#### III. Calculation: Normal Total Body Water (TBW)

1. Normal TBW (L) = fractionWater * wtKg
2. Where wtKg = Weight in kg
3. Where fractionWater
1. Child: 0.6 for males, 0.6 for females
2. Adult: 0.6 for males, 0.5 for females
3. Elderly: 0.5 for males, 0.45 for females

#### IV. Calculation: Current Total Body Water (TBW)

1. Current TBW = Normal TBW * (140 / sNa)
2. Where 140 mEq/L is the normal Serum Sodium, and sNa is the current Serum Sodium
3. Adjust measured Serum Sodium (sNa) for Hyperglycemia
1. See Corrected Serum Sodium for Glucose

#### V. Calculation: Free Water Deficit (Hypernatremia)

1. Free Water Deficit (FWD in L) = (Normal TBW) - (Current TBW)
2. Free Water Deficit (FWD in L) = (Normal TBW) * (sNa - 140) / 140
3. Free Water Deficit (FWD in L) = (Normal TBW) * (sNa/140 - 1)
1. Where 140 mEq/L is the normal Serum Sodium, and sNa is the current Serum Sodium
4. Adjustments to Total Body Water
1. Decrease Total Body Water by 10% in Hypernatremia (multiply TBW * 0.9)
2. Some equations use 0.5 * LbmKg as TBW for both men and women in Hypernatremia

#### VI. Calculation: Total Body Sodium Deficit or Water Excess (Hyponatremia)

1. Excess Total Body Water (TBW)
1. Excess TBW = Current TBW - Normal TBW
2. Sodium delivered by various crystalloid solutions (ivNa)
1. Hypertonic Saline (3% NaCl): 513 meq/L
2. Normal Saline or NS (0.9% NaCl): 154 meq/L
3. Lactated Ringers or LR: 130 meq/L
4. 1/2 Normal Saline or 1/2NS (0.45% NaCl): 77 meq/L
3. Total Body Sodium Deficit (NaDeficit)
1. Sodium deficit (meq) = Normal TBW * (140 - sNa)
2. Where 140 mEq/L is the normal or desired Serum Sodium, and sNa is the current Serum Sodium
4. IV Fluid rate (IVrate) to achieve a given rise in Sodium per hour (sNaChangePerH )
1. IVrate = (1000 * sNaChangePerH * (TBWnorm + 1)) / (ivNa - sNa)
2. where sNaChangePerH = Desired change per hour (typically <0.25 to <0.50 meq/h)
3. where TBWnorm = Normal Total Body Water (see above for calculation)
4. where ivNa = Sodium per Liter of IV fluid (see above)
5. where sNa = Serum Sodium
5. Serum Sodium change per liter of IV fluids (sNaChangePerL)
1. sNaChangePerL = (ivNa - sNa) / (TBWnorm + 1)
2. where TBWnorm = Normal Total Body Water (see above for calculation)
3. where ivNa = Sodium per Liter of IV fluid (see above)
4. where sNa = Serum Sodium

#### VIII. References

1. Marino (2014) ICU Book, Wolters Kluwer, p. 653-72
2. Rose (1989) Clinical Physiology of Acid Base and Electrolytes, p. 816-19
3. Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]

### Ontology: Total body water (C0429632)

 Concepts Finding (T033) SnomedCT 251837008 English total body water, Total body water, TBW - Total body water, Total body water (observable entity) Spanish agua corporal total (entidad observable), agua corporal total

### Ontology: body water loss (C0599707)

 Concepts Finding (T033) English body water loss

### Ontology: water; excess (C0599744)

 Concepts Finding (T033) English body water excess, excessive; water, water; excess Dutch overmatig; water, water; overmaat