The systems of medication measurement are the apothecary, metric, and household systems. Nurses should be proficient in the use of these systems of medication measurements to administer medications safely.
Incorrect measurements, conversions, or drug calculations will affect the dose of medication a patient receives. Mistakes in calculating medications often lead to fatal errors. These mistakes in measuring and calculating medications could cause harm to the patient.
It is important for a nurse to have knowledge of the systems of medication measurements to calculate drug dosages and prepare medications for administration safely. The nurse is responsible for checking conversions and calculations carefully before giving medications. This article will cover the systems of medication measurement (apothecary system, metric system, household system) and, dimensional analysis for medication calculations.
Systems of Medication Measurement – The Apothecary System
We will cover the apothecary system first. This system was used in the past but is no longer used for medication measurements. The apothecary system originated as a system for dispensing medications using weights and measures.
At one-time nursing students had to learn to convert this system to the metric system. These were the days of grains (gr), minims (m), drams (dr), and ounces (oz). I remember having to learn this conversion system but I don’t remember using it in clinical practice.
In, using this conversion system, grains convert to milligrams, minims (m) and drams (dr) convert to milliliter (mL), and ounces (oz) convert to milliliters. Also, below is a chart with some common apothecary to metric conversion just in case you are curious.
There are two systems of measurement in the United States for administering medications. These systems are the metric system and the household system.
Systems of Medication Measurement – Metric System
First, there is the metric system. The metric system is a decimal system most used in hospitals and clinics. This system is easy to convert using simple multiplication and division. Most consider this system easier because the measures differ from each other using powers of ten (10). This allows conversions to be made by moving the decimal point to the right or left. The decimal point moves to the right when you multiply and to the left when you divide.
When writing metric doses you should always INCLUDE a leading zero and EXCLUDE a trailing zero. This is the same system used for medication orders. You should round your calculations. Be very careful! Misplaced decimal points can change the drug dose by a multiple of 10. When testing in the classroom, your professor will normally tell you how many decimal points you need to round your answers.
Metric System Leading and Trailing Zeros
The ISMP’s Guideline for Standard Order Sets states medication orders should include:
“leading zeros (e.g., 0.1 mg) when expressing medication doses (or other numerical values, as appropriate)” when writing medication orders. And orders should exclude “trailing zeros (e.g., 1.0 mg) when expressing medication doses (or other numerical values, as appropriate).”
Metric System Rounding Rules for NCLEX
Metric doses can be rounded to one (or two) decimal points. The NCSBN states NCLEX calculation items do have decimal points:
“unless the item requests that the candidate records their answer using a whole number. If asked to record to one (or two) decimal places, the candidate must enter the decimal point for the answer to be correct, and answers to calculation items should be rounded at the end of the calculation.”
The Metric System of Measurements
The metric system measures length, volume, and weight. The measurement for length is meters, volume is liters, and weight is grams. Meters, liters, and grams are the basic units of measure. All have larger and smaller units of measure. This system uses prefixes along with the basic units of measure to identify the larger and smaller units.
For example, grams express weight. Most of you are familiar with kilograms to measure body weight. A kilogram is larger than a gram. 1 kilogram = 1,000 grams. You may also use the prefix kilo for meters and liters. Therefore a kilometer = 1000 meters and a kiloliter = 1000 liters.
You will most often see the kilogram in the clinical setting. A kilogram is the larger unit of measure and milli (milligram) and micro (microgram) are smaller units of measure. Below is a chart that has the original scale and the abbreviated scale that is most often used for medication administration.
Units of metric measure are abbreviated using the first initial. The first initial use a lowercase letter. The exception is liters. Liters use as an uppercase letter. You may often see the uppercase used with prefixes such as milliliter (mL). However, you may see it written with or without an uppercase. (ml or mL).
Also, notice that micro uses the abbreviation mc. The abbreviations µ (micro) and µg (micrograms) were previously used but now is on the ISMP’s List of Error-Prone Abbreviations orders due to the possibility of medication errors.
A milliequivalent is usually applied to electrolytes to express the amount of a medication in a normal solution. The abbreviation is mEq. You will see milliequivalent for parenteral solutions like potassium chloride.
Systems of Medication Measurement – Household System
Secondly, is the household system. Most people are familiar with the household system. This system is not widely used for medications administration in healthcare facilities because it is not as accurate as the metric system. You will find this system more often used in the home care setting.
Also, you may still see these measures in the clinical setting occasionally. Some disposable medication cups still have measurements for the household, metrics, and the apothecary system written on them.
Measures used for the household system include drops (gtts), teaspoons (tsp), tablespoon (tbsp, Tbsp, tbs), cups, pints, and quarts.
Consequently, the household system has a disadvantage because it uses items like teaspoons and cups. These items can vary greatly in size.
Systems of Medication Measurement – Medication Calculation
Dimensional Analysis is a method for solving dosage calculation problems. It is a way of setting up fractions using ratios and proportions.
Dimensional analysis is great for calculating oral doses, liquid doses, and injections. It is also a method of calculating hours of infusion, intravenous (IV) pushes, milliliters per hour (ml/hr), and drops per minute (gtt/minute).
This method can be easy but can also appear very difficult. Sometimes instructions are hard to figure out. And sometimes you find yourself digging through a thick calculation and dosage textbook to find a simple answer or example.
In order to administer medications properly, a nurse must calculate the dose correctly and also measure medications correctly. Below are some basic tips to consider when using dimensional analysis for dosage calculations.
Tip #1 Review Basic Math
When administering medications, you will need to understand decimals and fractions. Most dosage calculations problems involve solving an equation that contains fractions. These are simple equations to solve but it may have been a while since you have had to use that knowledge.
This free Basic Math eBook contains a quick and thorough review of basic math skills needed to understand dimensional analysis. It covers the basics of fractions, changing improper fractions, and mixed numbers. You will also learn how to add, subtract and multiply fractions. There are also sections on multiplying and dividing mixed numbers. There is also an introduction to ratio and proportions. Click below to download a copy.
Tip #2: Learn Conversion
Whether you realized it or not you use conversions all the time. When converting seconds to minutes, minutes to hours, and hours to days, you are using the metric system. You will need to learn metric conversions to correctly use dimensional analysis and dosage calculation.
Below is an excerpt from the Calculations and Dosage One Method eBook that has the abbreviated metric conversion scale. And, this book also teaches you how to use the scale in a few easy steps.
Tip #3: Dimensional Analysis and Dosage Calculation
You can use a number of formulas to calculate medication dosages. Although, the ratio and proportion method and variations of the ratio and proportion method is the best formula to use. Dimensional Analysis is easy when setting up ratio and proportion problems. Below is an example of setting up dimensional analysis for IV pushes.
Remember, always check and double-check your calculations. Some agencies require that you check with another nurse before administering medication.
Systems of medication measurement are useful for nursing students, especially during nursing school. Understanding the metric system is vital to safe medication administration. Learning how to convert and calculate dosages is a must. This article has covered a wealth of information about systems of medication measurements and dimensional analysis for medication calculation.
Taylor, PhD MSN, RN, Carol, et al. Fundamentals of Nursing: The Art and Science of Person-Centered Nursing Care. 8th ed., Philadelphia, Wolters Kluwer Health/Lippincott Williams & Wilkins, 2015.
Potter RN, MSN, PhD, FAAN, Patricia A., and Anne G. Perry RN, EdD, FAAN. Fundamentals of Nursing. 9th ed., St. Louis, Mosby Elsevier, 2017.
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