Calculating Fluid Deficit in Dehydration
Assessment-Based Calculation Method
Fluid deficit is calculated by multiplying the patient's body weight (kg) by the estimated percentage of dehydration determined through clinical assessment, with mild dehydration representing 3-5% fluid deficit, moderate dehydration 6-9%, and severe dehydration ≥10% body weight loss. 1
Clinical Assessment of Dehydration Severity
The degree of dehydration must be determined through physical examination findings:
- Mild dehydration (3-5% fluid deficit): Increased thirst and slightly dry mucous membranes 1
- Moderate dehydration (6-9% fluid deficit): Loss of skin turgor, dry mucous membranes, and prolonged capillary refill 1
- Severe dehydration (≥10% fluid deficit): Severe lethargy or altered consciousness, prolonged skin tenting, tachycardia, and orthostatic hypotension 1, 2
Calculation Formula
For a patient weighing 70 kg with moderate dehydration (estimated at 7% deficit), the fluid deficit = 70 kg × 0.07 = 4.9 liters. 1
Specific Context: Diabetic Ketoacidosis and Hyperosmolar States
In DKA and HHS, typical total body deficits are well-established:
- DKA: Total water deficit approximately 6 liters (100 mEq/kg sodium, 3-5 mEq/kg potassium) 3
- HHS: Total water deficit approximately 9 liters (100-200 mEq/kg sodium, 5-15 mEq/kg potassium) 3
Pediatric Considerations
In children, the most accurate method is comparing current weight to a recent known healthy weight, with the difference representing the fluid deficit. 4
For pediatric patients without recent weights:
- Mild dehydration: Administer 50 mL/kg ORS over 2-4 hours 1, 5
- Moderate dehydration: Administer 100 mL/kg ORS over 2-4 hours 1, 5
- Severe dehydration: Requires immediate IV boluses of 20 mL/kg until hemodynamic stabilization 1
Critical Replacement Principles
Fluid replacement should correct estimated deficits within 24 hours, with the induced change in serum osmolality not exceeding 3 mOsm/kg/hour to prevent complications. 3, 1
Monitoring Parameters
Success is judged by:
- Hemodynamic monitoring (blood pressure improvement) 3
- Fluid input/output measurement 3
- Clinical examination including mental status, skin turgor, mucous membrane moisture, and urine output 6
- Reassessment every 2-4 hours 6, 1
Common Pitfalls
In patients with renal or cardiac compromise, frequent assessment of cardiac, renal, and mental status must be performed during fluid resuscitation to avoid iatrogenic fluid overload. 3, 1
The traditional water-deficit equation [WD = 0.6 × body mass × (1 - 140/Na+)] significantly underestimates both total body water and free water losses and should not be relied upon for accurate deficit calculation. 7