…continuing the discussion of Standardization of Body Surface Area Standard Calculations
Back to BSA report page 1. by Thanh Vu, B.Sc. (pharm),
The Dubois Body Surface Area Standard Formula
Currently several institutions use the DuBois formula. There are several arguments for and against the use of the Du Bois formula for calculating body surface area.
In favor of the DuBois formula is the fact that the nine subjects used to derive it were of diverse body shapes.
The accuracy of the Dubois Body Surface Area Standard Formula?
However, many investigators have since questioned the accuracy of the formula. Results from a study by Jones et al5 indicate that the equation is not the most accurate of those available.
Jones (et al.) noted that for their formula DuBois and DuBois only measured one leg and one arm, assuming the body to be symmetrical. The assumption of bodily symmetry has since been found to be invalid6. Jones (et al.) also reported that the formula grossly overestimates the surface area of obese people.
Mitchell (et al.)7 compared the measured surface area with predicted surface area from the DuBois formula. The comparison shows that although the two methods highly correlate, the predicted area was systematically lower than the measured area.
The underestimate is higher in small individuals, thus the authors suggest that the Du Bois formula may be invalid when the predicted surface area is less than 1.3 m2 (i.e. in children).
Haycock (et al.)8 reported that the Du Bois formula underestimates the body surface area by up to 8% in infants especially when values fall below 0.7 m2.
Wand and colleagues support the above results in their 1992 study. Wand claims that the Du Bois formula is derived from a small number of subjects, several of whom had skeletal deformities and included only one child.
Wang questions the validity and accuracy of the formula for infants and for individuals with extremes of stature, such as those who are pregnant, obese, or very tall.
The Root Mean Squared Error
Wang (et al.) assessed the Du Bois formula for accuracy using the Root Mean Squared Error (RMSE) method of prediction.
The RMSE measures concordance between measured and predicted data. They found that the Du Bois formula systematically underestimates BSA by almost 5%. Furthermore, the tendency to underestimate BSA is slightly greater in infants than in other people. However, this difference is not likely to be clinically relevant9.
The results from the RMSE method of preduction by Wang shows that of the 15 different formulae for predicting BSA, 8 have an RMSE less than 8%.
The Du Bois formula ranks fifth for the lowest RMSE (Table 1).
However, the Du Bois formula continues to be in use, probably more due to tradition than to accuracy. Indeed, a number of major drug producers still provide their clients with nomograms based on this formula.
Turcotte observes that nomograms printed in several standard texts were reproduced incorrectly from the original. The incorrect nomograms lead to systematic under-estimation of surface area.
Furthermore, the compounded risks of using such nomograms may be quite large. To put this in perspective, a change in BSA from 1.87 to 1.60 is equivalent to a weight loss of 22kg in a woman who weight 80.5kg and is 158cm tall.
This would correspond to a change in a doxorubicin dose (50mg/m2) from 93.5mg to 80mg).
Why is it important to have a body surface area standard formula?
It is particularly important to standardize BSA calculation at an institution that conducts clinical trials.
There are several tools available to aid in the calculation of BSA including slide rules, nomograms, purpose built calculators and software.
Several types of slide rules and calculators are currently in use at the Cross Cancer Institute (CCI). However, The way BSA is determined at the CCI is still not consistent.
Some health care professionals use a BSA calculator while others use a slide rule or nomogram. The way to calculate body surface needs to be standardized in order to maintain consistency. Furthermore, it is necessary that the pharmacy receives a patient’s height and weight in order to confirm the dosage.
In a pharmacy setting, the BSA needs to be calculated to three significant figures.
Slide rules and nomograms are incapable of calculating with this degree of accuracy. In addition, slide rules and nomograms suffer from error based on the underlying formula. For example, after studying a three-scale nomographic chart designed by Boothby and Sandiford, that isbased on the Du Bois formula, Turcotte determined that BSA values obtained from it were smaller than the Du Bois values by 8%10.
The Need for Body surface Area Standard Formula
Recalling that the Du Bois formula underestimated actual BSA by 5%, according to Wang it is clear that this tool underestimates BSA by 13%.
Purpose built calculators are more accurate in their body surface area standard calculations. However, some calculators also rely on an inaccurate formulae.
The solution for an institution that enters all of its orders on a central computer system available to pharmacists, nurses, technicians, and physicians alike, is simple. Software is available that uses a reliable formula and also calculates BSA to three significant figures with unwavering certainty and accuracy.