentage fluid deficit categories (3-5% mild, 6-9% moderate, ≥10% severe) 4
• Use clinical sign clusters for volume depletion severity 1, 2

Common Pitfalls

Do not use "dehydration" and "hypovolemia" interchangeably—these represent distinct pathophysiological states requiring different treatments 3

Clinical signs alone are unreliable in older adults for detecting low-intake dehydration; serum osmolality measurement is essential 1, 3

Individual clinical signs have poor predictive value for volume depletion; use the validated 7-sign cluster requiring ≥4 positive findings 1

Renal function parameters are unreliable in older adults for detecting low-intake dehydration 1

Treatment Implications by Classification

Low-Intake Dehydration

• Requires hypotonic fluid replacement if unable to drink 2
• Oral intake is preferred when feasible 1

Volume Depletion (All Severities)

• Administer isotonic fluids (oral, nasogastric, subcutaneous, or intravenous) 1
• Mild: Oral rehydration solution 50 mL/kg over 2-4 hours 4
• Moderate: Oral rehydration solution 100 mL/kg over 2-4 hours 4
• Severe: Immediate IV rehydration with Ringer's lactate or normal saline boluses 4