What is the recommended method to compute body surface area?

How to Compute Body Surface Area

The Mosteller formula is the recommended method for calculating body surface area (BSA) in clinical practice: BSA (m²) = √[(Height(cm) × Weight(kg))/3600]. 1

Why the Mosteller Formula is Preferred

The Mosteller formula has become the standard for BSA calculation due to its simplicity, ease of use with basic calculators, and clinical applicability across diverse patient populations. 1 While the DuBois and DuBois formula (BSA = 0.007184 × Height^0.725 × Weight^0.425) is historically the most widely used, it was derived from only 9 subjects and systematically underestimates BSA in obese patients by 3-5%. 2

For chemotherapeutic dosing and other medications with narrow therapeutic indices, the Mosteller formula provides the most accurate and clinically validated approach. 1

Step-by-Step Calculation

Using the Mosteller Formula:

• Measure height in centimeters and weight in kilograms 1
• Multiply height × weight 1
• Divide the result by 3600 1
• Take the square root of that number to obtain BSA in m² 1

Example: For a patient who is 170 cm tall and weighs 70 kg:

• BSA = √[(170 × 70)/3600] = √[11,900/3600] = √3.31 = 1.82 m² 1

Clinical Applications Where BSA is Essential

• Chemotherapy dosing: Most cytotoxic agents are dosed per m² to account for variations in drug metabolism and clearance 1
• Cardiac output indexing: Normalizing hemodynamic parameters to body size 3
• Aortic diameter assessment: Indexing aortic measurements to BSA for accurate evaluation of aortic dilatation (ascending thoracic aorta >22 mm/m² indicates dilatation) 3
• Left ventricular mass indexing: Diagnosing left ventricular hypertrophy (LVM/BSA >115 g/m² in men or >95 g/m² in women) 3

Special Populations and Important Caveats

Patients with Abnormal Body Composition

For obese patients (BMI ≥30 kg/m²), the standard Mosteller formula may underestimate BSA, though it remains more accurate than DuBois. 2 The DuBois formula underestimates BSA in obese patients by up to 5% in females and 3% in males. 2

For patients with extremes of body weight (very low or very high), standard BSA formulas have limitations and may require alternative indexing methods. 3 In such cases, consider:

• Indexing by height alone (aorta-height index >32.1 mm/m indicates increased risk) 3
• Using z-scores or nomograms that account for age, sex, and body surface 3

Pediatric Patients

In pediatric populations, the Mosteller formula remains appropriate, though age-specific formulas like Gehan and George may provide additional accuracy. 1 BSA-based dosing is particularly critical in children due to developmental differences in drug metabolism. 1

Patients with Amputations

For amputees, BSA calculations require correction to account for lost body surface area. 3 The correction involves three steps:

1. Calculate the fraction of body weight lost using amputation nomograms 3
2. Calculate BSA at the hypothetical non-amputated weight 3
3. Subtract the fraction of BSA corresponding to the amputated limb(s) using published tables 3

When BSA Must Be Adjusted for Individual Patient Size

Drug Dosing Considerations

For medications requiring renal clearance, BSA-adjusted GFR (eGFRBSAadj in mL/min) should be calculated by multiplying standardized eGFR by the patient's BSA/1.73 m². 3 This adjustment is particularly important in the United States where overweight and obesity are common. 3

The formula is: eGFRBSAadj = eGFR × (patient's BSA/1.73 m²) 3

This allows comparison between eGFR and creatinine clearance for medication dosing decisions. 3

Imaging and Diagnostic Applications

When indexing aortic diameters or cardiac measurements to BSA, use the same imaging modality and measurement method for serial assessments. 3 This ensures consistency in monitoring disease progression or treatment response.

Common Pitfalls to Avoid

• Never use outdated formulas without understanding their limitations: The DuBois formula systematically underestimates BSA in obese patients 2
• Do not assume all BSA formulas are equivalent at extremes of body size: Differences of ≥4.5% in chemotherapy dosing can occur between Mosteller and DuBois formulas in patients at the 50th-95th percentiles for height and weight 4
• Avoid using BSA-indexed values for medication dosing when absolute clearance is needed: For drugs with narrow therapeutic windows, calculate individual BSA-adjusted values rather than using standardized 1.73 m² indexing 3, 1
• Do not forget to correct BSA calculations in amputees: Uncorrected formulas distort the relationship between body water content and degree of obesity 3

Alternative Formulas (For Reference Only)

While the Mosteller formula is recommended, other formulas exist:

• Gehan and George: Based on 401 subjects, uses least squares estimation, considered the most statistically rigorous 5
• Boyd formula: Historical formula with limited modern validation 2
• Haycock formula: Used in some pediatric contexts 2

All these formulas correlate highly (r ≥0.97) with each other in normal-weight individuals, but diverge at extremes of body composition. 2