Some medications have very narrow therapeutic ranges and the dosing must be very closely managed.  In these situations, rather than ordering a specified amount of a medication, the physician will order a dosage based on the patient's weight in either pounds or kilograms.  Before you can determne how many tablets to give or how many milliliters to prepare in an injection, you have to first calculate the dosage amount.

The left side of the ratio for these problems will be the physician's order.  As usual, you will put matching units on the right side, "plug in" numbers into the appropriate location. on the right, cross multiply, and solve for x.


Example problem #1:   Ordered is 3 mg/kg of a medication.  The patient weighs 75 kg.


     3 mg   =     X mg
      1 kg          75 kg

         X  =   225 mg


Example problem #2:  The physician orders  5 mcg/kg/min of dopamine.  The patient weighs 48 pounds.  How much dopamine will the patient receive in an hour?

Basic ratio:

     5 mcg/min   =       mcg/min
       1 kg                         kg


You need kg for the ratio and you have the patient' s weight in pounds.  Therefore, you must first convert the pounds to kilograms.

     1 kg     =   x kg
    2.2 lbs      48 lbs

      2.2 X   =   48

      2.2  X    =   48
      2.2             2.2

        X   =   21.81 = 21.8  kg   (PCC calculation rule:  less than 100 pounds, final answer to nearest tenth.)

Now back to problem:

      5 mcg/min   =    X  mcg/min
        1 kg                   21.8 kg

           X   =    109 mcg/min

Howver, this problem asked how much dopamine the patient would recieve in an hour.  So if receiving 108 mcg in a minute, then multiply by 60 to get the amount in an hour.  109  X  60  =  6540 mcg  in an hour.