22 Pediatric & Adult Dosage Calculations

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Drug references like the United States Pharmacopoeia can supply dosage information for adults, but not always for children. Doses that can be taken by children are typically provided by the manufacturer and if not, may be calculated by the prescriber or pharmacy using the following formulas.

Clark’s Rule: Child’s Dose Equals
$$\frac{\text{Adult Dose} \times \text{Child’s Weight (lb)}}{\text{Avg. Adult Weight (~150 lb)}}$$

Now this is not the most precise method as other factors may need to be taken into account such as condition, age, height, etc.

Young’s Rule: Child’s Dose Equals
$$\frac{\text{Adult Dose} \times \text{Child’s Age (Yr.)}}{\text{Child’s Age (Yr.) + 12}}$$
Fried’s Rule For Infants: Infant’s Dose Equals
$$\frac{\text{Adult Dose} \times \text{Infant’s Age (Mo.)}}{\text{Avg. Adult Weight (~150 lb)}}$$

Another formula uses body surface area (or BSA), which is based upon a person’s height & weight. BSA is frequently used with patients receiving chemotherapy as well and is measured in square meters (m^2), where doses are commonly given in mg/m^2. Body surface area is traditionally calculated by using a chart called a nomogram which has two inputs: the weight (in kg or lb) and the height (in cm or inches). There is an adult nomogram and a nomogram for children. To use these charts, just identify where a line drawn from the weight column to the height column intersects with the BSA column.

It is important to remember that the pharmacy should always check children’s doses to make sure they are appropriate since conversion formulas for pediatric doses have many variables. This can be checked with a suitable drug information resource.

Body Surface Area (BSA): Child’s Dose Equals
$$\frac{\text{Adult Dose} \times \text{Child’s BSA}}{\text{Avg. Adult BSA (~ 1.73 m}^2\text{)}}$$

Lastly, you should always pay special attention to the unit of weight being used to calculate the child’s dose. For example, if you’re asked to calculate a 22 lb child’s dose to be given 20mg/kg twice a day then make sure to convert the child’s weight from lb to kg first and vice-versa. Now just so you know, on the exam you may need to use 1 kg = 2.2 lb as the unit factor instead of 2.204624 lb:

Example of Calculating Dose by Weight: Dose Equals
$$\text{22 lb} \times \frac{\text{1 kg}}{\text{2.2 lb}} = \text{10 kg}$$
$$\text{10 kg} \times \frac{\text{20 mg}}{\text{1 kg}} = \text{200 mg}$$

Let’s practice: The recommended adult dose for daptomycin is 4 mg/kg IV infusion q24hr for 7-14 days. What would the dose be for a 7 month old infant in mcg using Fried’s Rule For Infants?

$$\frac{\text{4 mg} \times \text{7 months}}{\text{150 lb}} = \text{0.1866 mg}$$

$$\text{0.1866 mg} \times \frac{\text{1,000 mcg}}{\text{1 mg}} = \text{186.67 mcg}$$

The recommended adult dose for dramamine (dimenhydrinate) is 50 mg PO/IV/IM q4-6hr PRN, 30 minutes before exposure to motion; not to exceed 400 mg/day. What would the pediatric dose be for an 11 year old child using Young’s Rule?

$$\frac{\text{50 mg} \times \text{11 Years}}{\text{11 Years + 12}} = \text{24 mg}$$

How about for a 90 lb child using Clark’s Rule?

$$\frac{\text{50 mg} \times \text{90 lb}}{\text{150 lb}} = \text{30 mg}$$

An adolescent weighs 250 lbs, how much do they weight in kg? This time use 1 kg = 2.204624 lb as the unit factor instead of 2.2 lb.

$$\frac{\text{250 lb} \times \text{1 kg}}{\text{2.204624 lb}} = \text{113.398 lb}$$

Finally, what’s the dose for a child with a BSA of 0.7 m^2 if the adult dose is 300 mg.

$$\frac{\text{300 mg} \times \text{0.7 m}^2}{\text{1.73 m}^2} = \text{121 mg}$$

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