Patients may weigh the same but vary considerably in actual body size.  Therefore, an even more precise determinant of a safe dosage is body surface area (BSA).  Body surface is calculated in square meters (m2) using the patient's height and weight.  Your drug dosage workbook gives you formulas for calculating surface area.  YOU WILL NOT HAVE TO KNOW OR USE THESE FORMULAS.  In problems on drug dosage tests, you will either be given the patient's surface area or you will be given the patient's height and weight and will use a nomogram to determine the surface area.

The left hand side of the ratio will be the physician's order and as usual, the right hand side must match the left side.


Example problem#1:  Ordered is vinblastine 7.4 mg/m2.  The patient's BSA is 1.91 m2.


       7.4 mg   =     X   mg
        1 m2           1.91 m2

       X  =   14.134  = 14 mg



How to use a nomogram: You have a nomogram in the course packet.   Note that the nomogram has 4 columns.  The first column to the left is the height column.  Note that it has data in two columns with the left column in centimenters (cm) and the right column in inches (in).  The column to the far right is the weight column.  Note that it also has data in two columns with the left column in  pounds (lb) and the right column iin kilograms (kg).  You must make very, very sure that you use the correct columns for the data you are given!

To use the nomogram, find your patient's weight in the appropriate weight column (pounds or kilograms) and find the patient's height in the appropriate height column (centimeters or inches).  Carefully move a ruler (or edge of a piece of paper)  to connect these two points and draw a line to connect them.  This line will cross the third column labeled SA for surface area---this is the patient's surface area in square meters (m2).

There is a blocked column labeled "For children of normal height and weight".  That column is used only when you have a child of normal height for weight and only when the only data you have is weight.  If you are given height and weight, you will not use this column; you will use the height and weight columns.


Example problem#2:  The physician orders carmustine 150 mg/m2.  The patient weighs 65 pounds and is 32 inches tall.  What is the correct dose?

You do not have the patient's surface area so need to use the nomogram.  Plotting the points in the appropriate columns and drawing the line, you will find that the line crosses the SA line at 0.88.  Therefore, the patient's BSA is 0.88 m2.


     150 mg    =     X mg
        1 m2           0.88 m2

         X   =   132 mg