CALCULATING BODY SURFACE AREA (BSA).

Body Surface Area (BSA) is very important in pediatric drug dosage calculation.

There are two formulae that are generally used to calculate a child’s BSA.

1. BSA= Sq. Root [(weight in kg * height in cm) ÷ 3600]
This is used when the weight is expressed in kilogram (kg) and the height is expressed in centimeter (cm).

2. BSA= Sq. Root [(weight in pounds * height in inches) ÷ 3131]
This is used when the weight is expressed in pounds (lbs) and the height is expressed in inches (in)

Example 1:
Calculate the BSA of a child who weighs 65lbs and 5feet 8inches tall.

Weight= 65lb
Height= 5ft 8in
Convert 5ft to inches, then add it to 8in
1ft= 12in
5ft= 5*12= 60in
Height= 60 + 8= 68in

BSA= Sq. Root [(weight in pounds * height in inches) ÷ 3131]

BSA= Sq.root [(65*68)÷3131]
= sq.root [4420÷3131]
= sq.root [1.41]
= 1.19
BSA of the child is 1.19 sq meter

Example 2:
Calculate the BSA of a child who weighs 11.3kg and 75cm tall.

Weight= 11.3kg
Height= 75cm

BSA= Sq. Root [(weight in kg * height in cm) ÷ 3600]

= sq.root [(11.3*75)÷3600]
= sq.root [847.5÷3600]
= sq.root [0.24]
= 0.49
BSA of the child is 0.49 sq meter

Example 3:
A child is to receive dactinomycin 2.5mg/m2 IVP daily for 5days as treatment for Leukemia. The child weighs 48lbs and 4feet 6inches tall. Calculate the dose of dactinomycin to be given each day.

The first step is to calculate the BSA of the child.
Weight= 48lbs
Height= 4ft 6inches (1ft= 12in)
Height= (4*12) + 6= 48+6= 54in

BSA= Sq. Root [(weight in pounds * height in inches) ÷ 3131]

= sq root [(48*54)÷3131]
= sq root [2592÷3131]
= sq root [0.83]
= 0.91
BSA of the child is 0.91 sq meter.

Ordered dose of dactinomycin= 2.5mg per sq meter
Since BSA has been calculated to be 0.91 sq meter
Ordered dose of dactinomycin= 2.5mg*0.91
= 2.28mg

Example 4:
What is the single dose of Phenytoin for a child who is 112cm in length and weighs 25.3kg if the recommended dose is 250mg/m2 divided in 2 doses?
Phenytoin is available as 125mg/5ml. How much would you give for a single dose?

Weight= 25.3kg
Height= 112cm

BSA= Sq. Root [(weight in kg * height in cm) ÷ 3600]

= sq root [(25.3*112)÷3600]
= sq root [2833.6÷3600]
= sq root [0.79]
= 0.89
BSA of the child is 0.89 sq meter

Ordered dose of Phenytoin= 250mg per sq meter in two divided doses (BSA is 0.89 sq meter)
= 250mg*0.89
= 222.5mg in 2 divided doses
Each dose= 222.5mg÷2= 111.25mg

OGUNLABI, David (RN).