Module 3
docx
keyboard_arrow_up
School
University of Saskatchewan *
*We aren’t endorsed by this school
Course
111
Subject
Medicine
Date
Apr 3, 2024
Type
docx
Pages
26
Uploaded by ElderWallaby997
Module 3 Overview
O’Sullivan & Albrecht (2012) note that “dosing is one of the most common calculation functions that [pharmacists] will perform during [their] career[s]. It involves a series of steps, each of which potentially involves some calculations. Dosing begins with some guidelines, usually a dosage range” (p. 95).
The dose of a drug is the quantitative amount administered to, or taken by, a
patient for the intended medicinal effect.
Dosage calculations are involved in many activities of pharmacy practice, such as dispensing medications prescribed by a physician, compounding medications, or determining the amount of a medication to give a patient based on patient specific parameters (height, weight, etc.).
This module will cover the following topics:
Commercial dosage forms in compounding
Types of dosage calculations
Special dosing considerations (special populations)
Calculations of infusion time
Basic IV solutions
Dose adjustments (BSA)
Key Terms and Concepts:
Dose
Total dose
Dosage regimen
Total dose
Size of dose
Total amount
Usual Dosage Range
Usual Pediatric Dose
Usual Adult Dose
Loading Dose
Continuous Infusion
Reconstitution
Module 3 Learning Objectives
When you have finished this module, you should be able to do the following:
1.
Apply practical compounding principles in dosage calculations (S.1.1.c)
2.
Calculate total quantity needed, number of doses and/or size of dose for a given scenario (S 1.1.c)
3.
Identify and utilize specific patient characteristics that impact calculation parameters (S1.1.c, P 2.2)
4.
Calculate infusion time, rate of flow, infusion rate, or amount of active drug required to prepare a specified product (S1.1.c)
5.
Integrate information from the context of a given question (e.g. patient characteristics, size of dose, total amount, etc.) to make patient recommendations (S 1.1.c, P 2.2)
6.
Show an understanding of the importance of precision and accuracy
by identifying pharmacists’ actions that lead to calculation errors and /or patient adverse events (P 2.2)
Use of Commercial Dosage Forms in Compounding (L.O. 3.1)
Sometimes there is no bulk supply of a particular pharmacologically active ingredient to use in compounding. Therefore, pharmacists may use commercially available products to obtain the desired ingredient.
Use of Tablets and Capsules
Commercially available tablets and capsules may be used to compound other solid dosage forms (e.g., capsules or powders).
Commercially available products may also be used to compound semisolid or
liquid formulations. For example, if no liquid form is available and someone cannot swallow tablets, a suspension can be made.
When you are using commercial products for compounding there are a few rules of thumb to follow:
Use the simplest dosage form available (i.e., use uncoated tablets rather than coated ones, don’t use a modified release dosage form)
Use the least number of dosage units possible (i.e., use 5 tablets of 100 mg, rather than 20 tablets of 25 mg)
Prefer to use whole numbers of tablets (i.e., use 5 tablets instead of
4.5 tablets, make more product, and only dispense what’s necessary)
Learning Highlight
Calculate to use an exact number of dosage units (no half tablets), even if that results in making extra product - you would just dispense the necessary amount.
When calculating, the pharmacist may need to take into account the amount
of inactive ingredients in the commercial tablets as well.
Check Your Understanding 3-1
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
Use of Injections
An injectable solution can also be used as the source of an ingredient in compounding dosage forms other than those meant to be injected. When asked for the concentration of a medication, concentrations are usually expressed as a percentage, or quantity per unit volume (e.g.,
x
g / 1 mL).
What she is trying to say that it is either a percentage that is out of out
of 100 OR it is expressed as a per unit volume ratio.
-
Its not that they are equal to each other, but that its either or.
The use of injections as an ingredient source in compounding is useful in preparing liquid compounds or semi-solids which can absorb the added liquid. When using injectable solutions to make pharmaceutical compounds, pharmacists need to use a needle and syringe to accurately measure the amount of product to be used in the compound.
Some drugs must be stored in powdered form because they rapidly lose their
power or become unstable once they are mixed into a solution. These drugs will then have to be reconstituted, or mixed with a liquid, called the diluent, before they can be administered (Wladis, n.d.).
Check Your Understanding 3-2
Making Capsules
When you are making capsules in the pharmacy, you must ALWAYS calculate
for 2 extra capsules, due to loss of powder on equipment, and increased occurrence of errors.
Learning Highlight
You do NOT have to calculate extra ingredients when making tablets, solutions, or suspensions unless instructed to do so.
= 1000 mg / 1g = x mg / 0.06 g
Calculation of Doses (L.O. 3.2)
The
dose
of a drug is the quantitative amount administered to, or taken by, a patient for the intended medicinal effect.
The dose may be expressed as the
single dose
(amount taken at one time),
a
daily dose
(amount taken in entire day), or the
total dose
(amount taken
over the course of therapy.) A daily dose may be subdivided and taken in divided doses. The schedule of dosing is referred to as the
dosage regimen
.
Doses vary widely, depending on the drug’s pharmacologic and biochemical activity, its physical and chemical properties, the dosage form it is in, the route of administration, and various patient factors.
The
usual adult dose
of a drug is the amount that ordinarily produces the medicinal effect in adults. The
usual pediatric dose
is similar, but for children. These doses are only guides and can be adjusted based on various factors. The
usual dosage range
for a drug indicates the quantitative range
or amounts that would be prescribed in usual medical practice.
Pharmacists may need to complete (or check) dosage calculations for both prefabricated dosage forms and for compounded products. For liquid medications, patients may use household measures (e.g., teaspoonful or tablespoonful) or metric measures (in devices often provided by the pharmacist). In calculating doses, pharmacists and physicians accept the following:
Learning Highlight
one teaspoonful = 5 mL
(one teaspoonful is sometimes written as ℨ
i )
one tablespoonful = 15 mL (one tablespoonful is sometimes written as ss fl ℥
)
Any dropper, syringe, medicine cup, special spoon, or other device that is used to administer liquids should deliver 5 mL when one teaspoonful or one fluid dram is indicated.
Doses may also be indicated in number of drops. A drop does not indicate a definite quantity, since liquids and droppers differ greatly - therefore, each dropper must be calibrated to establish the volume represented by a certain number of drops.
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
Learning Highlight
Round drops to the nearest WHOLE number using standard rounding principles. There are NOT special rounding rules for drops. There are many patient parameters, which may need to be taken into account when establishing the appropriate dose of a drug for an individual. Some of these include body weight (actual or “ideal”), how the drug is handled in the body (pharmacokinetics), how the drug is to be given (e.g., orally vs intravenously), or the dosage form (e.g., immediate-release vs, sustained-release).
Types of Dosage Calculations (L.O. 3.2)
There are different types of information that a pharmacist may need in dealing with dosing problems. They can usually be categorized in one of three ways: size of dose, number of doses or total amount of drug.
Size of dose:
quantitative amount needed to deliver the prescribed amount of drug; measured by weight, volume or dosage units (e.g., number of tablets)
Number of doses:
doses available in a specified quantity
Total amount:
amount of drug or product needed to supply the prescribed dose
and dosage regimen
Basic Formula
Total amount = # of doses X size of dose
(NOTE: must use common denominations)
This equation can be modified depending on what value you are looking for.
# doses = total amount ÷ size of dose
Size of dose = total amount ÷ # of doses
Check Your Understanding 3-3
Check Your Understanding 3-4
Check Your Understanding 3-5
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
Pharmacists may also calculate the quantity of an ingredient needed either in a specified dose, or in the total amount dispensed or compounded.
Basic Formulae:
Quantity in each dose = quantity in total amount ÷ number of doses
Quantity in total amount = quantity in dose x number of doses
Check Your Understanding 3-6
Check Your Understanding 3-7
Special Dosing Considerations (e.g., Elderly and Pediatric Patients) (L.O. 3.3, 3.4)
To calculate some doses or adjust doses for certain patients, we may calculate a dosage based on:
age
body weight
body surface area
clinical laboratory test values
Dosage Based on Age
Before the physiologic differences between children and adults were clarified, various “rules” or formulae were developed to give fractions of the adult dose; these essentially treated children as miniature adults. Today these are no longer widely used because we have better methods which take
into account the variations in height, weight, and pharmacokinetics of children of the same age. However, if age is the only
variable that is known, several formulae exist which can be used to estimate a child’s dose, based on the recommended adult dose.
Dosage Based on Body Weight
The usual doses for adults are generally suitable for 70 kg (154 lb) individuals. Doses may be adjusted for extremely lean or obese individuals if needed. For infants and children, dosing based on body weight is much more
reliable than dosing based on age.
Clark’s Rule
is a
general rule
for calculating a pediatric dose based on an adult dose:
Some drugs must be dosed based on specific body weight.
Learning Highlight
Always use the LEAST number of steps when converting between units. I.e.,
to convert a patient’s weight form lbs to kg, use the conversion 2.2 lb/kg NOT 1 lb = 454g and then convert from grams to kilograms.
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
Check Your Understanding 3-8
Dosage Based on Body Surface Area
The body surface area (BSA in m2) method of calculating doses is widely used in cancer patients receiving chemotherapy and in all pediatric patients, with the exception of small or premature infants or those whose physiologic functions may require dosage adjustments. The most accurate method of calculating BSA takes into account both the height and weight of the patient.
A commonly used formula for BSA calculations is a formula by Mosteller, (published in NEJM 1987;317:1098):
BSA (m
2
) = square root of: the product of the weight (kg) times the height (cm) divided by 3600.
If available, nomograms can simplify the calculation of BSA (see for an adult
http://www.smm.org/heart/lessons/nomogram_adult.htm
or for a
child:
http://www.smm.org/heart/lessons/nomogram_child.htm
). The BSA can then be used directly to calculate a dose, if the recommended dose based on
BSA exists.
Learning Activity 3-1
Watch YouTube Video
: BSA via Nomogram
Check Your Understanding 3-9
Learning Highlight
Dosage adjustments
can also be made using a
BSA of 1.73 m
2
for an average adult.
Therefore, the dose for an adult can be estimated using the following equation:
Please note that BSA depends on more than just weight and height, so the following can also be used:
Average BSA for adult men: 1.9 m
2
Average BSA for adult women: 1.6 m
2
Average BSA for children (9 years): 1.07 m
2
Average BSA for children (10 years): 1.14 m
2
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
Average BSA for children (12-13 years): 1.33 m
2
(Medical Definition of Body surface area, 2016)
Check Your Understanding 3-10
IV Dosage Calculations
NOTE: You will learn about intravenous infusions and parenteral admixtures in upcoming courses. This section will deal only with the calculations used for
administration of specific doses.
Some things you need to know:
Commercially prepared infusions (such as 5% Dextrose in water [D5W] or 0.9% NaCl in water [normal saline or NS]) are available in volumes of 50 mL (a
minibag
), 100 mL, 250 mL, 500 mL and 1000 mL. These are common diluents used to administer medications and other additives by IV.
Learning Highlight
Unless you are directly asked about their concentration (NaCl or Dextrose), they are simply diluents.
An IV solution is delivered to the patient’s vein through an attached administration set.
Administration sets can be standard (macrodrip) or pediatric (microdrip). Depending on the set, the drip rate can vary from 10 - 15 drops / mL to 60 drops / mL. IV medication administration can be assisted by gravity (IV pole) or electronic volumetric infusion pump.
Continuous infusion
refers to large volumes of fluid (i.e., 250 - 100 mL) administered into a vein uninterrupted.
Intermittent infusions
are administered over a scheduled period (e.g., over 1 hour, repeated every 8 hours).
IV push
is the rapid infusion of a medication into a vein, and is usually conducted in less than a minute.
Loading Dose
is a comparatively large dose given at the beginning of treatment to start getting the effect of a drug, especially one with slow clearance, thus requiring a long period to achieve stable blood levels without
a high initial dose (
https://www.drugs.com/dict/loading-dose.html
).
Check Your Understanding 3-11
Calculations of Infusion Time (L.O. 3.5)
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
These types of questions can be solved using unit (or dimensional) analysis. However, you can also use formulas (if you like to memorize things).
To calculate
infusion time
:
To calculate flow rate in
drops/minute
:
To calculate flow rate in
mL / hour
when based on dose:
NOTES:
Units must be the same, top and bottom
If the dosage rate is stated in hours, the “60" is not needed
Learning Highlight
Reminder: Drops are rounded to the WHOLE number, using standard rounding principles.
Check Your Understanding 3-12
Check Your Understanding 3-13
If you had 10mg in 500 ml, what would be the ml to 1mg?
-
Then you would do 50 ml ok, it says 1 hour, so 50ml in 1 hour.
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
this makes more sense
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
The Importance of Accuracy (L.O. 3.6)
Accuracy in dosage calculations is of the utmost importance to ensure that patients receive the correct dose needed to illicit the intended therapeutic effect (e.g., correct dose of an antibiotic to treat an infection), and to ensure patient safety and eliminate adverse effects and patient harm due to overdose or toxicity.
Competency in pharmaceutical calculations is required to ensure public safety. Click on the following link to watch a video and review an article from
the NY Daily News about an 8-year-old who died after a pharmaceutical dose
error where a pharmacist dispensed a dose 1000 times higher than his usual dose.
Learning Activity 3-2
Watch this video and read the news article.
Colorado 8-year old dies after pharmacy allegedly gives him 1,000 times his usual medication dosage for sensory processing disorder
http://www.nydailynews.com/news/national/colorado-boy-8-dies-pharmacy-1-
000-times-dosage-article-1.2682631
Read the following article discussing ways to prevent errors.
Learning Activity 3-3
Read This Article
Medication Errors: Causes, Prevention and Reduction article
http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2141.2002.03272.x/full
After you have reviewed both activities, reflect on the possible cause(s) of the error that lead to the patient’s death. Based on information in article "Medication Errors: Causes, Prevention and Reduction", what activities can you incorporate in your practice as a pharmacist to reduce medication errors?
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
Learning Activity 3-4
Read This News Article
4 year-old ‘acting like a slobbering drunk’ after pharmacy dispenses wrong dose of antipsychotic drug
http://www.cbc.ca/news/canada/saskatoon/4-year-old-acting-like-a-
slobbering-drunk-after-pharmacy-dispenses-wrong-dose-of-antipsychotic-
drug-1.3801461
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
Module 3 Glossary
Continuous infusion:
refers to large volumes of fluid (i.e., 250 - 100 mL) administered into a vein uninterrupted
Dosage regimen
: the schedule of dosing
Dose
: the quantitative amount administered to, or taken by, a patient for the intended medicinal effect
Loading Dose
: is a comparatively large dose given at the beginning of treatment to start getting the effect of a drug, especially one with slow clearance, thus requiring a long period to achieve stable blood levels without
a high initial dose
Reconstitution
: a drug is mixed with a liquid (called the diluent) before it can be administered
Size of dose
: quantitative amount needed to deliver the prescribed amount of drug; measured by weight, volume or dosage units (e.g., number of tablets)
Total dose
: amount taken over the course of therapy
Usual Adult Dose
: the amount of a drug that ordinarily produces the medicinal effect in adults
Usual Dosage Range
: indicates the quantitative range or amounts of a drug that would be prescribed in usual medical practice
Usual Pediatric Dose
: the amount of a drug that ordinarily produces the medicinal effect in children
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
Module 3 References
Ansel, H. C., & Stockton, S. J. P. (2016). Pharmaceutical calculations (15th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.
MedicineNet.com. (2016, May 13). Medical Definition of Body surface area. Retrieved from: http://www.medicinenet.com/script/main/art.asp?
articlekey=39851
O'Sullivan, T. A., & Albrecht, L. S. (2012). Understanding pharmacy calculations. (2nd ed.). Washington, DC: American Pharmacists Association.
Wladis, C. (n.d.). Lecture 4: Reconstitution of powdered drugs and insulin dosages. Retrieved from http://www.cwladis.com/math104/lecture4.php
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help