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]

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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