Clinical Medicine & Physiology
Watson Total Body Water (TBW) Calculator: Dialysis Kt/V & Hydrophilic V_d Formula
Calculate adult Total Body Water V (Liters) for hemodialysis urea Kt/V adequacy, dysnatremia correction, and hydrophilic/ethanol volume of distribution using Patricia E. Watson et al.’s (1980) isotope-dilution equations.
Why KDOQI hemodialysis adequacy and hyponatremia formulas replace "0.60 * Weight" with Watson TBW!
Fixed rule-of-thumb water fractions (0.60 * Weight_kg in men, 0.50 * Weight_kg in women) severely overestimate Total Body Water V in obese and elderly patients because adipose tissue is ~90% anhydrous lipid! Calibrating against deuterium/tritium/antipyrine dilution across 458 men and 265 women, Patricia E. Watson, Ian D. Watson, and Richard D. Batt (1980, American Journal of Clinical Nutrition; NKF-KDOQI adult hemodialysis reference for urea distribution volume V in Kt/V) derived sex-specific linear anthropometric equations:
For Adult Men: TBW_men (L) = 2.447 - 0.09516 * Age_yr + 0.1074 * Height_cm + 0.3362 * Weight_kg! For Adult Women (where age was statistically insignificant after height and weight): TBW_women (L) = -2.097 + 0.1069 * Height_cm + 0.2466 * Weight_kg! Dividing TBW by Weight_kg immediately reveals true patient hydration fraction (% Body Weight)!
| Adult Patient Profile | Watson (1980) Total Body Water V (L) | Actual Hydration % (TBW / Weight) | vs Naive 0.60/0.50 Rule |
|---|---|---|---|
| 35-yr Male, 180 cm, 75 kg (Lean) | 43.66 L | 58.2% of body weight | 45.0 L (Close) |
| 70-yr Male, 172 cm, 110 kg (Obese Elderly) | 51.24 L | 46.6% of body weight | 66.0 L (+29% dangerous overestimate!) |
| 55-yr Female, 164 cm, 72 kg | 33.19 L | 46.1% of body weight | 36.0 L (+8.5% overestimate) |
Watson (1980) Sex-Specific Adult Total Body Water TBW (Liters)
Evaluates Male ("M") or Female ("F") Watson TBW (L) across Sex B2, Age B3 (18–105 yr), Height B4 (cm), and Weight B5 (kg).
=IF(OR(AND(B2 <> "M", B2 <> "F"), B3 < 18, B3 > 105, B4 < 120, B4 > 230, B5 < 30, B5 > 300), "Invalid Adult Sex (M/F), Age, Height (cm), or Weight (kg)", ROUND(IF(B2 = "M", 2.447 - 0.09516 * B3 + 0.1074 * B4 + 0.3362 * B5, -2.097 + 0.1069 * B4 + 0.2466 * B5), 2))Effective Body-Water Fraction (% of Total Body Weight)
Computes 100 * TBW_L / Weight_kg to display individualized water percentage for Adrogue-Madias sodium or Kt/V auditing.
=IF(OR(AND(B2 <> "M", B2 <> "F"), B3 < 18, B3 > 105, B4 < 120, B4 > 230, B5 < 30, B5 > 300), "Invalid Adult Sex (M/F), Age, Height (cm), or Weight (kg)", ROUND(100 * IF(B2 = "M", 2.447 - 0.09516 * B3 + 0.1074 * B4 + 0.3362 * B5, -2.097 + 0.1069 * B4 + 0.2466 * B5) / B5, 1))Try the example
Use the original values or change them to test your understanding.