Do you calculate estimated blood volume (EBV) using total body water (TBW)?

Estimated Blood Volume (EBV) is NOT Calculated Using Total Body Water (TBW)

EBV and TBW are distinct physiological compartments that should not be conflated—EBV represents only the intravascular plasma volume (approximately 7-8% of TBW), while TBW encompasses all body water including intracellular, interstitial, and intravascular compartments. 1

Understanding the Distinction

What TBW Actually Represents

• TBW is the entire water content of the body and serves as the volume of distribution for urea and other solutes, not blood volume 1
• TBW includes three major compartments:
  • Intracellular fluid: approximately 65-70% of TBW 1
  • Interstitial fluid: approximately 25% of TBW 1
  • Intravascular volume (plasma): only 7-8% of TBW 1

Proper Calculation Methods for TBW

When TBW calculation is needed (such as for dialysis adequacy), use anthropometric formulas:

Watson Formula (Recommended):

• Males: TBW = 2.447 - (0.09156 × age) + (0.1074 × height in cm) + (0.3362 × weight in kg) 2, 1
• Females: TBW = -2.097 + (0.1069 × height in cm) + (0.2466 × weight in kg) 2, 1

Hume Formula (Equally Acceptable):

• Males: TBW = (0.194786 × height) + (0.296785 × weight) - 14.012934 2, 1
• Females: TBW = (0.34454 × height) + (0.183809 × weight) - 35.270121 2, 1

Critical Pitfalls to Avoid

Never Use Fixed Fractions

• Do not use fixed fractions of body weight (0.60 for males, 0.55 for females) to estimate TBW—this method is inaccurate and overestimates TBW even in overhydrated patients 2, 1

Context Matters

• TBW calculations are appropriate for:

  • Dialysis adequacy assessments (Kt/V calculations) 2
  • Urea distribution volume estimation 2
  • Fluid replacement calculations in specific clinical contexts 3

• TBW calculations are NOT appropriate for:

  • Estimating blood volume for transfusion decisions
  • Calculating circulating plasma volume
  • Determining intravascular volume status

Special Corrections Required

• For edematous patients: Add the entire weight gain from fluid overload to the TBW calculated at dry weight 2, 1
• For amputees: Correct the anthropometric calculation using specific adjustment factors to restore proper body water content relationships 2, 1
• For malnourished patients: Calculate TBW using desired/target weight rather than actual weight when determining dialysis adequacy 2, 1

Clinical Bottom Line

If you need to estimate blood volume for clinical decisions (transfusion, hemorrhage assessment, etc.), use blood volume-specific formulas or clinical assessment tools—not TBW calculations. If you need TBW for dialysis adequacy or solute distribution, use the Watson or Hume anthropometric formulas with appropriate corrections for clinical conditions. 2, 1