Lean Body Weight Estimation: James vs Hume Formula
The Janmahasatian formula (often confused with "James") is preferred over the Hume formula for estimating lean body weight in clinical practice, particularly for anesthetic drug dosing and body composition assessment in patients across the adiposity spectrum. 1, 2
Evidence-Based Recommendation
Primary Formula Choice
The Janmahasatian formula demonstrates superior agreement with CT-derived reference standards compared to the traditional Hume equations, with a higher intraclass correlation coefficient (0.87 vs 0.77) and smaller mean differences (3.8 kg vs 6.3 kg) when validated against imaging-based measurements. 2
The Janmahasatian formula is specifically recommended by anesthesia guidelines for calculating lean body weight in clinical practice, using the following equations: 1
- Males: LBW (kg) = [9270 × TBW (kg)] ÷ [6680 + (216 × BMI)]
- Females: LBW (kg) = [9270 × TBW (kg)] ÷ [8780 + (244 × BMI)]
Hume Formula Performance
The Hume formula consistently overestimates lean body mass when compared to dual-energy X-ray absorptiometry (DXA), with a mean difference of 1.36 kg and limits of agreement ranging from -8.26 to 6.52 kg. 3
The Hume formula was originally derived using antipyrine space measurements and shows fair correlation (r = 0.913) with modern DXA measurements, but systematic bias exists. 3
Both Watson and Hume formulae provide similar estimates and were derived from populations including obese subjects, making them suitable for accounting for obesity, though they tend to underestimate body water in overhydrated patients. 4
Clinical Context and Applications
Drug Dosing in Anesthesia
For anesthetic drug dosing in obese patients, lean body weight or adjusted body weight should be used rather than total body weight to minimize the risk of relative overdose and hypotension. 1
The use of appropriate weight parameters is critical, as the NAP5 audit found a disproportionate number of obese patients experienced accidental awareness under general anesthesia due to inadequate dosing. 1
Body Composition Assessment
The Hume formula tends to overestimate lean body mass across different populations, including young and postmenopausal women, with overestimation ranging from 5.1 kg (Hume I) to underestimation of 16.9 kg (Hume II). 5
When validated against DXA in oncologic PET/CT studies, the Janmahasatian-derived lean body mass showed better precision and intercompatibility for standardized uptake value calculations. 2
Peritoneal Dialysis Applications
Both Watson and Hume formulae are recommended as reasonable approximations for estimating total body water volume in peritoneal dialysis patients, despite systematic error, based on ease of determination. 4
These formulae perform poorly in patients with abnormal hydration status, as they consistently underestimate isotopic body water in both lean and obese peritoneal dialysis patients with overhydration. 4
Important Caveats
Population-Specific Limitations
All predictive body weight formulas show reduced accuracy at extremes of body weight, with approximately 45% accuracy in severely malnourished patients (anorexia nervosa) and around 60% accuracy at best in patients with obesity. 6
Both Watson and Hume formulae provide unrealistic estimates in subjects whose height and/or weight differ greatly from ordinary values. 4
Hydration Status Considerations
The error of estimates increases in subjects with abnormalities in body water (hydration), as such subjects were systematically excluded from the studies used to derive the formulae. 4
A proposed correction for edema requires careful assessment of dry weight, where total body water equals the body water at dry weight (from Watson or Hume formulae) plus the weight gain secondary to edema. 4
Clinical Implementation
Predictive equations must be reported and justified when used in clinical studies, as they are technology-specific and population-specific, and different formulas can yield different results for the same parameter. 4
For obese patients requiring nutritional assessment or drug dosing, all four weight parameters (total body weight, ideal body weight, lean body weight, and adjusted body weight) should be calculated for proper clinical decision-making. 1