Dose by Weight

Many medications according to the patient's weight.  This is especially true with pediatric dosages.  Safe dosage can also be assessed when the ordered dose is known along with the patient weight.  Most drug handbooks use weights in kilograms.  Patients are often weighed in pounds, so a conversion from pounds to kilograms is often the first step in calculating dose by weight or assessing safe doses.

To convert pounds to kilograms, an important equivalency to remember is 2.2 lb. = 1 kg.  Divide pounds by 2.2 to get the weight in kg.

Weight in pounds divided by 2.2 lb = weight in kg

Example: 

If a child weighs 88 lb, what is his weight in kg?
88 / 2.2 = 40 kg

To calculate a patient's dose using his weight, consider this example.

Example:

Order reads: Give "Drug X" 3 mg/kg per day in two divided doses.
Patient weighs 22 lbs. or 10 kg

Step 1. Calculate the daily dose (3 mg X 10 kg = 30 mg per day)

Step 2. Divide the daily dose (30 mg) by number of doses per day (2) to = mg per dose
              30 / 2 = 15 mg per dose

For a dosage of medication to be safe, it must fall within the safe range as listed in a Drug Handbook, PDR or other reliable drug reference.  Several steps are needed:

1. Calculate the daily dose ordered
2. Calculate the low and high parameters of safe range
3. Compare the patient's daily dose to the safe range to see if it falls within it

Example:

A child who weighs 66 lb is receiving 125 mg of Drug "Q" tid
The safe range is 10 - 20 mg/kg/day

Patient weight in kg is 66 / 2.2 = 30 kg

Step 1. The daily dose is 125 mg x 3 doses per day = 375 mg/day
Step 2. The low safe dose is 10mg x 30 kg/day = 300 mg/day; 
              The high safe dose is 20 mg x 30 kg/day = 600 mg/day
                
Step 3. Compare - Does 375 mg per day (the pt. dose) fall between 300 and 600 mg/day?  
             Yes. Therefore, dose is safe

Test your knowledge in Quiz 6

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