calculations for sterile compounding

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Chapter 5

Calculations for Sterile Compounding

2012 Paradigm Publishing

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Learning Objectives Explore the evolution of mathematical formulas used in sterile

compounding. Understand the principles of pharmacy dosage calculations. Practice several types of pharmaceutical calculations using a

basic formula, ratio and proportion, dimensional analysis, intravenous flow rates, intravenous drip rates, and alligations.

Determine the best method of solving pharmaceutical dosage questions based on the medication labeling and sterile compounding procedure required.


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Topics Learning Objectives Introduction Calculations as Part of the Anteroom Protocol Dosage Calculations Special Considerations Common Pharmacy Equivalents and Conversions Basic Formula Calculations Ratio and Proportion Calculations Dimensional Analysis Calculations IV Flow Rate Calculations IV Drip Rate Calculations Alligation Calculations Calculation Skills in the Sterile Compounding Environment Chapter Summary


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Introduction

The majority of the CSPs assembled will require the application of one or more pharmacy calculation methods to determine the volume of the medication, additive, or solution component needed to prepare a CSP or to resolve the number of CSPs necessary for a 24-hour supplynumerous other calculation methods are used by

pharmacy personnel to determine information such as day’s supply, quantity to be dispensed, and average wholesale price (AWP)


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Calculations as Part of the Anteroom Protocol

The prescriber includes the names of the desired medications, the desired doses, and the dosing intervals as part of the CSP orderthe prescriber also determines the desired base solution

and infusion rate for large-volume parenterals (LVPs)most small-volume parenterals (SVPs), however, are

prepared using standard base solutions and volumes and are administered using standard administration times


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Calculations as Part of the Anteroom Protocol…/2

Once the medication order is received, personnel then enter the information into the computer system and generate a label for each CSPthe pharmacist and IV

technician must verify that the CSP label matches the medication order and is 100% accurate


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Looking for words or abbreviations that identify a number, dose, volume, amount, or route can help in identifying the medications needed for sterile compounding


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Dosage Calculations

The calculations are based on information on the CSP label and the label on the stock medication containerbegin the calculation process

by determining the concentration; this information can be found on the medication label

these calculations determine the drug volume


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Dosage Calculations…/2

Locating and understanding label information is an important initial step in ensuring accuracy in pharmacy calculations and in sterile compounding


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Special Considerations

One consideration is that the volume (in milliliters [mL] or liters [L]) and strength (in percent [%]) of the base solution are not to be included in any equation in which personnel are asked to determine the amount or volume of drug that is to be drawn up for a desired dose

Another consideration when performing pharmacy calculations is that an IV technician chooses a method or formula based on the type of CSP being prepared as well as a personal calculation preference


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In addition to understanding basic math principles, the IV technician must memorize the most commonly used equivalents and metric conversions


Common Pharmacy Equivalents and Conversions

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Common Pharmacy Equivalents and Conversions…/2


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Sterile compounding personnel must also memorize the conversions from standard time (clock time) to military time


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Your Turn


1) For dosage calculations, begin the calculation process by determining this. a. weightb. volumec. concentrationd. thickness

2) Standard time is called this.a. regular timeb. clock timec. average timed. military time

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Basic Formula Calculations

The basic formula method is a simple process to calculate most parenteral medication dosages and may be used when the concentration on the drug label is provided in units per one milliliter

D/H = x mLD = desired doseH = concentration on handx = unknown volume of drug needed to be drawn up


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Basic Formula Calculations…/2

The basic formula method is best used in sterile compounding scenarios that meet the following criteria:the prescriber has ordered a CSP with a single medication

additivethe drug is in an injectable formthe medication label provides you with the concentration

of the drug in the form of number of units per one milliliter


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Basic Formula Calculations…/3

Performing a Basic Formula Calculation1. Find the desired dose (D)2. Determine the

concentration on hand (H)3. Divide D by HThe resulting answer equals x mL


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Ratio and Proportion Calculations

The ratio and proportion method may be used whenever three of the four values in a proportion are known, thus allowing the IV technician to solve for the fourth, unknown value of the proportion

H mg = D mg Y mL x mL

H mg = number of milligrams on handY mL = number of milliliters in the concentration on handD mg = desired dosex mL = unknown volume of drug needed to be drawn up


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Ratio and Proportion Calculations…/2

The ratio and proportion method is best used in sterile compounding scenarios that meet the following criteria:the stock drug is already in a liquid, injectable form (does

not need to be reconstituted; the medication label identifies the concentration of the drug in a form other than the form milligrams per one milliliter

the stock drug is in a powdered form and requires reconstitution prior to drawing it up for injection

either a liquid or powdered drug requires the completion of several additional steps to solve the problem


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Ratio and Proportion Calculations…/3

Performing a Ratio and Proportion Calculation1. Set up the ratio2. Cross-multiply Y × D3. Divide the answer

from step 2 by H The resulting answer

equals x mL


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Dimensional Analysis Calculations

The dimensional analysis method, also known as “calculation by cancellation,” is another approach that some IV technicians prefer to use when a dosage calculation involves multiple stepsbased on the principle that any number can be multiplied

by one without changing its valuefirst determine the answer that is needed, and then use

the values provided to perform a series of calculations


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Dimensional Analysis Calculations…/2

The dimensional analysis method for solving pharmacy dosage calculations is best used in sterile compounding scenarios that meet either of the following criteria:to determine the volume of drug needed to prepare the

CSP, the IV technician must answer several questions by performing multiple calculations

the IV technician must make one or more conversions between various units of measure


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Performing a Dimensional Analysis Calculationput the unknown quantity (the x you want to define) on its

own on the left side of the equation, followed by an equal sign

on the right side, record a series of ratios, using both standard equivalents and information provided by the medication label and the CSP label

then multiply the ratios together to provide the answer to the problem


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The unit of measure in the numerator of the first ratio must be the same as the unit of measure you are solving, and the unit of measure in each successive numerator must match the unit of measure in the preceding denominatoras a result of this setup, the units of measure in each

alternate numerator–denominator will cancel out, such that the only unit of measure remaining is the one you must solve


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IV Flow Rate Calculations A daily IV run, or the batch typically requires a 24-hour

supply of CSPs Several types of IV flow rate calculations will determine the

answer to one or more of the following:How long will this bag last?What time will the next bag be needed?How many bags will be needed for the patient for a 24-hour

period? These may all be solved using IV flow rate calculations that

employ simple addition, division, and/or multiplication


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IV Flow Rate Calculations…/2

The three methods for solving IV flow rate questions are best used in sterile compounding scenarios that meet the following criteria:the infusion rate in mL/hour is provided on the medication

order or CSP labelthe total volume is provided on the medication order or

CSP labelthe current time is known or may be easily determined


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IV Flow Rate Calculations…/3

Performing an IV Flow Rate Calculation The answers to the three questions can be

determined by setting up a division or multiplication problemwhen calculating IV flow rates, you must know the total

volume of the LVP and either 1) the infusion rate in milliliters per hour (mL/hr) or 2) the number of hours over which the LVP is to be infused

this information can be found on the medication order or the CSP label


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IV Drip Rate Calculations

The drop factor method relies on the specific drop factor (gtts/mL), or the number of drops per milliliter, that a certain type of IV tubing delivers

Each of the different types of IV tubing delivers a different drop factormacrodrip IV tubing may deliver either 10 gtts/mL or 20

gtts/mLmicrodrip IV tubing delivers 60 gtts/mL

An equation using drop factor is sometimes called an IV drip rate calculation


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IV Drip Rate Calculations…/2

The drop factor method is best used in situations that meet the following criteria:the total volume to be infused is knownthe total infusion time is knownthe drop factor of the IV tubing is knownthe desired answer is to be provided in drops per minute

(gtts/min)

total volume in mL × drop factor = drops infusion time in minutes minute


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Alligation Calculations

The pharmacy may be required to mix two different strengths of the same active ingredient of a drug or solution to make the desired strengtha higher-percent strength of a drug or solution is mixed

with a lower-percent strength of a drug or solution in order to make the desired strength

This scenario requires you to employ a calculation called the alligation method, or simply alligationrarely performed by IV technicians


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Alligation Calculations…/2

The alligation method of solving pharmacy dosage calculations is best used in compounding situations that meet any of the following criteria:two or more strengths of the same active ingredient or

solution must be used to prepare the desired strength of a drug or solution

the desired strength of the drug or solution is knownthe total volume of the CSP is known


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Alligation Calculations…/3

Performing an Alligation Calculation1. Identify the variables by determining the component

concentrations2. Fill in the concentration strengths3. Find the values4. Set up a ratio5. Use a formula


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Calculation Skills in theSterile Compounding Environment

Accurate calculation and measurement of the components or ingredients of the formulation are criticalthe pharmacy technician must have an understanding of

historical practices and a working knowledge of current systems of measurement and pharmacy calculations

in order to perform accurate calculations, you must write down the equation and verify that you have the correct information in the correct place within the formula


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Your Turn


3) This is the basic formula. a. H/D = x mLb. X/D = h mLc. D/H = x mLd. X/H = d mL

4) An equation using drop factor is sometimes called this.a. ratio and proportion calculationb. IV flow rate calculationc. alligation calculationd. IV drip rate calculation

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calculations for sterile compounding

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