2. Calculating Doses of Nonparenteral Medications
Chapter 9
Calculating Doses of Nonparenteral Medications

3. Nonparenteral Medications

4. Calculating Solid Doses
Three methods used
ratio and proportion
formula method
dimensional analysis
Two parts
dose ordered (DO)
dosage available (DA)

5. Calculating Doses Identify available unit to be used for desired dose
tablets, capsules, mL, ounces
Ensure all units are alike
conversions must be done prior to calculation of the dose
Verify dose calculation with medication order

6. Ratio and Proportion Terminology

7. Proportional Method First set of comparisons Other side of equation
dosage available (DA)
located right side
Other side of equation
dose ordered (DO)
located left side
Dosage available (DA) : Dosage form = Dose ordered (DO) : Dose to be given
Always perform dose calculations in writing

8. Proportional Method Ordered: Glucophage 850 mg qd po with meal
Dosage available: Glucophage 850 mg tablets
Are the units in the same measurement?

9. Proportional Method Ordered: Glucophage 850 mg qd po with meal l
Dosage available: Glucophage 850 mg tablets
Are the units in the same measurement?
Yes

10. Proportional Method Ordered: Glucophage 850 mg qd po with meal
Dosage available: Glucophage 850 mg tablets
Are the units in the same measurement?
Yes
850 mg (DA) : 1 tablet (form) :: 850 mg (DO) : # tablet(s) (dose to be given)

11. Proportional Method
850 mg (DA) : 1 tablet (form) :: 850 mg (DO) : # tablet(s) (dose to be given)
x stands for dose to be given
850 is to x as 850 is to 1
850 : 1 :: 850 : x
850x = 850
x = 1
Answer: one 850 mg tablet daily

12. Proportional Method Ordered: phenobarbital gr 1/2 po tid
Dosage available (DA): phenobarbital gr 1/4 tablet
Are the units in the same measurement?

13. Proportional Method Ordered: phenobarbital gr 1/2 po tid
Dosage available (DA): phenobarbital gr 1/4 tablet
Are the units in the same measurement?
Yes

14. Proportional Method Ordered: phenobarbital gr 1/2 po tid
Dosage available (DA): phenobarbital gr 1/4 tablet
Are the units in the same measurement?
Yes
gr 1/4 (DA) : 1 tablet (dosage form) :: gr 1/2 (DO) : x # tablet(s) (dose to be given)

15. Proportional Method
gr 1/4 (DA) : 1 tablet (dosage form) :: gr 1/2 (DO) : x # tablet(s) (dose to be given)
1/4 : 1 :: 1/2 : x
1/4 is to x as 1/2 is to 1
1/4 x = 1/2
Invert fraction before multiplying
x = 1/2 x 4/1
x = 2
Desired dose is two gr 1/4 tablets three times daily

16. Proportional Method Ordered: phenobarbital gr iii po hs
Dosage available (DA): phenobarbital 100 mg scored tablet
Are the units in the same measurement?

17. Proportional Method Ordered: phenobarbital gr iii po hs
Dosage available (DA): phenobarbital 100 mg scored tablet
Are the units in the same measurement? No

18. Proportional Method Ordered: phenobarbital gr iii po hs
Dosage available (DA): phenobarbital 100 mg scored tablet
Are the units in the same measurement? No
What is the equivalency?

19. Proportional Method Ordered: phenobarbital gr iii po hs
Dosage available (DA): phenobarbital 100 mg scored tablet
Are the units in the same measurement? No
What is the equivalency?
60 mg = 1 grain (gr i)

20. Proportional Method Conversion
60 mg = 1 grain (gr i)
60 mg : 1 gr :: 100 mg : x
100 mg = 1½ grains gr iss
100 mg : gr iss :: x mg : gr iii
100 is to 1½ as x is to 3
1½ x = 300
Change 1½ to 3/2
100 mg : 3/2 = x : 3
3/2x = 300
x = 300 x 2/3
x = 200 mg

21. Proportional Method
100 mg (DA) : 1 tablet (form) :: 200 mg (DO) : x (dose to be given)
100 : 1 :: 200 : x
100 is to x as 200 is to 1
100x = 200
x = 2
Desired dose is two 100 mg tablets every day at bedtime

22. Proportional Method Ordered: oxybutynin HCl 10 mg po bid
Available: 5 mg scored tablets
Are the units in the same measurements?

23. Proportional Method Ordered: oxybutynin HCl 10 mg po bid
Available: 5 mg scored tablets
Are the units in the same measurements?
Yes

24. Proportional Method Ordered: oxybutynin HCl 10 mg po bid
Available: 5 mg scored tablets
Are the units in the same measurements?
Yes
5 mg (DA) : 1 tablet (form) :: 10 mg (DO) : # tablet(s) (dose to be given)

25. Proportional Method
5 mg (DA) : 1 tablet (form) :: 10 mg (DO) : # tablet(s) (dose to be given)
5 is to 1 as 10 is to x
5x = 10
x = 2
Desired dose is two 5 mg tablets, twice daily

26. Formula Method
Dosage ordered (DO) / Dosage available (DA) x Quantity or Form (Qty) = Dose to be given
DO / DA x Qty = Dose to be given
Always perform dosage calculations in writing

27. Formula Method Ordered: Glucophage 850 mg po qd with meal
Available: Glucophage 850 mg tablets
Are the units in the same measurements?

28. Formula Method Ordered: Glucophage 850 mg po qd with meal
Available: Glucophage 850 mg tablets
Are the units in the same measurements?
Yes

29. Formula Method Ordered: Glucophage 850 mg po qd with meal
Available: Glucophage 850 mg tablets
Are the units in the same measurements?
Yes
What is the desired dose?

30. Formula Method Ordered: Glucophage 850 mg po qd with meal
Available: Glucophage 850 mg tablets
Are the units in the same measurements?
Yes
What is the desired dose?
850 mg tablet daily

31. Formula Method Ordered: Glucophage 850 mg po qd with meal
Available: Glucophage 850 mg tablets
Are the units in the same measurements?
Yes
What is the desired dose?
850 mg tablet daily
DO (850 mg) / DA (850 mg) x Qty (1 tablet) = 1 tablet (dose to be given)

32. Formula Method Ordered: Glucophage 850 mg po qd with meal
Available: Glucophage 850 mg tablets
Are the units in the same measurements?
Yes
What is the desired dose?
850 mg tablet daily
DO (850 mg) / DA (850 mg) x Qty (1 tablet) = 1 tablet (dose to be given)
Answer: One 850 mg tablet

33. Formula Method Ordered: phenobarbital gr iii po qd hs
Available: phenobarbital 100 mg scored tablets
Are the units in the same measurement?

34. Formula Method Ordered: phenobarbital gr iii po qd hs
Available: phenobarbital 100 mg scored tablets
Are the units in the same measurement? No

35. Formula Method Ordered: phenobarbital gr iii po qd hs
Available: phenobarbital 100 mg scored tablets
Are the units in the same measurement? No
What is the equivalency for conversion?

36. Formula Method Ordered: phenobarbital gr iii po qd hs
Available: phenobarbital 100 mg scored tablets
Are the units in the same measurement? No
What is the equivalency for conversion?
gr I = 60 mg

37. Formula Method Conversion
60 mg = 1 grain (gr i)
60 mg : 1 gr :: 100 mg : x
100 mg = 1½ grains (gr iss)
100 mg : gr iss :: x mg : gr iii
100 is to 1½ as x is to 3
1½ x = 300
Change 1½ to 3/2
100 mg : 3/2 = x : 3
3/2x = 300
x = 300 x 2/3
x = 200 mg

38. Formula Method
200 mg (DO) / 100 mg (DA) x 1 tablet (Qty) = 2 tablets (Dose to be given)
Answer: Two 100 mg tablets

39. Formula Method Ordered: oxybutynin HCl 10 mg po bid
Available: oxybutynin HCl 5 mg scored tablets
Are the units in the same measurement?

40. Formula Method Ordered: oxybutynin HCl 10 mg po bid
Available: oxybutynin HCl 5 mg scored tablets
Are the units in the same measurement?
Yes

41. Formula Method Ordered: oxybutynin HCl 10 mg po bid
Available: oxybutynin HCl 5 mg scored tablets
Are the units in the same measurement?
Yes
10 mg (DO) / 5 mg (DA) x 1 tablet (Qty)

42. Formula Method Ordered: oxybutynin HCl 10 mg po bid
Available: oxybutynin HCl 5 mg scored tablets
Are the units in the same measurement?
Yes
10 mg (DO) / 5 mg (DA) x 1 tablet (Qty)
Answer: Two 5 mg tablets

43. Calculating Liquid Doses
Must convert to household system—patients more familiar with household than metric
Two numbers for drug dose calculation
amount of active ingredients (usually mg)
amount of liquid mixed with drug (usually mL)
mg/mL

44. Reconstitution
Must reconstitute some medications from powder to liquid form prior to administering
Label gives total volume in bottle, medication strength
Figure 9-1. Label for Zithromax. Courtesy of Pfizer Labs, New York.

45. Reconstitution
Different dosage strengths created with different amounts of water
Always document strength, date, time of reconstitution on medication label, and initial

46. Two Active Ingredients
Protuss
Each 5 mL (1 teaspoon) contains:
hydrocodone bitartrate 5 mg
potassium guaiacolsulfonate 300 mg
Remember: 5 mL = 1 tsp = 1 fluid dram (f )

47. Liquid Medications Ordered: Protuss f iss po prn
Available: Protuss, 5 mg hydrocodone bitartrate, and 300 mg potassium guaiacolsulfonate in each 5 mL.
Are the units in the same measurement?

48. Liquid Medications Ordered: Protuss f iss po prn
Available: Protuss, 5 mg hydrocodone bitartrate, and 300 mg potassium guaiacolsulfonate in each 5 mL.
Are the units in the same measurement? No

49. Liquid Medications Conversion
1 fluid dram (f ) = 1 teaspoon (tsp) = 5 mL
f (DA) : 5 ml (form) :: f iss (DO) : x mL (dose to be given)
1 : 5 :: 1½ : x
1 is to 5 as 1½ is to x
5 x 3/2 = x
x = 7.5 mL
Desired dose is 7.5 mL = 1½ tsp

50. Liquid Medications Ordered: Augmentin 400 mg po q 12 hr
Available: Augmentin 400 mg/5 mL oral suspension
Are the units in the same measurement?

51. Liquid Medications Ordered: Augmentin 400 mg po q 12 hr
Available: Augmentin 400 mg/5 mL oral suspension
Are the units in the same measurement?
Yes

52. Liquid Medications
400 mg (DA) : 5 mL (form) :: 400 mg (DO) : x mL (dose to be given)
400 : 5 :: 400 : x
400 is to 5 as 400 is to x
400x = 2000
x = 5 mL
Desired dose is 5 mL = 1 tsp

53. Liquid Medications Ordered: cetirizine HCl 10 mg po
Available: cetirizine HCl 1 mg/1 mL
Are the units in the same measurement?

54. Liquid Medications Ordered: cetirizine HCl 10 mg po
Available: cetirizine HCl 1 mg/1 mL
Are the units in the same measurement?
Yes

55. Liquid Medications Ordered: cetirizine HCl 10 mg po
Available: cetirizine HCl 1 mg/1 mL
Are the units in the same measurement?
Yes
1 mg (DA) : 1 mL (form) :: 10 mg (DO) : x mL (dose to be given)
1 : 1 :: 10 : x
1 is to 1 as 10 is to x
x = 10
Answer: desired dose is 10 mL = 2 tsp

56. Formula Method Ordered: Protuss f iss po qid prn
Available: Protuss 5 mg hydrocodone bitartrate and 300 mg potassium guaiacolsulfonate in each 5 mL
Are the units in the same measurement?

57. Formula Method Ordered: Protuss f iss po qid prn
Available: Protuss 5 mg hydrocodone bitartrate and 300 mg potassium guaiacolsulfonate in each 5 mL
Are the units in the same measurement? No

58. Formula Method 1 dram = 5 mL i / 5 mL = iss / x mL
Cross multiply 1x = 1 1/2 x 5
x = 7.5 mL

59. Formula Method 5 mL/1 tsp = 7.5 mL / x tsp Cross multiply 1 1/2 tsp
Answer: Administer 1 1/2 tsp up to four times a day as needed

60. Formula Method Ordered: Augmentin susp 400 mg po q12hr
Available: 400 mg/5 mL
Are the units in the same measurement?

61. Formula Method Ordered: Augmentin susp 400 mg po q12hr
Available: 400 mg/5 mL
Are the units in the same measurement?
Yes

62. Formula Method Ordered: Augmentin susp 400 mg po q12hr
Available: 400 mg/5 mL
Are the units in the same measurement?
Yes
400 mg (DO) / 400 mg (DA) x 5 mL (Qty) = 5 mL (Dose to be given)
1 tsp every 12 hours

63. Formula Method Ordered: cetirizine HCl syr 10 mg po qd
Available: cetirizine HCl syrup 1 mg/1 mL
Are the units in the same measurement?

64. Formula Method Ordered: cetirizine HCl syr 10 mg po qd
Available: cetirizine HCl syrup 1 mg/1 mL
Are the units in the same measurement?
Yes

65. Formula Method Ordered: cetirizine HCl syr 10 mg po qd
Available: cetirizine HCl syrup 1 mg/1 mL
Are the units in the same measurement?
Yes
10 mg (DO) / 1 mg (DA) x 1 mL = 10 mL
5 mL/ 1 tsp = 10 mL/ x tsp
2 tsp daily dose to be given

66. Measuring Liquid Medications
Pour at eye level
Read bottom of meniscus
Meniscus
Figure 9-3. The meniscus

67. Liquid Measurement Instruments
Oral dispenser (A)
Oral syringe (B)
Liquid dropper (C)
Figure 9-4.

68. Body Surface Area (BSA)
Calculate precise pediatric and geriatric dosages
Based on weight and height using nomogram
Nomogram—set up as graph; shows relationships among numerical values
height—left side, cm and inches
weight—right side, lb and kg

69. West BSA Nomogram
Figure From Behrman RE et al (2000). Nelson textbook of pediatrics (16th ed.). Philadelphia: Saunders.

70. Calculating BSA Determine BSA BSA of adult weighing 140 lb is 1.7
BSA of patient in m2 / 1.7 x adult dose = desired dose

71. Calculating BSA Child weighs 55 lbs and is 85 cm.
Find and mark height in first column
Find and mark weight in second column
Use ruler to align marks
Read BSA
0.82 m2

72. Calculating BSA
Formula:
BSA = 1.2 m2
1.2 m2 / 1.7 m2 x 4 mg = 2.8 mg (3 mg)
Child weighs 70 lbs and is 45 in. tall. Physician orders albuterol based on the BSA. Normal adult dose is 4 mg.
BSA in m2 x adult dose = desired dose 
1.7

73. Calculating BSA Albuterol syrup available in 2 mg/5 mL
3 mg (DO) / 2 mg (DA) x 5 mL = 7.5 mL
Answer: administer 7.5 ml or 1 1/2 tsp of albuterol