14 Dosage Calculations

Delivering medications is another critical function that Jolene performs many times on every shift and mistakes in dosage can have very serious consequences. Dosage calculations ensure patients receive the correct amount of each medication and Jolene uses the chain-link method of unit analysis to ensure that her patients receive correct dosages. “Three primary methods for calculation of medication dosages exist, and these include dimensional analysis, ratio proportion, and formula or desired-over-have method. Commonly used in solving chemistry and physics problems, dimensional analysis is fast becoming the go-to method for dosage calculations in nursing and the medical profession. Chances for error are diminished, thus increasing the popularity of these dosing calculations.”[1]

Everyday Examples: Dosage Calculation

An MD orders 300 mg of Ibuprofen to be taken by a 6 kg infant every 4 hours. The label
shows 75 – 150 mg/kg per day max. Is the physician’s order within normal range?

First let’s calculate the max dosage for this infant:

    \begin{equation*} 150 \,\bold{mg/kg/day}= \left(150 \,\frac{\bold{mg}}{\cancel{\bold{kg}}\,day}\right)\left(6 \,\cancel{\bold{kg}}\right) = 900 \,\frac{\bold{mg}}{day} \end{equation}

Now let’s calculate the ordered dose.

    \begin{equation*} 300 \,\bold{mg}/(4 \, \bold{hrs})= \left(300 \,\frac{\bold{mg}}{4\,\cancel{\bold{hrs}}}\right)\left(\frac{24 \,\cancel{\bold{hrs}}}{1 \,day}\right) = 1800 \,\frac{\bold{mg}}{day} \end{equation}

The dosage is not within the normal range.

[2]

Exercises

  1. "Dose Calculation" by Tammy J. Toney-Butler and Lance Wilcox, National Library of Medicine, National Institutes of Health ↵
  2. Adapted from "Medication Calculation Examination Study Guide " by Public Health Nursing, County of Los Angeles ↵
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