1. Calculating Pediatric Dosage
Unit 47
Calculating Pediatric Dosage

2. Basic Principles of Calculating Pediatric Dosage
Correct medication dosages for infants and children are based on weight, height, body surface area (BSA), and age.
An accurate dose is usually a fraction of the amount of medication given to an adult.
It is extremely important to calculate dosages accurately: a minor error can be extremely serious, causing an overdose and even death.

3. Basic Principles of Calculating Pediatric Dosage
There are only two acceptable ways to determine pediatric dosage:
Calculating dosage based on kilograms or pounds of body weight.
Calculating dosage based on body surface area (BSA).

4. Calculation of Dosage Based on Body Weight
Most drug manufacturers and medication reference books such as the Physician’s Desk Reference (PDR) provide a recommended pediatric dosage based on kilograms (kg) of body weight.

5. Calculation of Dosage Based on Body Weight
The infant or child must be weighed.
Usually weight is recorded in pounds and ounces or pounds and fractions of pounds.
The pounds must first be converted to kilograms.
Dosage can be calculated based on the child’s weight in kilograms.

6. Calculation of Dosage Based on Body Surface Area (BSA)
The BSA method is considered to be one of the most accurate methods of calculating dosages because it relies on both the height and weight of the child.
A nomogram is used to determine the BSA.

7. Calculation of Dosage Based on Body Surface Area (BSA)
To determine the BSA, the height of the child in inches (in) or centimeters (cm) is located on the left column of the nomogram.
The weight of the child in kilograms (kg) or pounds (lb) is located on the right column.
A straight edge is used to connect the two points.
The point of intersection on the middle column provides the BSA in m2 (meters squared).

8. Calculation of Dosage Based on Body Surface Area (BSA)
Many drug manufacturers now provide a recommended dose per m2.
The formula used to calculate dosage when this information is available is:

9. Calculating Pediatric Dosage
Example: A physician orders methotrexate 25 mg IM for an 8-year-old girl with cancer. The manufacturer’s recommended dose is 30 mg/m2. If a nomogram shows the child’s BSA is 0.9 m2, is 25 mg IM a safe dose for this girl?
Use the formula:
1X = 30 x 0.9
X = 27
The child’s dose is 27 mg; the ordered dose of 25 mg is safe for this child.

10. Practical Problem
A physician orders Vantin 50 mg po (by mouth) q12h (every 12 hours) for a child who weighs 22 pounds. The manufacturer recommends a dose of 5 mg/kg q12h.
Is 50 mg a safe dose for the infant?

11. Practical Problem
Convert the infant’s weight to kilograms, set up the formula and solve:
22 lbs = 10 kg
1X = 50
50 mg q12 is the correct dose.
50 mg q12 is a safe dosage for this child.