Dosage Calculation Using the Formula Method

CHAPTER 15


Dosage Calculation Using the Formula Method



Objectives


After reviewing this chapter, you should be able to:


1. Identify the information from a calculation problem to place into the formula given


2. Calculate medication dosages using the formula DH×Q=ximage


3. Calculate the number of tablets or capsules to administer


4. Calculate the volume to administer for medications in solution


This chapter shows how to use a formula for dosage calculation, which requires substituting information from the problem into the formula. The nurse should use the formula consistently and in its entirety to avoid calculation errors.


Errors can be made if you totally rely on a formula to determine a dosage rather than asking yourself whether the answer is reasonable. You will learn, for example, that the maximum number of tablets or capsules for a single dosage is usually three. Anything exceeding that should be a red flag to you, even if the answer is obtained from the use of a formula. Use formulas to validate the dosage you think is reasonable, not the reverse. Think before you calculate. Always estimate before applying a formula. Thinking first will allow you to detect errors and alert you to try again and question the results you obtained.



Critical Thinking


Avoid Dosage Calculation Errors


Do not rely solely on formulas when calculating dosages to be administered. Use critical thinking skills such as considering what the answer should be, reasoning, problem solving, and finding rational justification for your answer. Formulas should be used as tools for validating the dosage you THINK should be given.




FORMULA FOR CALCULATING DOSAGES


The formula presented in this chapter can be used when calculating dosages in the same system of measurement. When the dosage desired and the dosage on hand are in different systems, convert them to the same system before using the formula, using one of the methods learned for conversion. It is imporant to learn and memorize the following formula:


DH×Q=x


image

Let’s examine the terms in the formula before using it.


D = The dosage desired, or what the prescriber has ordered, including the units of measurement. Examples: mg, g, etc.


H = The dosage strength available, what is on hand, or the weight of the medication on the label, including the unit of measurement. Examples: mg, g, etc.


Q = The quantity or the unit of measure that contains the dosage that is available, in other words, the number of tablets, capsules, milliliters, etc. that contains the available dosage. “Q” is labeled accordingly as tablet, capsule, milliliter, etc.


x = The unknown, the dosage you are looking for, the dosage you are going to administer, how many milliliters, tablets, etc. you will give.



NOTE


It is important to note that the unknown “x” and “Q” will always be stated in the same unit of measure.



When you are solving problems that involve solid forms of medication (tabs, caps), Q is always 1 and can be eliminated from the equation. For consistency and to avoid chances of error when Q is not 1, always include Q even with tablet and capsule problems. When you are solving problems for medications in solution, the amount for Q varies and must always be included.


The available dosage on the label for liquid medications may indicate the quantity of medication per 1 milliliter or per multiple milliliters of solution, such as 80 mg per 2 mL, 125 mg per 5 mL. Some liquid medications may also express the quantity in amounts less than a milliliter, such as 2 mg per 0.5 mL. Because the amount for “Q” can vary with liquid medications and is not always 1, omission of the amount for “Q” can render an error in dosage calculation.



CAUTION


Label all terms of the formula, including “x,” as a safeguard to prevent errors in calculation. Think about what is a reasonable amount to administer, and calculate the dosage using the formula.




STEPS FOR USE OF THE FORMULA


Now that we have reviewed the terms in the formula, let’s review the steps for using the formula (Box 15-1) before beginning to calculate dosages using the formula.



Box 15-1   Steps for Using the Formula




Now we will look at sample problems illustrating the use of the formula.





Example 1:

Order: 0.375 mg p.o. of a medication.


Available: Tablets labeled 0.25 mg



Solution:

The dosage 0.375 mg is desired; the dosage strength available is 0.25 mg per tablet. No conversion is necessary. What is desired is in the same system and unit of measure as what you have on hand.



image Formula Setup



DH × Q=x


image


The desired (D) is 0.375 mg. You have on hand (H) 0.25 mg per (Q) 1 tablet. The label on x is tablet. Notice that the label on x is always the same as Q.


(D) 0.375 mg(H) 0.25 mg × (Q) 1 tab=x tabx=0.3750.25 × 1x=0.3750.25x=1.5=1½ tabs


image


Therefore x = 1.5 tabs, or 1½ tabs. (Because 0.375 mg is larger than 0.25 mg, you will need more than 1 tab to administer 0.375 mg.) Note: Although 1.5 tabs is the same as 1½ tabs, for administration purposes, it would be best to state it as 1½ tabs.


Example 2:


Order: 7,000 units IM of a medication.


Available: 10,000 units in 2 mL




CAUTION


Omitting Q here could result in an error. A liquid medication is involved; Q must be included because the amount varies and is not always per 1 mL.




RULE


Rule for Different Units or Systems of Measure


Whenever the desired amount and the dosage on hand are in different units or systems of measure, follow these steps:





Tips for Clinical Practice


When converting before calculating a dosage, convert apothecary and household measurements to their metric equivalents when possible. The metric system is the principal system used in measurement for medications.



NOTE


What is desired and what is available must be in the same units and system of measure. Remember that it is usual to convert what is desired to what is available. Therefore change gr to mg; this will also eliminate the fraction and decrease the chance of error in calculation.




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Feb 11, 2017 | Posted by in PHARMACY | Comments Off on Dosage Calculation Using the Formula Method

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