Pharmacology · chapter 2 · Drug Administration

Dosage Calculations

The 14 things this section of the textbook says that you are most likely to be asked about — quoted word for word, not retold. Then 5 practice questions with the reasoning.

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Key points

Dosage calculation safety
Dosage calculations are an important part of drug administration, and the safety of the client depends on the nurse’s ability to correctly calculate the dosage needed at any point in time.
When calculation is needed
Drugs are not always available in the exact dosage or units of measurement that the client needs or the provider ordered. Knowing how to accurately calculate the amount needed is vital for the safety of the client.
Common measurement systems
The apothecary system, the household system, and the metric system are three of the most common.
Apothecary system precautions
This system is very difficult to use, and the Joint Commission, the FDA, and the Institute of Safe Medication Practice (ISMP) have recommended that it be discontinued. It is now seldom used, and the metric system was adopted.
Household measurement errors
Unfortunately, there has been much confusion in using this system, which leads to dosing errors. Thus, it is no longer used.
Metric conversion basis
The basic units are multiplied or divided by multiples of 10, which increases the ease of use when converting from one unit of measure to another.
Micro abbreviation risk
Note that the abbreviation for “micro” is sometimes shown as the Greek mu (µ, as in µg or µL); however, that practice is considered to be error prone by the Institute for Safe Medication Practices and has been supplanted by “mc,” as in “mcg” for microgram or “mcL” for microliter.
Drug label components
Components of a drug label (see Figure 2.15 and Figure 2.16) include drug generic and brand names, the strength of the drug, drug form, route of administration (if indicated), dosage and administration instructions, expiration date, controlled substance notification (if indicated), drug reconstitution (if indicated), and if the drug is a single or multidose vial (if indicated).
Calculation method options
There are three general methods for drug calculation: basic formula method, ratio and proportion, and dimensional analysis.
Matching units in formulas
Note: It is extremely important that the units of measure are the same when doing these calculations. If the desired dose is in mg, the on-hand amount must also be in mg.
Ratio unit conversion
Note that if they are not the same, you must convert them to the same unit of measurement prior to solving for X.
Dimensional analysis purpose
Dimensional analysis uses a series of equivalent measurements to change one unit of measurement to another to solve a problem.
Body weight dosing use
This method is commonly seen with medications with a narrow therapeutic index, drugs being adjusted due to the individual’s ability to metabolize the drug, and pediatric clients.
Medication dose rounding
Nurses must know when and how to round medication doses. Caplet and tablet doses can be rounded to the nearest half-tablet if they are scored.

Terms to know

apothecary system: One of the older systems of measurement is the apothecary system, and it has been used by apothecaries or pharmacists for the last couple of centuries.
household system: The household system (also known as the customary system) includes units of measure such as tablespoons, teaspoons, pounds, and ounces.
volume: Volume measures a liquid and is commonly used in dosage calculation when measuring liquid drugs, in the reconstitution of drugs, or with intravenous therapy.
basic formula method: The basic formula method, also known as Desire Over Have, is the simplest of the drug calculation methods.
ratio and proportion method: The ratio and proportion method uses a linear equation to solve the dosage calculation problem.
dimensional analysis: Dimensional analysis uses a series of equivalent measurements to change one unit of measurement to another to solve a problem.
body weight method: The body weight method allows dosage calculation for individualization.
body surface area (BSA) method: The body surface area (BSA) method allows dosage calculation for individualization.

Practice questions

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A nurse receives a medication order written using an apothecary measurement with Roman numerals. What is the safest action before administering the medication?
1. Ask another nurse to estimate the dose based on experience with similar orders.
2. Convert the order to a household measurement such as teaspoons for easier administration.
3. Collaborate with the pharmacist or use a table of weights and measures to adapt the order to the metric system.
4. Administer the medication as written because apothecary measurements are still commonly used.
Answer: Collaborate with the pharmacist or use a table of weights and measures to adapt the order to the metric system.
The section states that apothecary measurements are seldom used and that the system has been recommended for discontinuation. The safe nursing action is to adapt the order to the metric system through pharmacist collaboration or a table rather than estimating or using household measures.
A nurse is checking a medication label before administration. Which label components should the nurse verify to safely administer the drug? Select all that apply.
1. Generic and brand names
2. Client’s body surface area
3. Strength of the drug
4. Whether the vial is single or multidose, if indicated
5. Expiration date
Answer: Generic and brand names, Strength of the drug, Whether the vial is single or multidose, if indicated, Expiration date
Drug labels include identifying information such as names, strength, expiration date, and single- or multidose vial status when indicated. Body surface area may be used in dosage calculations, but it is not listed as a drug label component in this section.
A provider orders furosemide 60 mg IV push now. The medication available is furosemide 20 mg in 2 mL. How many mL should the nurse administer?
1. 2 mL
2. 12 mL
3. 3 mL
4. 6 mL
Answer: 6 mL
Using the basic formula method, the ordered dose is divided by the dose on hand and multiplied by the available volume: 60 mg ÷ 20 mg × 2 mL = 6 mL. The other options result from not applying the full ordered-to-available dose relationship.
A nurse is preparing calculated medication doses for administration. Which rounding or administration decisions are consistent with the rounding rules in this section? Select all that apply.
1. Round a medication ordered in drops to the nearest whole number.
2. Round a scored tablet dose to the nearest half-tablet.
3. Round a liquid medication dose to the nearest tenth.
4. Round a capsule dose to the nearest whole number.
5. Split or crush an extended-release tablet if the calculated dose is one-half tablet.
Answer: Round a medication ordered in drops to the nearest whole number., Round a scored tablet dose to the nearest half-tablet., Round a liquid medication dose to the nearest tenth., Round a capsule dose to the nearest whole number.
The section allows scored tablets to be rounded to appropriate scored portions, capsules to whole numbers, liquids to tenths, and drops to whole numbers. Extended-release, controlled-release, sustained-release, or enteric-coated tablets should not be split or crushed.
A client weighing 110 lb has an order for amoxicillin 10 mg per kg orally QID. The available medication is amoxicillin 125 mg/5 mL. Which steps or results match the body weight method example? Select all that apply.
1. Calculate 10 mg × 50 kg = 500 mg per dose.
2. Convert 110 lb to 50 kg.
3. Administer 20 mL per dose.
4. Use 110 kg as the client’s weight because the order is already in mg/kg.
5. Calculate the dose as 125 mg per dose because that is the concentration on hand.
Answer: Calculate 10 mg × 50 kg = 500 mg per dose., Convert 110 lb to 50 kg., Administer 20 mL per dose.
The body weight method first converts pounds to kilograms, then multiplies the drug dose by body weight, then uses a dosage calculation method to find the volume. Using 110 kg skips the required conversion, and 125 mg is the available concentration, not the ordered client dose.

Where every quote comes from

Section 2.4 Dosage Calculations of Pharmacology for Nurses by Tina Barbour-Taylor, Leah Mueller (Sabato), Donna Paris, Dorie Weaver, OpenStax, 2024. Read the whole section free at openstax.org.

The textbook is licensed under CC BY-NC-SA 4.0. This page quotes it word for word and adds the headings, the order and the practice questions; references to the book's figures and stray spaces left by its formatting are removed. The page is shared under the same licence. Nothing here is sold.

Key points