Math for Medications Some terms youll see: Desired Dose Available Dose Ratio & Proportion Cross Product.
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Math for Medications
Some terms you’ll see:
Desired Dose
Available Dose
Ratio & Proportion
Cross Product
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Math for Medications
• The purpose of this class is for the learner to be able to calculate drug dosages of tablets and liquids.
• You will calculate the drug dosages using the formula or ratio & proportion method.
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Math for Medications- the Formula Method for Tablets
Desired dose X Vehicle = ATA
Available dose
• dd x v = ATA
ad (amount to administer)
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Problem
• The doctor ordered Benadryl 75 mg. The drug label reads Benadryl 0.025 grams. How many tablets are needed?
• Read the problem and identify what you’ve been given.
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Problem
• dd = 75 mg
• ad = 0.025 grams
• v = 1 tab
• Have both dd & ad in the same unit of measure 0.025 g = 25 mg.
• ad = 25 mg
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Problem
• Put the numbers into the formula:
75 mg x 1 tab = 3 tabs
25 mg
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Math for Medications- The Ratio & Proportion Method for Tablets
• We will use the same problem but will cross multiply.
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Problem
• The doctor ordered Benadryl 75 mg. The drug label reads Benadryl 0.025 grams. How many tablets are needed?
• Read the problem and identify what you’ve been given.
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Problem
• The known ratio: 25 mg
1 tablet
• The unknown ratio: 75 mg
N tablets
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Problem
• Write the proportion :
25 mg x 75 mg
1 tablet N tablets
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Problem
• Cross Multiply:
25 mg x 75 mg
1 tablet N tablets
25mg X N tab = 1 X 75 mg
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Problem
• Solve for N by dividing both sides of the equation by 25:
• 25 N = 75
25 25
or N = 3
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Problem
• Substitute 3 for N in the original proportion and your answer is : You would administer 3 tablets to give a dosage of 75 mg.
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Math for Medications for Liquids
• Calculate drug dosages using the formula method or ratio & proportion method for liquids.
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Problem
• The doctor has ordered Gentamycin Sulphate 25 mg.
• The label reads Gentamycin Sulphate 40 mg/mL. How much Gentamycin will you administer?
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Problem - Using the Formula Method
• Identify:
dd = 25 mg
ad = 40 mg
v = 1 mL
• Put the numbers in the formula:
25 x 1 mL = 0.625 mL
40
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Problem
• Round off to the nearest decimal place:
• 25 x 1 mL= 0.625 mL or 0.6 mL
40
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Problem- Using the Ratio & Proportion method
• Cross product ( Cross Multiply) :
• The know ratio: 40 mg
1 mL
• The unknown ratio: 25 mg
N mL
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Problem
• Proportion: 40 mg = 25mg
1 mL N mL
• Cross Multiply:
40 x N = 1 x 25
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Problem
• Solve for N:
40N = 25
40 40
N = 0.625
N = 0.625
Round off: N = 0.6 mL
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• Substitute for N in the original proportion:
• Proportion: 40 mg = 25mg
1 mL 0.6 mL
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Math for Medications
Another way to calculate
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Math for Medications The “Math Chart”
• Lets look at another way to calculate
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Math for MedicationsThe “Math Chart”
• When we are calculating drug dosages of tablets and liquids we always have 2 pieces of information.
• 1) What the label reads
• 2) What has the doctor ordered
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Math for MedicationsThe “Math Chart”
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Math for MedicationsThe “Math Chart”
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Math for MedicationsThe “Math Chart”
• Once we have our chart filled in, all we do is
MULTIPLY ON THE DIAGONAL AND DIVIDE BY WHAT’S LEFT
Ex: Mg 30 45 Mg
Tablets 1 Tablets
Lets do our calculations:45 X 1 ÷ 30 = 1.5 tablets
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Math for MedicationsThe “Math Chart”
• Lets look at the first example
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Math for MedicationsThe “Math Chart”
• The doctor ordered Benadryl 75 mg. The drug label reads Benadryl 0.025 grams. How many tablets are needed?
**The first thing we have to do is to look at what information we have.
1)What the label reads – 0.025g for 1 tablet
2)What the doctor ordered – 75mg
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Math for MedicationsThe “Math Chart”
***Before me make our chart we have to make sure both units are the same.
• In this question, we have 0.025 g & 75 mg
• Lets change 0.025g to mg by multiplying by 1000
• Now both units are the same 25mg and 75mg and we can make our chart
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Math for MedicationsThe “Math Chart”
• Step 1 - Make the chart and label it• Step 2 – Fill in the information that we have
Mg Mg25 75
1Tablets Tablets
• Step 3 – Do the calculations MULTIPLY ON THE DIAGONAL AND DIVIDE BY WHAT’S LEFT
75 X 1 ÷ 25 = 3 tablets
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Math for MedicationsThe “Math Chart”
• Lets try the next example:
• The doctor has ordered Gentamycin Sulphate 25 mg.
• The label reads Gentamycin Sulphate 40 mg/mL. How much Gentamycin will you administer?
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Math for MedicationsThe “Math Chart”
**The first thing we have to do is to look at what information we have.
1)What the label reads – 40mg for 1 mL
2)What the doctor ordered 25mg
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Math for MedicationsThe “Math Chart”
• Step 1 - Make the chart and label it• Step 2 - Fill in the information we have
Mg Mg
40 25
1mL mL
• Step 3 – Do our calculations• 25 x 1 ÷ 40 = 0.625 mL
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Rules to Remember
• Example: 0.66mg = 0.7 mg
1.Put 0 to the left of the decimal if there is no whole number.
2.Adults: round of drug doses to the nearest 1/10 or 0.1.
3.Pediatrics: Round off drug doses to the nearest 1/100 or 0.01
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Rules to Remember
• Do not round off until your final answer:
For example: 100 mg x 15 mL
80 mg
10=1.25 x 15 mL = 18.75 mL
8
Not 1.3 x 15 mL which would =19.5mL
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References
• AOM coursepack
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