Dimensional Analysis
Calculations in pharmacy can be performed by dimensional analysis. This method involves placing the ratios of the data and the equivalence between the units in a way that like units will cancel out and only the desired terms will be left.
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Solved Problem: If the adult dose of a solution is 0.2 mL/kg of body weight to be administered once daily, how many teaspoonful (tsp) should be administered to a person weighing 220 lb? (1 kg = 2.2 lb, 1 tsp = 5 mL) Approach: $${0.2ml \over kg} \times {1kg \over 2.2lb} \times {220lb} \times {1 \over 5 \ ml}$$ Answer: 4 tsp. |
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Solved Problem: A medication order calls for 500 mL of D5W/NS solution to be infused over 6 hours using an administration set that delivers 15 drops per milliliter. How many drops per minute should be delivered to the patient? Approach: $${500ml \over 6 \ hours} \times {1hr \over 60 \ minutes} \times {15drops \over ml}$$ Answer: 21 drops |