Metric System

CHAPTER 6


Metric System



Objectives


After reviewing this chapter, you should be able to:


1. Express metric measures correctly using rules of the metric system


2. State common equivalents in the metric system


3. Convert measures within the metric system


The metric system is an international decimal system of weights and measures that was introduced in France in the late seventeenth and eighteenth centuries. The system is also referred to as the International System of Units, abbreviated as SI units. SI is the abbreviation taken from the French Système International d’Unit’s. The metric system is more precise than the apothecary and household systems; therefore it has become the system of choice for the calculation of dosages. The apothecary system is being phased out and gradually being replaced by the metric system. The benefit of the metric system lies in its simplicity and accuracy because it is based on the decimal system, unlike the apothecary system, which is based on fractions and could result in multiple errors. More and more in everyday situations, we are encountering the use of metric measures. For example, soft drinks come in bottles labeled in liters; engine sizes are also expressed in liters.


The metric system has become the system of choice for medications and measurements used in the health care setting. For example, newborn weights are recorded in grams and kilograms, and adult weights are expressed in kilograms, as opposed to pounds and ounces. In obstetrics we express fundal height (upper portion of the uterus) in centimeters. Although a few medications are still prescribed in apothecary and household terms, the nurse will find that the majority of medication calculation and administration skills involve familiarity and accuracy with the metric system to administer medications safely.



PARTICULARS OF THE METRIC SYSTEM




1. The metric system is based on the decimal system, in which divisions and multiples of 10 are used. Therefore a lot of math can be done by decimal point movement.


2. Three basic units of measure are used:



3. Common prefixes in this system denote the numerical value of the unit being discussed. Memorization of these prefixes is necessary for quick and accurate calculations. The prefixes in bold in Table 6-1 are the ones used most often in health care for dosage calculations. However, some of the prefixes may be used to express other values, such as laboratory values. Kilo is a common prefix used to identify a measure larger than the basic unit. The other common prefixes used in medication administration are smaller units: centi, milli, and micro.



Let’s look at the following example to see how the prefixes may be used.















Example 2:

Microgram—abbreviated as mcg. Microgram is also written using the symbol μ in combination with the letter g from the basic unit gram (μg). However, use of the abbreviation μg should be avoided when writing orders. It can be mistaken for “mg” when handwritten and result in an error when orders are transcribed.



CAUTION


The abbreviation μg for microgram is listed on the official “Do Not Use” List of TJC (The Joint Commission). Confusion of the symbol used for microgram with the abbreviation for milligram could cause a critical error in dosage calculation. These units differ from each other in value by 1,000.



Although the symbol for microgram is no longer approved for use when writing medication orders, it may be seen on some medication labels (e.g., Lanoxin tablets, Lanoxin injectable, Lanoxin elixir for pediatric use), and you should be familiar with the meaning.



Example 3:

Milliliter—abbreviated as mL. Note that when L (liter) is used in combination with a prefix, it remains capitalized.



Tips for Clinical Practice


You may see gram abbreviated as Gm or gm, liter as lowercase l, or milliliter as ml. These abbreviations are outdated and can lead to misinterpretation. Use only the standardized SI abbreviations. When in doubt about an abbreviation being used, never assume; ask the prescriber for clarification. See Box 6-1 for common metric abbreviations.



BOX 6-1   Common Metric Abbreviations


gram = g


microgram = mcg


milligram = mg


kilogram = kg


liter = L


*deciliter = dL


milliliter = mL


*Seen in the expression of laboratory values (e.g., hemoglobin, creatinine levels).





CAUTION


It is critical to differentiate between the SI abbreviations for milligram (mg) and milliliter (mL). At a quick glance these abbreviations appear similar; however, confusing these two units can result in lethal consequences for a client.



RULES OF THE METRIC SYSTEM


Certain rules specific to the metric system are important to remember (Box 6-2). These rules are critical to the prevention of errors and ensure accurate interpretation of metric notations when used in medication orders.



BOX 6-2   Metric System Rules




Example:


1, 1,000, 0.5




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Feb 11, 2017 | Posted by in PHARMACY | Comments Off on Metric System

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