1. 52
Dosage Calculation

2. Learning Outcomes
52.1 Explain the role of the medical assistant to ensure safe dosage calculations.
52.2 Identify systems of measurements and their common uses.
52.3 Convert among systems of measurements.

3. Learning Outcomes 52.4 Execute dosage calculations accurately.
52.5 Calculate dosages based upon body weight and body surface area.

4. Introduction Administer medications Close attention to detail
Dosage calculation must be accurate
Basic math
Systems of measurement
Conversions within or between measurement systems
Calculations for special patient populations
Learning Outcome: Explain the role of the medical assistant to ensure safe dosage calculations.
As a medical assistant, you may be called upon to administer medications.
Dosage calculations must be completed carefully and accurately in order to prevent medication errors.
This chapter will provide the basics of safe dosage calculations.
Remember to check the scope of practice in your state and at your place of employment before working with dosages.

5. Ensuring Safe Dosage Calculations
Minor math errors major error in dosage
Each time you calculate a dosage
Analyze the problem
Estimate a range
Is the answer reasonable?
Learning Outcome: Explain the role of the medical assistant to ensure safe dosage calculations.
You must understand and be able to perform basic math accurately.
A minor mistake in basic math can mean major errors in the patient’s medication.
Think about your answer and determine if it is reasonable.
Every time you perform a calculation, analyze the problem and try to estimate a reasonable range for the answer.
This critical thinking skill can help you to detect errors and should become a part of every calculation you perform.

6. Measurement Systems Three systems Focus on the basic units of
Metric ~ most common
Apothecary
Household
Focus on the basic units of
Volume
Weight
Learning Outcome: Identify systems of measurements and their common uses.
Three systems of measurement are used in the United States for pharmacology and drug administration.
You need to remember the basic unit of
Volume – the amount of space a drug occupies.
Weight – refers to its heaviness.

7. Metric System Based on multiples of 10 Basic units
Volume = liter (L)
Weight = gram (g)
Prefixes indicate multiples of each
Learning Outcome: Identify systems of measurements and their common uses.
Like the decimal system, the metric system is based on multiples of 10.
Refer to the Caution: Handle with Care feature, Working with Decimals
The basic units of volume and weight
Liters (L) to measure volume
Grams (g) to measure weight.
Prefixes are added to these basic units of measurement to indicate multiples.

8. Metric System Prefix Kilo- Base unit Centi- Milli- Micro- Value × 1000--
÷ 100
÷ 1000
÷ 1,000,000
Weight
kilogram (kg)
1000 g
gram (g)
1 g
centigram (cg)
0.01 g
milligram (mg)
0.001 g
microgram (mcg) g
Volume
kiloliter (kL)
1000 L
liter (L)
1 L
centiliter (cL)
0.01 L
milliliter (mL)
0.001 L
microliter (mcL) L
Learning Outcome: Identify systems of measurements and their common uses.
A cubic centimeter (cc) is the amount of space occupied by 1 mL.
Although these two measurements are equal, the accepted medical abbreviation is mL.
Do not use the abbreviation “cc,” even though you may sometimes see it in practice.
Additionally, note that the abbreviation for liters is a capital L instead of a small l. The small l can be confused with the numeral 1.

9. Apothecary and Household Systems
Volume
Fluid ounces
Fluid drams
Pint quart
Weight
Drams
Ounces
Pounds
Learning Outcome: Identify systems of measurements and their common uses.
The apothecary system uses
Fluid ounces, fluid drams, pints, and quarts for volume
Drams, ounces, and pounds for weight.

10. Apothecary and Household Systems
Household – only volume
Drops
Teaspoons and tablespoons
Ounces
Cups
Pints, quarts, and gallons
Learning Outcome: Identify systems of measurements and their common uses.
The only household units used for measurement are units of volume: drops, teaspoons, tablespoons, ounces, cups, pints, quarts, and gallons.
Units of measurement found in both the apothecary and the household systems are equal: an apothecary ounce equals a household ounce.

11. Apothecary and Household Systems
Apothecary Units
Equivalent
Measures of Volume
8 fluid drams (fl dr)
1 fluid ounce (fl oz)
16 fl oz
1 pint (pt)
2 pt
1 quart (qt)
4 qt
1 gallon (gal)
Measures of Weight
60 gr
1 dram (dr)
8 dr
1 ounce (oz)
16 oz
1 pound (lb)
Learning Outcome: Identify systems of measurements and their common uses.

12. Apothecary and Household Systems
Household Units
Equivalent
Measures of Volume
60 drops* (gtt) =
1 teaspoon (tsp)
3 tsp =
1 tablespoon (tbsp)
6 tsp =
1 ounce (oz) or 2 tbsp
8 oz =
1 cup (c)
2 c =
1 pint (pt)
4 c =
1 quart (qt) or 2 pt
Learning Outcome: Identify systems of measurements and their common uses.

13. Impressive! Apply Your Knowledge ANSWER:
Match the following: units of measurements are the same as in the household system drops multiples of ten only uses units of volume fluid dram microliter
Metric
Apothecary
Household B C A
Learning Outcome: Identify systems of measurements and their common uses. C
Impressive! B A

14. Conversions Within and Between Measurement Systems
Rules for converting within the Metric System
Move the decimal point to the right when you convert from a larger to a smaller unit.
This is dividing.
Move the decimal point to the left when you convert from a smaller to a larger unit.
This is multiplying.
Learning Outcome: Convert among systems of measurements.
Most commonly you will convert within the metric system.
For example, you may need to determine how many milligrams of medication to give a patient when the medication only comes in grams.
Sometimes you may need to convert from one measurement system to another.
For example, a patient may need to take five milliliters of medication and the only device she has is a teaspoon.

15. Converting Within the Metric System
Determine the direction and number of places to move the decimal point
A milliliter is three decimal places to the right of liter
A kilogram is three decimal places to the left of a gram
Learning Outcome: Convert among systems of measurements.

16. Converting Within the Metric System (cont.)
Learning Outcome: Convert among systems of measurements.
LEFT 3

17. Converting Between Systems
Be familiar with equivalent measures
Conversion charts – check carefully before administering a drug
1 teaspoon = 5 mL
1 tablespoon = 15 mL
1 pint = 480 mL
1 kg = 2.2 lbs
1 fl oz = 30 mL
1 fl oz = 2 tbsp
Learning Outcome: Convert among systems of measurements.
When converting between systems, you must be familiar with their equivalent measures.
Conversions between systems are only approximate equivalents.
If you use a conversion chart, read it carefully before administering a drug.
Check it several times, and place a ruler under the line you are reading to be absolutely sure you are reading the chart properly.
Table 52-4 Common Equivalent Measures for the Metric, Apothecary, and Household (on slide)

18. Converting Between Systems (cont.)
Learning Outcome: Convert among systems of measurements.
You may need to convert between systems of measurement by doing a calculation.
The proportion method is one way to calculate these conversions.

19. Converting Between Systems (cont.)
Learning Outcome: Convert among systems of measurements.
You may need to convert between systems of measurement by doing a calculation.
The proportion method is one way to calculate these conversions.

20. Apply Your Knowledge Right!
Which measuring system is most commonly used?
ANSWER: The metric system.
Convert 25 grams to milligrams.
ANSWER:
Add a decimal point to the measurement: 25. g
Add 3 zeros so you can move the decimal point three places to the right: g
Move the decimal point to the right three places: 25,000
Change the unit: 25,000 mg
Learning Outcome: Convert among systems of measurements.

21. Dosage Calculations Must be accurate Use Double check
Have coworker check
Use
Proportion method
Formula method
Learning Outcome: Execute dosage calculations accurately.
Follow your scope of practice.
You may be able to calculate these using either the proportion method or a formula method.
The patient’s health or life can depend on your calculations. Always take the time to check and recheck your arithmetic.
If you have a question or you are not sure about your calculations, check the problem again and then have a coworker check
Refer to Points on Practice: Math Review for a quick review of basic math.

22. Proportion Method for Dosage Calculations
Learning Outcome: Execute dosage calculations accurately.

23. Proportion Method for Dosage Calculations (cont.)
Learning Outcome: Execute dosage calculations accurately.

24. Proportion Method for Dosage Calculations (cont.)
Learning Outcome: Execute dosage calculations accurately.

25. Formula Method for Dosage Calculations
Learning Outcome: Execute dosage calculations accurately.
You can use this formula to calculate drugs that do not require a conversion between systems.
The basic formula that you would use looks like this:
D/H × Q
Using this formula, you will need to know the following:
D = desired dose of medication the physician has ordered the patient to take.
H = dose on hand or the amount of medication in each unit of the drug; for example, the number of mcg, mg, or g in each unit dose.
Q = quantity or dosage unit of the dose on hand; for example, a pill or an amount of liquid.

26. Formula Method
Learning Outcome: Execute dosage calculations accurately.

27. Dosage Calculations (cont.)
Prevent errors
During calculations
When selecting medication
When reading the label
Learning Outcome: Execute dosage calculations accurately.
Medication errors are a serious problem in health care.
Errors can occur when performing calculations, when selecting the medication to administer, and when reading the label to perform the calculation.
Check and recheck the ordered form of the drug as well as the amount of drug per dose of the drug.
Refer to PREVENTING ERRORS DURING DOSAGE CALCULATIONS

28. Apply Your Knowledge Super!
The physician has ordered ampicillin 500 mg, on hand 250 mg capsules. How much would you give?
You have 50 mg metoprolol as a scored tablet on hand and the doctor tells you to give 25 mg. How much would you give?
2 capsules
Learning Outcome: Execute dosage calculations accurately.
Problem 1 D = 500 mg, H = 250 mg, Q = 1 capsule
500𝑚𝑔/250𝑚𝑔 𝑥 1𝑐𝑎𝑝𝑠𝑢𝑙𝑒=2/1 𝑥 1 𝑐𝑎𝑝𝑠𝑢𝑙𝑒=2 𝑐𝑎𝑝𝑠𝑢𝑙𝑒𝑠
Problem 2 D = 25 mg, H = 50 mg, Q = 1 scored tablet
25𝑚𝑔/50𝑚𝑔 𝑥 1 𝑡𝑎𝑏𝑙𝑒𝑡=25/50 𝑥 1 𝑡𝑎𝑏=1/2 𝑡𝑎𝑏𝑙𝑒𝑡
½ tablet
Super!

29. Body Weight and Body Surface Area Calculations
More common for pediatric or geriatric dosages
Greater risk of harm
Calculations must be precise
Learning Outcome: Calculate dosages based upon body weight and body surface area.
Calculating a drug dose based on the body surface area (BSA) or the weight of the patient is common with pediatric and geriatric patients.
They are at greater risk of harm from medication because of the way they break down and absorb medications.
Calculations for these individuals must be precise. The medical assistant may perform these based on area of practice.

30. Dosages Based on Weight
Order
Dosage of medication / weight of patient
Over a unit of time
Weight must be calculated in kilograms
Learning Outcome: Calculate dosages based upon body weight and body surface area.

31. Dosages Based on Weight (cont.)
Learning Outcome: Calculate dosages based upon body weight and body surface area.

32. Dosages Based on Weight (cont.)
Learning Outcome: Calculate dosages based upon body weight and body surface area.

33. Dosages Based on Weight (cont.)
Learning Outcome: Calculate dosages based upon body weight and body surface area.

34. Dosages Based on Weight (cont.)
Calculate the amount to administer. Refer to label for medication on hand
Learning Outcome: Calculate dosages based upon body weight and body surface area.

35. Dosages Based on Weight (cont.)
Learning Outcome: Calculate dosages based upon body weight and body surface area.

36. Dosages Based on Weight (cont.)
Learning Outcome: Calculate dosages based upon body weight and body surface area.
Round answer to nearest tenth.

37. Dosages Based upon Body Surface Area
Precise dosages
Pediatric patients
Burn patients
Chemotherapy patients
Nomogram
Plot height and weight
Third scale will give the BSA
Learning Outcome: Calculate dosages based upon body weight and body surface area.
The total surface area of the body, or body surface area (BSA), is used to calculate very precise medication dosages.
Pediatric patients, as well as burn victims or patients undergoing chemotherapy or radiation therapy, may need BSA dosage calculations.
A complex formula or a nomogram may be used to determine the BSA.
A nomogram – a set of scales arranged so that a ruler aligned with two of the values shows the corresponding value on the third scale.
Aligning the ruler with a person’s height and weight shows the body surface area.

38. 20 kg 250 mg Apply Your Knowledge Very Good!
A 5-year-old child weighs 44 lbs. The physician orders him to receive Zinacef 50 mg/kg/day IM q6h. What is the child’s weight in kg? How many milligrams of medication should the child receive in one dose?
20 kg
Learning Outcome:52.5 Calculate dosages based upon body weight and body surface area.
A 5-year-old child weighs 44 lbs. The physician orders him to receive Zinacef 50 mg/kg/day IM q6h.
What is the weight in kilograms? 44𝑙𝑏/𝑋𝑘𝑔=2.2𝑙𝑏/1𝑘𝑔(𝑥𝑘𝑔 𝑥 2.2𝑙𝑏)/2.2𝑙𝑏=(44 𝑙𝑏 𝑥 1 𝑘𝑔)/2.2𝑙𝑏X = 20 kg
How many milligrams of medication should the child receive in one dose?
50 mg x 20 kg = mg / day
24 hr/day ÷ 6 = 4 doses/day
1000 mg ÷ 4 doses = 250 mg/dose
250 mg
Very Good!

39. In Summary
52.1 A medical assistant must be proficient in math and determine if the answer to every calculation he performs is reasonable.
52.2 The metric system is based on 10 and is the most common system of measurement for dosage calculations.
Metric units commonly used for dosage calculations include g, mg, mcg, and mL.
The apothecary and household systems have some equal measures, but they are used rarely.

40. In Summary (cont.)
52.3 To convert among systems of measurements, you can refer to a conversion chart or perform a proportion method calculation.
Keep in mind that measurements between the metric and apothecary and household systems are only approximations

41. In Summary (cont.)
52.4 Use the formula method or proportion method to perform dosage calculations.
Formula Method D/H × Q
Proportion Method D/X = H/Q
52.5 Dosages based on body weight and BSA are used when precise amounts of medication must be administered.
Body weight calculations are usually ordered in mg/kg/day. BSA calculations use special formulas or a nomogram
52.4 Use the formula method or proportion method to perform dosage calculations.
Formula Method D/H × Q
D = desired dose or the amount of medication the physician has ordered the patient to take.
H = dose on hand or the amount of medication in the each unit of the drug.
Q = quantity of the dose on hand or dosage unit.
Proportion Method
Set up a fraction with the amount of the drug ordered over the unknown amount.
Set up a fraction with the amount of drug in a single dose (dose on hand) over the dosage unit.
Set up the proportion using both fractions, making sure the same units of measure are on the top and bottom of each side of the proportion.
Cross multiply.
To solve for x, do the arithmetic, then cancel out like terms in the top and bottom of each fraction

42. Math is like love -- a simple idea but it can get complicated.
End of Chapter 52
Math is like love -- a simple idea but it can get complicated.
~ Unknown