Total Body Water Calculation
For this 66-year-old male (167.6 cm, 64.3 kg), use the Watson or Hume anthropometric formulae to calculate total body water, which will yield approximately 35-37 liters. 1
Recommended Calculation Method
The NKF-K/DOQI guidelines explicitly recommend using either the Watson or Hume anthropometric formulae for estimating total body water (V), as both provide similar and reliable estimates. 1
Watson Formula for Males:
- TBW (L) = 2.447 - 0.09156 × age (years) + 0.1074 × height (cm) + 0.3362 × weight (kg) 1
- For this patient: 2.447 - 0.09156(66) + 0.1074(167.6) + 0.3362(64.3) ≈ 36.4 liters
Hume Formula for Males:
- TBW (L) = 0.194786 × height (cm) + 0.296785 × weight (kg) - 14.012934 1
- For this patient: 0.194786(167.6) + 0.296785(64.3) - 14.012934 ≈ 37.7 liters
Why These Formulae Are Preferred
Both Watson and Hume formulae were validated against isotopic dilution techniques (the gold standard) and included obese subjects in their derivation populations, making them applicable across body habitus ranges. 1
The fixed fraction method (0.60 × body weight for males) is explicitly NOT recommended by NKF-K/DOQI guidelines, as it overestimates total body water even in overhydrated patients. 1
The Watson and Hume formulae provide estimates that are generally close to isotopic body water measurements in clinical populations. 1
Important Clinical Caveats
These formulae systematically underestimate total body water in patients with fluid overload/edema, as they were derived from euvolemic subjects. 1
For edematous patients, add the entire weight of edema fluid to the calculated TBW from dry weight to correct the estimate. 1
Both formulae provide unrealistic estimates in subjects whose height and/or weight differ greatly from ordinary ranges. 1
This patient's weight (64.3 kg) is slightly below the median for his age group (65-69.9 years: 70.3-78.9 kg for males), suggesting he may be on the leaner side, where these formulae remain accurate. 1