Transcript Document

Pharmacology Math
Chapter 33
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
Drug Management
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The medical assistant must be absolutely certain that
the medication prepared and administered to the
patient is exactly what the physician ordered.
Although drugs often are delivered by the pharmacy in
unit dose packs, the dosage ordered may differ from
the dose on hand.
In this case the medical assistant must be prepared to
calculate the correct dose accurately.
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
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Dosages
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There is no margin of error in drug calculations.
Even minor mistakes may result in serious
complications.
The MA must take meticulous care in calculating all
drug dosages.
If the dosage ordered by the physician is different
from the dosage on hand, the MA must complete
three basic steps.
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
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Three Steps for Correct Dosage
1. Based on the type of system printed on the label,
determine if the physician order is in the same
mathematic system of measurement.
 If the systems vary (the order is in teaspoons but
the label states the medication is to be prepared in
milliliters), then accurately convert the order to
match the system on the label.
2. Perform the calculation in equation form, using the
appropriate formula.
3. Check your answer for accuracy, and ask someone
you trust to confirm your calculations.
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
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Accurate Dose
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All three of these steps must be completed before the
medication is dispensed and administered.
Confirm your calculations with the physician if you have
any doubt about their accuracy.
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
Slide 5
Accurate Dose (cont’d)
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Slide 6
Drug Labels
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Accurately read the drug label to determine if the
physician order and the packaged drug use the same
system of measurement.
Examples:
– Physician orders 10 ml of a drug and the drug label
states there are 20 ml/cc.
– Physician orders 4 g of a drug and the medication
label states there are 2 g/tab.
– Physician orders 6 mg of a drug and the medication
label reads there are 12 mg/scored tab.
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Drug labels
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
Slide 8
Label Terms
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Strength: The potency of the drug. Stated as a
percent of drug in the solution (2% epinephrine); as a
solid weight (g, mg, lb, gr); or as a milliequivalent or
unit.
Dosage: The size or amount of medication in the
drug unit. Could be in ml, tsp, or a number of tabs. For
example, label reads “Imitrex, 6 mg/0.5 ml,” which
means there are 6 mg of Imitrex in each 0.5 ml.
Solute: Pure drug dissolved in a liquid to form a
solution.
Solvent (Diluent): The liquid, usually sterile water,
that dissolves the solute.
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
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Systems of Measurement
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Physician may order medication in a strength that is
different than the one identified on the drug label.
– Example: orders 2 gr of the drug but the label states there are
120 mg/tab
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Before determining how many tablets to administer,
the MA must first convert the strength of the physician
order to match the strength of the dose on the label
since that is the medication that is available for
administration.
3 different systems of measurement: metric,
apothecary, and household.
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
Slide 10
Metric System
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Metric system of weights and measures is used
throughout the world as the primary system for weight
(mass), capacity (volume), and length (area).
Based on units of 10
– Each larger unit of measure is 10 times the previous unit of
measure.
– Fractions are written as decimals (1½ L = 1.5 L).
– Cubic centimeter = milliliter (1 cc holds 1 ml).
– Amount or volume of a liquid medication is expressed in
milliliters.
– Weight or strength of a solid medication is expressed in
grams.
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
Slide 11
Conversions to Smaller Units
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Units in the metric system are converted by moving the
decimal point in multiples of 10.
When going from larger to smaller units of measure,
as in converting grams to milligrams, the answer will
be a larger number, so move the decimal point to the
right.
– 0.35 g = 350 mg
– OR multiply 0.35 g × 1000 = 350 mg
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Conversions to Larger Units
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If converting smaller units of measurement to larger
ones as in milliliters to liters, the answer will be a
smaller number, so move the decimal point to the left.
– 150 ml = 0.15 L
– OR divide 150 ml by 1000 = 0.15 ml
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
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Metric System Equivalents
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The following equivalents can be used to make
conversions in the metric system.
1 kg = 1000 g
1 g = 1000 mg
1 mg = 0.001 g or 1/10,000 g
1 kl = 1000 liters
1 L = 1000 ml
1 ml = 0.0001 L or 1/1000 L
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
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Apothecary System
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In the apothecary system the basic unit of weight for a
solid medication is the grain (gr) and the basic unit of
volume for a liquid medication is the minim (M).
As in the metric system, these two units are related:
the grain is based on the weight of a single grain of
wheat, and the minim is the volume of water that
weighs 1 gr.
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
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System to System Equivalents
15 gr = 1 g = 1000 mg
5 gr = 0.3 g = 300 mg
1 quart = 1000 cc
1 fl oz = 30 ml
1 fl dr = 4 ml
15 M = 1 ml
1 M = 0.06 ml
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Household Measurements
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The household system is used in most U.S. households.
Although this system of measurement is important for
the patient at home who has no knowledge of the
metric or apothecary systems, it is not completely
accurate. The household system should never be used
in the medical setting.
The basic measure of weight is the pound (lb) and of
volume is the drop (gtt).
– 1 gtt = 1 M
– 60 gtt = 1 tsp
– 3 tsp = 1 Tbsp (tablespoon)
– 2 Tbsp = 1 oz
– 8 oz = 1 cup
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Conversions between Systems
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Formula:
Drug Have × Wanted = Unit wanted in new system
Have
• Drug Have—unit of measure that is on label (1 g/tab)
• Wanted—amount or strength ordered by physician
• Have—conversion (15 gr = 1 g)
Order: Administer 30 gr of Lasix
Label: 1 g Lasix/tab
1 g × 30 gr = 2 g = 2 tabs
15 gr
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
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Calculating Drug Dosages for
Administration
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Based on the strength of the drug and the amount as
stated on the label
– Example: 15 mg/tab means that each tablet in the
bottle contains 15 mg of the drug, OR 250 mg/cc
means that for each cc in the vial there is 250 mg of
the drug.
– Standard Formula
Available Strength = Available Amount
Ordered Strength
Amount to Give
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Pediatric Doses and Fried's Law
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Most important factor in determining the amount of a
drug that should be administered to children is the
weight of the child.
Even a minor miscalculation may be dangerous.
This calculation is for children younger than 1 year and
is based on the age of the child in months compared
with a child 12½ years old.
Pediatric dose = Child's age in months  Adult dose
150 months
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Young's Rule
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Young's rule is for children older than 1 year
Pediatric dose = _Child's age in years__  Adult dose
Child's age in years + 12
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Clark's Rule
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This rule is based on the weight of the child.
This system is much more accurate, because children of
any age can vary greatly in size and body weight.
Pediatric dose = Child's weight in pounds  Adult dose
150 pounds
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
Slide 22
West's Nomogram
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West's nomogram uses a calculation of the body
surface area of infants and young children to determine
the pediatric dose.
Pediatric dose = BSA of child in m2 × Adult dose
1.7 m2
(Average adult BSA = 1.7 m2.)
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West’s Nomogram
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Reconstitution
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Reconstituting powdered injectables requires adding an
amount of solvent (as recommended on the drug label)
to a vial of powdered or crystal medication.
Once the solute and solvent are mixed in the vial, a
solution of medication is formed with a strength based
on equivalents printed on the drug label.
Once the medication is mixed, carefully read the label
to determine how much of the drug must be withdrawn
to equal the physician’s order.
Use the standard conversion formula to determine the
accurate dose for administration.
Copyright © 2007 by Saunders, Inc., an imprint of Elsevier Inc.
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