Brush up on
Your Drug Calculation
Skills
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Many nurses are weak with drug calculations of all sorts.
This article will help to review the major concepts related
to drug calculations, help walk you through a few
exercises, and provide a few exercises you can perform on
your own to check your skills. There are many reference
books available to review basic math skills, if you find that
you have difficulty with even the basic conversion
exercises.
Common Conversions:
1 Liter = 1000 Milliliters
1 Gram = 1000 Milligrams
1 Milligram = 1000 Micrograms
1 Kilogram = 2.2 pounds
Methods of Calculation
Any of the following three methods can be used to
perform drug calculations. Please review all three
methods and select the one that works for you. It is
important to practice the method that you prefer to
become proficient in calculating drug dosages.
Remember: Before doing the calculation, convert units of
measurement to one system.
I.
Basic Formula: Frequently used to calculate drug
dosages.
D (Desired dose)
H (Dose on hand)
V (Vehicle-tablet or liquid)
D
x V = Amount to Give
H
D = dose ordered or desired dose
H = dose on container label or dose on hand
V = form and amount in which drug comes (tablet,
capsule, liquid)
Example: Order-Dilantin 50 mg p.o. TID
Drug available-Dilantin 125 mg/5ml
D=50 mg
H=125 mg
V=5 ml
50
250
x5=
= 2 ml
125
125
II.
Ratio & Proportion: Oldest method used in
calculating dosage.
Known
H
:
Desired
V ::
D :
X
Means
Extremes
III.
IV.
V.
Left side are known quantities
Right side is desired dose and amount to give
Multiply the means and the extremes
HX = DV
X=
DV
H
Example: Order-Keflex 1 gm p.o. BID
Drug available-Keflex 250 mg per
capsule
VIII.
IX.
X.
D=1 gm (note: need to convert to milligrams)
1 gm = 1000 mg
H=250 mg
V=1 capsule
250
: 1
XI.
::
: X
250X = 1000
X=
XIII.
1000
1000
250
XII.
X = 4 capsules
Fractional Equation
H D
=
V X
XIV.
Cross multiply and solve for X.
H D
=
V X
HX = DV
X=
DV
H
Example: Order - Digoxin 0.25 mg p.o. QD
Drug Available - 0.125 mg per tablet
D=0.25 mg
H=125 mg
V=1 tablet
0.125 0.25
=
1
X
XX.
0.125X = 0.25
X=
XXII.
0.25
0.125
XXI.
X = 2 tablets
Intravenous Flow Rate Calculation (two methods)
Two Step
Step 1 - Amount of fluid divided by hours to
administer = ml/hr
ml/hr x gtts/ml(IV set)
Step 2 = gtts/min
60 min
One Step
amount of fluid x drops/milliliter (IV set)
hours to administer x minutes/hour (60)
Example: 1000 ml over 8 hrs
IV set = 15 gtts/ml
Two Step
Step 1 -
1000
= 125
8
125 x 15
= 31.25 (31 gtts/min)
60
One Step
1000 x 15 15,000
=
= 31.25 (31gtts/min)
8 hrs x 60
480
Step 2 -
XXIII.
How to Calculate Continuous Infusions
A. mg/min (For example - Lidocaine, Pronestyl)
Solution cc x 60 min/hr x mg/min
= cc/hr
Drug mg
Drug mg x cc/hr
= mg/hr
Solution cc x 60 min/hr
Rule of Thumb
Lidocaine, Pronestyl
2 gms/250 cc D5W
1 mg = 7 cc/hr
2 mg = 15 cc/hr
3 mg = 22 cc/hr
4 mg = 30 cc/hr
D. mcg/min (For example - Nitroglycerin)
Solution cc x 60 min/hr x mcg/min
= cc/hr
Drug mcg
Drug mcg x cc/hr
= mcg/hr
Solution cc x 60 min/hr
Rule of Thumb
NTG 100 mg/250 cc
1 cc/hr = 6.6 mcg/min
NTG 50 mg/250 cc
1 cc/hr = 3.3 mcg/min
G. mcg/kg/min (For example - Dopamine,
Dobutamine, Nipride, etc.)
1. To calculate cc/hr (gtts/min)
Solution cc
x 60 min/hr x kg x mcg/kg/min = cc/hr
Drug mcg
Example: Dopamine 400 mg/250 cc D5W to start
at 5 mcg/kg/min.
Patient’s weight is 190 lbs.
250 cc
400,000
mcg
x 60 min x 86.4 x 5 mcg/kg/min = 16.2
cc/hr
4. To calculate mcg/kg/min
Drug mcg/ x cc/hr
= mcg/kg/min
Solution cc x 60 min/hr x kg
Example: Nipride 100 mg/250 cc D5W was
ordered to decrease your patient’s blood
pressure.
The patient’s weight is 143 lbs, and the
IV pump is set at 25 cc/hr. How many
mcg/kg/min of Nipride is the patient
receiving?
100,000 mcg x 25
cc/hr
2,500,000
=
250 cc x 60 min x 65
975,000
kg
= 2.5
mcg/kg/min
A. How to calculate mcg/kg/min if you know the rate
of the infusion
Dosage (in mcg/cc/min) x rate on
pump
Patient’s weight in kg
=
mcg/kg/min
B. For example:
400mg of Dopamine in 250 cc 1600 mcg/cc
D5W = 60 min/hr
=
26.6
mcg/cc/min
C. 26.6 is the dosage concentration for Dopamine in
mcg/cc/min based on having 400 mg in 250 cc of IV
fluid. You need this to calculate this dosage
concentration first for all drug calculations. Once
you do this step, you can do anything!
D. NOW DO THE REST!
E. If you have a 75 kg patient for example...
26.6 mcg/cc/min x 10 cc
on pump
= 3.54 mcg/kg/min
Patients’s weight in kg
(75 kg)
= 3.5 mcg/kg/min
(rounded down)
F. How to calculate drips in cc per hour when you
know the mcg/kg/min that is ordered or desired
mcg/kg/min x patient’s weight in
kg
dosage concentration in
mcg/cc/min
= rate on
pump
G. For example:
H. 400 mg Dopamine in 250 cc D5W = 26.6 mcg/cc/min
3.5 mcg/kg/min x 75 kg
= 9.86 cc
26.6 mcg/cc/min
= 10 cc rounded up
I. ALWAYS WORK THE EQUATION
BACKWARDS AGAIN TO DOUBLE CHECK
YOUR MATH!
J. For example:
10 cc x 26.6 mcg/cc/min
= 3.5 mcg/kg/min
75 Kg
Dosage (in mcg/cc/min) x rate on
pump
Patient’s weight in kg
=
mcg/kg/min
L. For example:
M. 400mg of Dopamine in 250 cc D5W = 1600 mcg/cc
60 min/hr = 26.6 mcg/cc/min
N. 26.6 is the dosage concentration for Dopamine in
mcg/cc/min based on having 400 mg in 250 cc of IV
fluid. You need this to calculate this dosage
concentration first for all drug calculations. Once
you do this step, you can do anything!
O. NOW DO THE REST!!
P. If you have a 75 kg patient for example
26.6 mcg/cc/min x 10 cc on
pump
Patients’s weight in kg (75 kg)
= 3.54
mcg/kg/min
Now do some practice exercises to check what you learned
A. Practice Problems:
1. 2.5 liters to milliliters
2. 7.5 grams to milligrams
3. 10 milligrams to micrograms
4. 500 milligrams to grams
5. 7500 micrograms to milligrams
6. 2800 milliliters to liters
7. 165 pounds to kilograms
8. 80 kilograms to pounds
B. Practice Problems: Use the method you have
chosen to calculate the amount to give.
1. Order-Dexamethasone 1 mg
Drug available-Dexamethasone 0.5 mg per
tablet
2. Order-Tagamet 0.6 gm
Drug available-Tagamet 300 mg per tablet
3. Order-Phenobarbital 60 mg
Drug available-Phenobarbital 15 mg per
tablet
4. Order-Ampicillin 0.5 gm
Drug available-Ampicillin 250 mg per 5 ml
5. Order-Dicloxacillin 125 mg
Drug Available-Dicloxacillin 62.5 mg per 5
ml
6. Order-Medrol 75 mg IM
Drug Available-Medrol 125 mg per 2 ml
7. Order-Lidocaine 1 mg per kg
Patient’s weight is 152 pounds
8. Order- 520 mg of a medication in a 24 hour
period. The drug is ordered every 6 hours.
How many milligrams will be given for each
dose?
C. Practice Problems:
1.
Order-1000 ml over 6
hrs
2.
Order-500 ml over 4
hrs
3.
Order-100 ml over 20
min.
IV set 15
gtts/ml
IV set 10
gtts/ml
IV set 15
gtts/ml
D. Practice Problems:
1. Dopamine 400 mg in 250 cc D5W to infuse at
5 mcg/kg/min. The patient’s weight is 200
pounds. How many cc/hour would this be on
an infusion pump?
2. A Dopamine drip (400mg in 250 cc of IV
3.
4.
5.
6.
fluid) is infusing on your 80 kg patient at 20
cc/hour. How many mcg/kg/min are
infusing for this patient?
A Nitroglycerin drip is ordered for your
patient to control his chest pain. The
concentration is 100 mg in 250 cc D5W. The
order is to begin the infusion at 20 mcg/min.
What is the rate you would begin the
infusion on the infusion pump?
A Nitroglycerin drip (100mg in 250 cc D5W)
is infusing on your patient at 28 cc/hour on
the infusion pump. How many mcg/min is
your patient receiving?
A procainamide drip is ordered (2gms in
250 cc D5W) to infuse at 4 mg/min. The
patient weighs 165 pounds. Calculate the
drip rate in cc/hour for which the infusion
pump will be set at.
A Lidocaine drip is infusion on your 90 kg
patient at 22 cc/hour. The Lidocaine
concentration is 2 grams in 250 cc of D5W.
How many mg/min is your patient
receiving?
Summary
Many nurses have difficulty with drug calculations.
Mostly because they don’t enjoy or understand math.
Practicing drug calculations will help nurses develop
stronger and more confident math skills. Many drugs
require some type of calculation prior to administration.
The drug calculations range in complexity from requiring
a simple conversion calculation to a more complex
calculation for drugs administered by mcg/kg/min.
Regardless of the drug to be administered, careful and
accurate calculations are important to help prevent
medication errors. Many nurses become overwhelmed
when performing the drug calculations, when they require
multiple steps or involve life-threatening drugs. The main
principle is to remain focused on what you are doing and
try to not let outside distractions cause you to make a
error in calculations. It is always a good idea to have
another nurse double check your calculations. Sometimes
nurses have difficulty calculating dosages on drugs that
are potentially life threatening. This is often because they
become focused on the actual drug and the possible
consequences of an error in calculation. The best way to
prevent this is to remember that the drug calculations are
performed the same way regardless of what the drug is.
For example, whether the infusion is a big bag of vitamins
or a life threatening vasoactive cardiac drug, the
calculation is done exactly the same way.
Many facilities use monitors to calculate the infusion
rates, by plugging the numbers in the computer or
monitor with a keypad and getting the exact infusion
titration chart specifically for that patient. If you use this
method for beginning your infusions and titrating the
infusion rates, be very careful that you have entered the
correct data to obtain the chart. Many errors take place
because erroneous data is first entered and not identified.
The nurses then titrate the drugs or administer the drugs
based on an incorrect chart. A method to help prevent
errors with this type of system is to have another nurse
double check the data and the chart, or to do a hand
calculation for comparison. The use of computers for drug
calculations also causes nurses to get “rusty” in their
abilities to perform drug calculations. It is suggested that
the nurse perform the hand calculations from time to
time, to maintain her/his math skills.
Answers to Practice Problems
A. Practice Problems
1. 2500 mL
2. 7500 mg
3. 10,000 mcg
4. 0.5 gm
5. 7.5 mg
6. 2.8 L
7. 75 kg
8. 176 lbs
B. Practice Problems
1. 2 tablets
2. 2 tablets
3. 4 tablets
4. 10 mL
5. 10 mL
6. 1.2 mL
7. 69 kg = 69 mg
8. 130 mg for 4 doses
C. Practice Problems
1. 41.6 (42)
2. 20.8 (21)
3. 75
D. Practice Problems
1. 17 cc/hr
2. 6.65 mcg/kg/min
3. 3 cc
4. 186.5 mcg/min
5. 30 cc/hr
6. 3 mg/min
Reference: Dosage Calculations Made Incredibly Easy! by
Springhouse Corporation, 1998
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