Transcript DOSAGE CALCULATIONS

DOSAGE
CALCULATIONS
Fundamentals of nursing pg 736-738
Medical Dosage Calculations
Olsen, et al
Objectives
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Goal
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Nursing students will demonstrate dosage
calculations competency by passing a required
dosage calculation test with a 90% or better.
Objectives
Students will understand how to correctly convert
numbers between and within apothecaries,
household, and metric measurement systems.
 Students will demonstrate ability to accurately
calculate drug dosages
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Dosage Calculation Test
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Must pass with a 90% or higher
Three attempts allowed to pass
You WILL NOT proceed in the program if you
do not pass the dosage calculation exam
MUST complete remediation exercises prior to
second or third attempt
May use testing center issued calculator for test
(basic function calculator)
Testing in the Testing Center
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Must have CSI ID card or can’t get in
You must be checked in 1 hour prior to testing
center closing to receive test
Identify test as following:
Class – NURP 106 (Fundamentals)
 Instructor: Paula Hart
 Test: Dosage Calculation Exam # (1, 2 or 3)
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Question- Why do we care about getting 100%
on our dosage calculations test?
Answer- Because a human being could die from
a wrongly placed decimal point or an accidental
extra zero placed during a dosage calculation…
It’s that important.
“3 babies died from the wrong dose of heparin
given. Dosage calculation errors were made.”
Indianapolis Star/ Sept 19, 2006
What do you need to be
successful?
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Review the content in this power point, your
text book, text book site, and Kaplan site
Ask for help if you need it from your
instructors, math lab, internet sites
Use ALL available resources
PRACTICE, PRACTICE, PRACTICE
Ready to begin?
Math Review – Roman Numerals
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Read from left to right
A smaller numeral to the right of a larger
numeral is added to the larger numeral
A smaller numeral to the left of a larger numeral
is subtracted from the larger numeral
Easily translated to Arabic numbers
Roman Numeral Practice
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XXX = _________
XIV = __________
iii = ____________
XXV = ___________
XIX = ____________
VI = _____________
XXIV =___________
Math Review cont
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You will need to review/refresh how to multiply
and divide whole numbers, fractions and
decimals, how to convert minutes to hours, and
any basic arithmetic in your Medical Dosage
Calculations book.
The Metric System
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The preferred system of measurement for
prescribing and administering meds.
Is a decimal system based on powers of 10
Base units are
Grams- measures mass or weight, such as the dose
of a drug
 Liters- measures the volume- such as the amount of
a drug or solution to administer
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Metric System cont.
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The order of the six metric units from largest to
smallest can be remembered by:
King Henry Died of a Disease Called Mumps
(kilo-)(hecta-)(deka-)(base unit)(deci-)(centi-)(milli-)
-gram
-liter
-meter
Value to base: k-1000, h-100, d-10, base-1, d-0.1, c-0.01, m-0.001
Metric System cont
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Moving from one metric unit equivalent to the
next involves moving the decimal point one
place value to the left or right depending on
whether you are moving up or down from the
base.
If you are moving from a smaller to a larger
metric unit, such as mg to grams(gm) the
decimal moves to the left. (Left = Larger)
If your moving from larger to smaller, gm to
mg, then move the decimal to the right.
Metric System cont
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Examples- To state 1 mg in gm you would move
the decimal 3 place values to the left. So, 1 mg=
0.001 gm.
To state 1 kg in gm you would move the decimal
point 3 places to the right. So, 1 kg=1000 gm.
Cubic centimeters (cc)=milliliters(ml)
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cc is no longer an acceptable abbreviation, however you
will see it used.
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Convert within a system to make it easier to do
calculations. Ex. 1000ml=1L
Apothecary and Household
systems
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Common US systems of measurement but not
used worldwide.
Need to convert apothecary and household units
of measurement into the metric system.
Household conversions you need to memorize
1tsp=5ml
 1Tbsp=3tsp
 1 ounce=30ml
 16ounces=1lb
 2.2lb=1kg
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Apothecary and Household
systems cont.
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Apothecary measurements
gr xv is 15 grains
 gr ii is 2 grains
 gr 1/6 is 1/6th of a grain
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Apothecary conversions you need to memorize
gr i=60mg
 gr XV=1gm
You will be provided with charts to help you with
memorizing.
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Apothecary and Household
systems cont.
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Examples
2tsp=10ml (2tsp x 5ml=10ml)
 8oz=240ml
 100kg=220lb
 gr ¾=45mg
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Calculating Dosages
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Three ways:
Ratio & Proportion (solving for X)
 Dimensional Analysis (Preferred method)
 Formula
We will be teaching Dimensional Analysis in this
program.
You may choose the method you are most
comfortable with and use it consistently.
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Calculating Dosages cont.
We all learn differently. It very important that you identify the
method that makes sense to you. If you don’t know which
method makes sense, try out each method on the same set of
practice problems and see which way makes sense. Once you
find a method stick with it. Resist the temptation to “formula
hop.”
Whichever method you choose follow these 3 steps:
1.Convert
2. Think
3. Calculate
Always ask yourself does my answer make sense.
It is normal to feel nervous, so try to remember to “KISS”
(Keep It Simple Students)
Ratio and Proportion:
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Solve for X
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Dose on hand
volume/tablet
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Example: The doctor orders 10 mg of a drug and you
have 5 mg tabs on hand. You will give ____ tabs.
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5mg = 10 mg
1 tab X tab
5X = 10 x 1 = 5X = 10
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=
What the Dr. ordered
X
X = 2 Tablets
Dimensional Analysis:
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Set up units of measurement so that they cancel
out, until you arrive at your correct
measurement
Always start with the dose ordered
10mg
1
X
1 Tab
5mg
=
10 Tab
5
=
2 Tablets
Formula:
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Do conversions first, and then plug it into the
equation to formulate the answer
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Dose ordered X Amount
Dose on hand
on hand
Ie: 10 mg X 1 Tab
5 mg
=
=
2 Tablets
Amount to
administer
Conversion practice:
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1 gram = _________ mg
1 cc = ___________mL
1 oz = ___________ mL
1 cc = ____________ mL
3 t = _____________ T
1 T = ____________ mL
1 t = _____________ mL
Conversion practice:
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1 gram = ___________grains
60 mg = ____________ grains
1 kg = _____________ lbs
1 in = ______________ cm
120 mg = ____________ grains
0.5 gram = ____________ grains
Rounding
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Do not round until you get to the answer. You
should always carry at least two decimal places
(hundredths 0.01)throughout the problem.
Answers must be rounded to the tenths (0.1)
Follow the rules of rounding: if the number
being rounded is from 1-4 then round down, 5-9
round up
Rounding cont
Answers in drops (gtt) must be rounded to the
nearest whole number.
 Capsules must be rounded to the nearest whole
 Enteric coated tablets must be rounded to the
whole.
 Unscored tablets should be rounded to the
whole.
In other words, round to the nearest deliverable
dose. Ex. If your answer is 1.9 suppositories or
21.7 gtts do these answers make sense?
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Diluting tube feeding solutions
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Occasionally a client may not be able to tolerate
full strength feeding solution. You may need to
dilute a feeding solution to an ordered strength,
such as ½ strength, 2/3 strength, etc.
Formula to dilute a feeding solution
Amount in can = Strength
X
300ml = 3
X
4
300ml x 4 =3x
1200ml=x
3
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400ml-300=100ml
of water
General guidelines and tips when
taking test
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Write everything you know at the top of your
scratch paper at the start of the test.
Identify the specific question being asked and
then identify the components that will help you
to arrive at the answer.
Set up the equation carefully, label all units in
the equation, make all conversions needed.
Follow the rules of math for order of
operations: multiply, divide, add and/or subtract
General guidelines and tips when
taking test cont.
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Use your calculator, even if you think you can
do it in your head.
Follow guidelines for rounding.
Show all your work.
Recheck your answer when submitting it to the
computer for keystroke errors.
Don’t second guess yourself!!!
Practice Problem #1
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The physician orders digoxin (Lanoxin) 5 mg.
The pharmacy sends 15 mg/5mL. How many
mL will you administer?
5mg x 5mL/15mg = 1.7mL
Practice Problem #2
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The physician orders Mycostatin 250,000 units
oral swish and swallow QID
You have on hand: 100,000 units/mL
How many mL will you administer?
250,000 units x mL/100,000 units= 2.5mL
Practice Problem #3
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You have an order for gr (grain) X
Acetaminophen (Tylenol) PO daily
You have Tylenol 15 mg/mL
How many mL will you administer?
10gr x 60mg/gr x mL/15mg= 40mL
Practice Problem #4
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Your patient has just had lunch and you must
calculate the total amount in mL consumed by
the patient:
4 oz Juice
 8 oz Milk
 6 oz Broth
1oz=30mL so….
120mL +240mL +180mL= 540mL
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Practice Problem #5
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The order is to start a continuous tube feeding
of 2/3 strength Jevity at 50ml/hr. How much
water will you add to the entire can to get the
desired strength if each can contains 300ml?
300ml = 2
x
3
300ml x 3= 2x
900ml=x
2
450ml=x
450ml-300ml=150
Reconstitution
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Some medications are unstable when stored in
solution or liquid form, therefore they are
supplied as a powder.
Before they can be administered they must be
dissolved or “reconstituted” by mixing with a
diluent (liquid)
The volume of diluent used to dissolve the
powder determines its unit concentration or the
dose of drug contained in 1ml (mg/ml)
Reconstitution cont.
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Vials usually will have reconstituting directions
printed on them so you must determine the type
and amount of diluent to be added in order to
reconstitute the drug correctly.
The resulting solution strength or concentration
(mg/ml) will be found in the instructions on the
vial. The mg/ml is now the available dose of
the drug ordered.
Reconstitution ex pges 237-244
Dosages based on body weight
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Sometimes dosages are ordered by body weight
of the pt., especially when giving pediatric doses.
Remember all medication math is done in the
metric system, so weight for dosages must be
converted and calculated in kg.
Remember 1lb=16oz, 1kg=2.2lb
When converting from lbs to kg, divide by 2.2
When converting ounces to lbs divide by 16
Dosages based on body weight
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Ex. A baby weighs 10lb 12oz. How many lbs is this?
First divide 12oz by 16= 0.75lb. So this baby weighs
10.75 lbs. Now how many kg does this baby weigh?
10.75/2.2=4.88kg (leave at 4.88 for calculations)
Now we must multiply the ordered mg/kg dose by the
weight of the pt in kg to get the desired dose for that
pt. Then do your dosage calculations.
Ex. Ordered drug is 2mg/kg, available is 10mg/ml, the
baby weighs 10lb 12oz, how much will you give?
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Wt=4.88 kg
2mg/kg=9.8mg
You will give 0.98ml or 1ml
Practice problem
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You have an order for cefaclor (Ceclor) 20
mg/kg/day in three divided doses. The child
weighs 20 lbs. How much will you give in each
dose?
Wt=9.09
 Daily dose=181.8
 Each dose=60.6
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Calculating IV flow rates
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IV flow rate is the speed at which the IV fluid infuses
in the body.
Usually measured in drops per minute (gtt/min)
The number of gtts required to deliver 1mL of fluid
varies with the type of administration set used.
The administration set is made of plastic tubing, a drip
chamber, and a “spike,” which is inserted into the
bag/bottle of fluid/medication.
There are 2 types of sets: Macrodrip=10,15,or 20
gtt/mL or Microdrip=60gtt/ml
Calculating Flow rates for IVs
IV flow rate formula:
 Volume of infusion in mL x gtt factor = Flow rate
time of infusion (usually in min)
(usually in
gtt/min)
Ex: Administer D5 1/2NS at 30mL/hr using a microdrip.
30mL x 60gtt/mL =30gtt/min
60 min
Administer 3,000mL D5 1/2NS in 24 hrs with a drop
factor of 15gtt/mL
3000mL x 15gtt/mL = 31.25gtt/min
1440 min
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Drugs Infusing in Less than 1 hour:
If a drug is ordered to be infused in 1 hour or
less what do you do?
Example: Mandol 1 Gm in 50 mL NS
Infuse in 30 minutes. Adm. Set: 15gtt/cc
50 mL x 15gtt/mL = 25gtt/min
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30 min
Example:
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You have an order for Aldomet 250 mg IVPB in
100 mL D5W to be infused over 45 minutes.
The administration set is labeled 10 gtt/mL
What is the drop rate?
100 mL x 10gtt/mL = 22.22 or 22gtt/min
45 minutes
Practice Problems
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The nurse hangs 1 Liter D5W to infuse over 8
hours. The administration set is labeled 60
gtt/mL.
What is the gtt/min?
1000 mL x 60gtt/ml =125gtt/min
480 min
Dosage Calculations
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The doctor orders 50 mL of Normal Saline (NS)
to infuse over 30 minutes. Your tubing is
labeled 15 gtt/mL.
50mL x 15gtt/mL = 25gtt/min
30 minutes
Do you now feel better about your
knowledge of dosage calculations?
Well, you should.
Looking to the future
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This semester we covered the basic knowledge
of dosage calculations. Each semester the math
test will add more advanced problem solving
along with the basic concepts learned here.
So….. Again ask for help if you need it and
PRACTICE, PRACTICE, PRACTICE