Archie who weighs 44 lb has been given an order for amoxicillin 500 mg b.i.d. The drug text notes that the daily dose of amoxicillin is 50 mg/kg/day in two divided doses. What dose in milligrams is safest for this child?
A. 1000 mg
B. 750 mg
C. 500 mg
D. 250 mg
Correct Answer: C. 500 mg
First, calculate the child’s weight in kg: 44/2.2 = 20 kg. Then calculate the appropriate daily dose according to the drug text: 50 mg/kg/day = 50 mg x 20 kg = 1,000 mg/day. the abbreviation b.i.d. means twice daily; therefore 1,000 divided by 2 equals 500 mg.
Option A: There are 3 primary methods for the calculation of medication dosages, as referenced above. These include Desired Over Have Method or Formula, Dimensional Analysis, and Ratio and Proportion (as cited in Boyer, 2002).
Option B: Desired over Have or Formula Method is a formula or equation to solve for an unknown quantity (x), much like ratio proportion. Drug calculations require the use of conversion factors, such as when converting from pounds to kilograms or liters to milliliters. Simplistic in design, this method allows us to work with various units of measurement, converting factors to find our answer.
Option D: It is useful in checking the accuracy of the other calculation methods as mentioned above, thus acting as a double or triple check. A basic formula, solving for x, guides us in the setting up of an equation: D/H x Q = x, or Desired dose (amount) = ordered Dose amount/amount on Hand x Quantity.