Chapter 10Chapter 10

Dosage Calculations

ObjectivesObjectives

Define key terms.Use dimensional analysis to calculate

dosages for administering drugs accurately.

Use the formula method to calculate dosages accurately.

Use fractions to solve dosage calculations accurately.

ObjectivesObjectives

Use ratios and proportions to solve dosage calculation problems by cross-multiplying to calculate or check a dosage given.

Correctly solve word problems.Accurately calculate dosages ordered by

weight.Use a child’s weight to calculate the desired

dosage correctly.

ObjectivesObjectives

Use a body surface area nomogram to calculate pediatric dosages.

Discuss how pediatric patients differ from adults. Correctly reconstitute powdered medication and

calculate desired dosage. Use dimensional analysis to calculate IV drip

rates accurately. Calculate fluid intake accurately.

Methods for Calculating Drug Methods for Calculating Drug DosagesDosages

Dimensional analysisFormulaFractionsRatio and proportionsWord problems

Dimensional AnalysisDimensional Analysis

Write the unit of the dose ordered.Write the units that are on the label and

the unit you plan to give the patient.Fill in the numbers, after you set up the

problem.Calculate.Check your work for reasonableness.

Formula MethodFormula Method

D/H × Q = AnswerD is the dimension ordered.H is what you have on hand in that

dimension.Q is the quantity of the unit you are giving.The answer is in the same unit as Q.

Fraction MethodFraction Method

Dosage on hand (H)/Dosage unit (Q) = Desired dose(D) /Dose given

OROrdered dosage unit (D)/label dosage unit

(H) = Dose given/Quantity on hand (Q)

Critical ThinkingCritical Thinking

Does it make any difference if you align the units on one side and desired units on the other? Explain.

Using Several MethodsUsing Several Methods

Healthcare professionals usually find a method they prefer to use and use it for all calculations.

Preference may depend on how your brain may have first learned how to calculate.

You can use a different method to check your work.

Critical ThinkingCritical Thinking

Can you use the fraction method to check the accuracy of other methods?

Ratio and ProportionsRatio and Proportions

Set the equation up with label on one side and desired on the other.

e.g., 250 mg : 1 mL :: 500 mg : ?Multiply the means (middle section) and

the extremes (far ends) and solve for ?e.g., 250 × ? = 1 × 500? = 2 mL

Word ProblemsWord Problems

Dimensional analysis If the prescriber orders ___ mg, multiply

the order by the amount of tablets, caplets, milliliters, etc. on the label and divide by the number of milligrams on the label.

Formula MethodFormula Method

If the prescriber orders _________ mg, divide that number by the milligrams on the label and multiply by the unit per milligram.

Fraction MethodFraction Method

If the prescriber orders ______ mg, place it over the milligrams on the label. Place the quantity of units on the label and write it as the denominator of a fraction on the other side of the equals sign. Then find what numerator makes the two fractions equal.

Ratio and ProportionsRatio and Proportions

Express the label quantities as a ratio of mg : units. On the other side of the two colons, place the prescriber’s order in milligrams. Solve for the units that you should give by multiplying the extreme (far right and left) amounts together and the mean (inside right and left) amounts together—solving for what you need to give.

Special CircumstancesSpecial Circumstances

Weight-Based Calculations in Weight-Based Calculations in Adults and ChildrenAdults and Children

Find weight in kg.Multiply by ordered mg/kg/day.Adjust if dosage is required:bid 2 × per day tid 3 × per dayqid 4 × per day

Critical ThinkingCritical Thinking

You may find it bothersome that dosages sometimes come out to be odd numbers relative to the usual dosages—for example, when the dose is 337.5 mg/dose. If the medication comes in 200 mg tablets, what would you do?

Calculations Using Body Surface Calculations Using Body Surface AreaArea

Find child’s height and weight on BSA chart (Figure 10.4).

Put a ruler or piece of paper on those two values.

Record the point at which they intersect.Place that number over the average 1.7

and multiply by the adult dose to get the child dose.

Reconstituting PowdersReconstituting Powders

Reconstitute as specified on the label and then use the reconstituted ratio for the calculations.

Calculations of IV DosagesCalculations of IV Dosages

Electronic regulator pumpsTotal mL ordered/Total time ordered in

hours = mL/hour (rounded to whole number)

Critical ThinkingCritical Thinking

In the above case, where the flow rate has not been consistent, you see that the IV insertion site is swollen. Would you increase the flow rate to make up the difference? What would you do?

Manual IV SetsManual IV Sets

Total volume (V) to be infused (mL) divided by total time in minutes

×

Drop factor (D) in gtts/mL

=

Rate of flow (R) in gtts/minute

Critical ThinkingCritical Thinking

Because the actual order is the same, the real factor in these calculations is the drip rate. Which would drip faster at the same hourly rate—a macrodrip or a microdrip?

Fluid BalanceFluid Balance

Add up the total fluids consumed in oz.Multiply this number by 30 to get mL. (1 oz = 30 mL)Compare this total with the fluid balance

order. If greater than order, patient has

exceeded it. If less than order, patient has not.

Critical ThinkingCritical Thinking

What instructions do you need to give this child’s parents? (page 196)

HydrationHydration

Patients may have restricted fluids if they are storing fluids inappropriately, such as with congestive heart failure.

Patients who are dehydrated may have, instead, a plan to increase fluids to exceed a certain threshold. This patient may be required to drink more than 1,500 mL per day, for example.

Critical ThinkingCritical Thinking

Coffee, caffeinated sodas, and beer have a diuretic effect on the kidneys, meaning that they increase urination. Although consumption of these fluids does count as hydration, what would be better choices for dehydrated patients?

SummarySummary

What new piece of information in this chapter were you most interested to learn?

What questions do you still have about the information in this chapter?

Return to Objectives to determine extent of learning.

CreditsCredits

Publisher: Margaret Biblis

Acquisitions Editor: Andy McPhee

Developmental Editor: Yvonne N. Gillam

Production Manager: Samuel A. Rondinelli

Manager, Electronic Development: Kirk Pedrick

Technical Project Manager, EP: Frank Musick

Design Associate, EP: Sandra Glennie

The publisher is not responsible for errors or omission or for consequences from application of information in this presentation, and makes no warranty, expressed or implied, in regards to its content. Any practice described in this presentation should be applied by the reader in accordance with professional standards of care used with regard to the unique circumstances that may apply in each situation.