Standards for Obtaining Accurate Weight Prior to General Anesthesia
All adult patients must have their actual total body weight measured directly using a calibrated scale prior to general anesthesia, as visual estimates by healthcare providers are inaccurate in approximately 43-49% of cases and should never be used for medication dosing. 1, 2
Direct Weight Measurement is Mandatory
- Measured weight on a calibrated scale is the only acceptable method for obtaining weight prior to anesthesia, as healthcare workers (including consultants, nurses, and support staff) correctly estimate weight within ±10% in only 57% of patients 2
- Visual estimates by physicians are accurate within 10% in only 54% of cases, while nurse estimates achieve 51% accuracy 1
- Patient self-reported weight is more accurate than provider estimates (86% accuracy within 10%), but remains too unreliable for drug dosing and should not be used as a substitute for measured weight 1, 2
Critical Pitfall: Electronic Bed Scales Are Inadequate
- Electronic weighing beds should not be used as the primary method for obtaining preoperative weight, as they demonstrate poor agreement with calibrated scales (95% CI: -7.6 to 7.6 kg difference) 3
- Electronic bed weights are confounded by nonremovable items averaging 3.5 kg (SD 1.4 kg) and lack sufficient accuracy for medication dosing 3
Weight Documentation Requirements
Once total body weight is measured, calculate and document the following weight parameters for obese patients (BMI ≥30 kg/m²):
Ideal Body Weight (IBW) 4
- Males: IBW (kg) = height (cm) ÷ 100
- Females: IBW (kg) = height (cm) ÷ 105
Lean Body Weight (LBW) using Janmahasatian formula 4, 5
- Males: LBW (kg) = [9270 × TBW (kg)] ÷ [6680 + (216 × BMI)]
- Females: LBW (kg) = [9270 × TBW (kg)] ÷ [8780 + (244 × BMI)]
- Note: LBW rarely exceeds 100 kg in men or 70 kg in women regardless of total body weight 4, 5
Adjusted Body Weight (ABW) 4, 5, 6
- ABW (kg) = IBW (kg) + 0.4 × [TBW (kg) - IBW (kg)]
- This accounts for increased lean body mass in obese individuals 6
Clinical Application for Drug Dosing
- For most anesthetic agents in obese patients, use lean body weight or adjusted body weight—never total body weight—as dosing to total body weight increases the risk of relative overdose and hypotension 4, 5
- The Association of Anaesthetists of Great Britain and Ireland specifically recommends against using total body weight for anesthetic drug calculations in obese patients 5
- This recommendation is particularly critical given that the NAP5 audit found a disproportionate number of obese patients experienced accidental awareness under general anesthesia, with half occurring during induction when inadequate dosing based on lean/adjusted body weight was not quickly followed by maintenance anesthesia 4
Special Populations
Obese Patients (BMI ≥30 kg/m²)
- Automatic acceptance for day surgery if BMI <40 kg/m² 4
- Case-by-case review required for BMI ≥40 kg/m² 4
- Calculate all four weight parameters (TBW, IBW, LBW, ABW) for proper drug dosing 4