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Chapter 52
Measurement Systems,
Basic Mathematics,
and Dosage Calculations
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
The Metric System
• Primary system of measurement in medicine
• Three basic units:
– Gram (g), which measures mass (weight)
– Liter (L), which measures volume (liquid)
– Meter (m), which measures length (size)
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
The Metric System
• Based on multiples of 10
– Add prefixes to the basic units to indicate the
value (multiple or submultiple) of the unit
– Kilo = 1000
– Kilo+gram = 1000 grams
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
The Metric System
Prefix
Unit
Decimal Value
Micro-
1/1,000,000 of a unit
0.000001
Milli-
1/1,000 of a unit
0.001
Centi-
1/100 of a unit
0.01
Deci-
1/10 of a unit
0.1
No prefix
1 unit
1
Deka-
10 units
10
Hecto-
100 units
100
Kilo-
1,000 units
1,000
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
Household Measures
• Patients at home typically use household
measurements
• You may need to use these measurements to
explain to patients how to take their
medications after discharge
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
Household Measures
•
•
•
•
•
Drop (gtt)
Teaspoon (t or tsp)
Tablespoon (T or tbs)
Ounce (fluid) (fl oz)
Cup
•
•
•
•
Pint (pt)
Quart (qt)
Ounce (weight) (oz)
Pound (lb)
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
Household Measures
60 gtt = 1 tsp
3t=1T
2 T = 1 fl oz
1 cup = 8 fl oz
1 pt = 2 cups
1 qt = 4 cups (2 pt)
1 lb = 16 oz
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
The Apothecary System
• The first system of medication measurement,
but not commonly used today
• Basic units:
– Grain (gr)
– Quart (qt)
– Pint (pt)
– Ounce or fluid ounce (oz)
– Dram
– Minim
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
Review of Basic Math
• Basic math skills are necessary for accurately
calculating and verifying medication dosages
to ensure patient safety before administering
any medication
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
Review of Basic Math
• A fraction indicates a part of a whole number
– Numerator: top of fraction
– Denominator: bottom of fraction
• A percentage expresses a value that is part of
100
Example >> 35% is 35/100 (or 0.35)
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
Review of Basic Math
• A ratio expresses a relationship between two
components
– It is another way to express a fraction
Example >> 1 : 5 is the same as 1/5
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
Review of Basic Math
• A proportion expresses the relationship
between two ratios
Example >> 3 : 5 = 600 : 1000
– Means: 5 and 600
– Extremes: 3 and 1000
– The product of the means always equals the
product of the extremes
– You can solve for X if you know the other 3 values
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
Dosage Calculations
• Calculating the right dose of medication is a
responsibility not to be taken lightly
• The wrong dose of any medication can have
profound adverse consequences
Copyright ©2012 Delmar, Cengage Learning.
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Basic Formula
• (Needed/Available) x Vehicle = Dose
– Needed > Amount prescribed by provider
– Available > Supply on hand
– Vehicle > Form and amount of supply
Copyright ©2012 Delmar, Cengage Learning.
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Ratio and Proportion Method
• Dosage on hand : amount on hand =
Dosage desired : amount desired (x)
• When using this method, you may need to
convert between units of measurement
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.
Converting Between Units of
Measurement
• Identify the equivalent
• Set up a proportion of two equivalent ratios
• Solve for x
• Calculating dosage by weight
– Conversion factor: 1 kg = 2.2 lbs
Copyright ©2012 Delmar, Cengage Learning.
All rights reserved.