Formulas for Calculating the Desired Dose - Pharmacology

In the ideal world, the prescriber will write a medication prescription that has a dose that is available in the hospital. For example, the medication prescription is for a 15-mg tablet of Inderal and the hospital has on hand a 15-mg tablet of Inderal.

In the real world, the dose specified in the medical prescription may not be available. The hospital might have 10-mg tablets of Inderal and not the 15-mg tablets prescribed. Instead of asking the prescriber to change the medication order, the nurse calculates the proper medication to give the patient based on the medication order and the dose that is on hand.

In this example, the nurse calculates that the patient should receive 1.5 tablets of Inderal. This means that the nurse must divide one tablet into halves.

There are two methods nurses can use to calculate the desired dose. These are the formula method or the ratio-proportion method. Either method will produce same result. When applying either method, make sure that all the terms are in the same units before calculating the desired dose. For example, the medication order might be in grams and the dose on hand might be in milligrams. The nurse will need to convert the grams to milligrams before calculating the desired dose to give. Always convert to the unit of the “have” dose.

Desired Dose Formula

The formula method uses the following formula to determine the correct dose.

Desired Dose Formula

Quantity (Desired dose divided by dose you have
multiplied by vehicle of drug you have equals
the amount calculated to be given to the patient)

D = desired dose
H = dose you have
V = vehicle you have (tablets or liquids)
A = amount calculated to be given to the patient

Ratio and proportion method

Ratio-and-proportion-method--equation

H is the drug on hand (available)
V is the vehicle or drug form (tablet, capsule, liquid)
D is the desired dose (as prescribed)
x is the unknown amount to give, and
:: stands for “as” or “equal to.”

Multiply the means and the extremes. Solve for x; x is the divisor.

Example: Give 500 mg of ampicillin sodium by mouth when the dose on hand
is in capsules containing 250 mg.
500 mg divided by 250 mg multiplied by 1 capsule = 2 capsules

Formula method:

Formulas-Desired-example

Example: Give 375 mg of ampicillin when it is supplied as 250 mg/5mL. 375 mg divided by 250 mg multiplied by 5 mL = 7.5 mL

Example


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