12. Plotting and interpreting BMI-for-age using the metric system

A Case Study of ‘Sam’

Step 1. Obtain accurate weights and height measurements

Date of Birth (DOB): 9/15/94

Date of Visit (DOV): 4/4/98

Weight = 16.9 kg

Height = 105.4 cm

Step 2. Select the appropriate growth chart

Because Sam is a normal 3.5 years old, a standing height was obtained. The appropriate chart to use is the ‘Boys 2 to 20 BMI-for-age’ chart.

Step 3. Record the data

On the data entry table found on the clinical growth chart, information relevant to the growth chart is recorded. Here data has been recorded for the date of visit, the child’s age, weight and height at the visit.

Step 4. Calculate BMI

Calculate BMI at the time of the clinic visit.

Calculations:

BMI: (16.9 kg/ 105.4 cm/ 105.4 cm) x 10,000 = 15.2

Then the BMI of 15.2 is entered on the data entry table.

Step 5. Plot measurements

All the necessary information is recorded and Sam’s BMI can be plotted. On the BMI-for-age chart, find Sam’s age on the horizontal axis and visually draw a vertical line up from that point. Then find his BMI on the vertical axis and visually draw a horizontal line across from that point. The point where the two intersect represents Sam’s BMI-for-age.

When plotted on the growth chart, Sam’s BMI-for-age falls just below the 25th percentile curve.

Step 6. Interpret the plotted measurements

Since Sam’s BMI-for-age falls just below the 25th percentile curve, this means that of 100 children with the same gender and age as Sam’s, fewer than 25 children have a BMI-for-age lower than his and more than 75 have a BMI-for-age higher than his. Sam is not overweight, underweight or at risk of overweight.

When a child’s plotted measurement falls between the 5th and 85th percentiles it is considered to be within the normal range. Sam’s BMI-for-age is in the normal range based on this measurement.