1. Measurement Systems, Basic Mathematics, and Dosage Calculations
Chapter 52
Measurement Systems, Basic Mathematics, and Dosage Calculations
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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)
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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
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The Metric System
Prefix
Unit
Decimal Value
Micro-
1/1,000,000 of a unit
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
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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
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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)
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Household Measures
60 gtt = 1 tsp
3 t = 1 T
2 T = 1 fl oz
1 cup = 8 fl oz
1 pt = 2 cups
1 qt = 4 cups (2 pt)
1 lb = 16 oz
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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
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Review of Basic Math
Basic math skills are necessary for accurately calculating and verifying medication dosages to ensure patient safety before administering any medication
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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)
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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
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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
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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
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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
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15. 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
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16. 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
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