1. Posology
&
Dosage Calculations

2. DEFINITIONS
• Posology
– the pharmacological study of drug
dosage.
– The study of dosage quantity and
prescription.

3. Usual Recommended dose
– the amount of drug that will ordinarily
produce the effect for what is intended.

4. •Minimum dose
- the smallest dose of drug that produces
therapeutic effect.
•Maximum dose
-the largest dose that can safely administered.
Ex. Acetaminophen
10 to 15 mg/kg/dose

5. Toxic dose
•amount of drug that cause harmful effect.
Lethal dose
•amount of substance that will cause death.

6. Single dose
- to be taken at one time.
Daily dose
- amount to be taken in 24 hour period.

7. Maintenance dose
- amount to be taken to maintain the therapeutic effective
dose.
Loading dose
- first dose given to achieve maintenance drug level
quickly.

8. System of Measurement
1. Metric system
2. Apothecary system
3. Household system

9. System of Measurement
1. Metric System
– Most widely used system of measurement
– A decimal system based on the power of ten
– Units:
• Gram (weight)
• Liter (volume)
• Meter (length)

10. System of Measurement
2. Apothecary System
– Uses Roman numerals to express quantity
– Old system of measurement
– Uses:
• minim = liquid
• Grains = solid

12. System of Measurement
3. Household System
– Not as accurate as metric system due to lack of
standardization of spoons, cups and glasses
– Teaspoon (tsp) = liquid
– Pound (lb) = solid

14. System of Measurement
ml- milliliter
cc- cubic centimeter
gm- gram
mg- milligram
gr- grain
mEq- milliequivalent
mcg- microgram

15. GENERAL METHODS
FOR DRUG CALCULATION
1. BASIC FORMULA
2. RATIO& PROPORTION
3. FRACTIONAL EQUATION
4. BODY WEIGHT
5. BODY SURFACE AREA

16. 1. Basic Formula:
DxV=A
H
D- desired dose
(drug dose ordered by health care provider)
H- on-hand
(stock on hand/on label of container)
V- vehicle
( Q- quantity/drug form in which drug comes)
A- Amount calculated to be given to client

17. Examples:
1. Order:
Avandia 2mg OD. How much tablet will you give?
DxV=A
H
Solution:
2mg x 1 tablet
4mg
= .5 tablet

18. Paracemol 500mg. Q 4 hour Round the Clock
(RTC) p.o. The stock on hand is 250mg/5ml in
60ml bottle. How many ml should be
administered?
DxV=A
H
500 mg x 5 ml =
250 mg
10 ml

19. 2. Dilute Terramycin to 50ml of sterile water. Give
5000mg po Q12.

20. 3. Banophen 50mg. TID p.o.
a. How many tablet will you give in 1 dose?
b. How many tablets will you give in a day?

21. 2. Ratio and Proportion
H : V = D : x
Where:
H = drug on hand (available)
V = vehicle/drug form (capsule/tablet/liquid)
D = desired dose (as ordered)
x = unknown amount to give

22. Example
Order:
Cefdinir (3rd gen cephalosporin) 100 mg PO q.i.d
*How many mL should the client receive?

23. Order:
Cefdinir (3rd gen cephalosporin) 100 mg PO q.i.d
*How many mL should the client receive?
Solution:
H : V = D : X
250mg : 5mL = 100mg : xmL

24. • Your drug order is for morphine sulfate 5 mg, IV, q3h, PRN. The
drug is available as morphine sulfate 10 mg/mL. Your instructions
are: infuse morphine sulfate 5 mg; not to exceed 10 mg/4mins
– How many mL equal morphine sulfate 5 mg?
– What is the number of minutes to administer morphine sulfate?
Solution:
• a. 0.5 ml
• b. 2 min

25. 3. Fractional Equation
• Same as R & P except it is written as a
fraction.
H = D
V X

26. Example
Order:
Ciprofloxacin 500 mg PO q12h
*how many tablets should the client receive?
H = D
V X

27. Order:
Ciprofloxacin 500 mg PO q12h
*how many tablets should the client receive?
Solution:
H=D 750mg = 500mg
V X 1 tablet x
750(x) = 500
x = 0.67 tablets

28. 4. Body Weight (BW)
• Allows individualization of the drug dose
• Involves 3 steps:
1. Convert pounds to kg
– 1 Kg = 2.2 lbs
2. Determine drug dose per BW
– Drug dose x body weight = clients dose per day
3. Follow basic formula, R & P

29. Example
Order:
Fluorouracil (5-FU), 12 mg/kg/day IV, not to exceed 800 mg/day. The
adult weighs 132 lb.
1. Convert pounds to Kg :
132/2.2 = 60kg
2. mg x kg = client’s dose:
12 x 60 = 720 mg/kg/day
Answer:
fluorouracil 720 mg/kg/day

30. 5. Body Surface Area (BSA)
• Most accurate to calculate drug dose for:
– Infants
– Children
– Older adults
– clients who are on antineoplastic agents
– low body weight
• In m2, determined by where the person’s height and weight intersect
the nomogram scale.
• To calculate the drug dosage using the method, multiply the drug
dose by # of square meters.

31. Example
Order:
Cyclophosphamide (Cytoxan) 100 mg/m2/day, IV; available dosage is 200mg;
client’s height is 70 inches, weight is 160 lbs.
1. 70 inches and 160 lbs intersect the nomogram scale at 1.97 m2 (BSA).
2. 100 mg x 1.97 = 197mg
Answer:
Administer cyclophosphamide 197 mg/day

32. Example
The doctor has ordered an antibiotic whose average adult dose is 250
mg per day. What would the dosage for this medication be on a
child who has a length of 120 cm and weight of 40 kg?
Determine the BSA.

35. The doctor has ordered an antibiotic whose average adult dose is 250 mg per day. What would the dosage
for this medication be on a child who has a length of 120 cm and weight of 40 kg?
Child dose :
surface area in sq m (m2) x ave adult dose (mg)
1.73
Solution:
1.2 m2 x 250 mg
1.73
= 173 mg of medication
would be given.

36. Pediatric considerations
Fried’s rule
– applies to the child younger than 1 year of age.
Child’s dose:
infant age (months) X adult average dose
150 months

37. Pediatric considerations
Young’s Rule
- applies to children 1-12 y.o
Child’s dose:
child’s age (years) X adult average dose
child’s age (yrs) +12

38. Pediatric considerations
Clark’s Rule
– Uses the child’s weight to calculate the appropriate
dose and assumes that the adult dose is base on 150
lb person.
Child’s dose:
child’s weight (pounds) X adult average dose
150 lbs (pounds)

39. Calculation of Intravenous Fluid
– 3 different methods
– gtt/min
METHOD I: Three-Step
METHOD II: Two-Step
METHOD III: One-Step

40. Calculation of Intravenous Fluid
• METHOD I: Three-Step
Step 1:
Amt of soln = mL/hr
hrs o administer
Step 2:
mL per hr = ml/min
60 minutes
Step 3:
Ml/min x gtt/ml = gtt/min

41. Calculation of Intravenous Fluid
• METHOD II: Two-Step
Step 1:
Amnt of fluid = ml/hr
Hrs to administer
Step 2:
mL per hr x drops per mL = gtt/min
60 mins

42. Calculation of Intravenous Fluid
• METHOD III: One-Step
Amnt fluid x gtt/mL = gtt/min
Hrs to admin x min/hr

43. Drop factors:
Macro drip – 15/60
Micro drip – 60/60
No. of drops/min= no. of fluids (cc) X drop factor
no. of hours X 60
No of hours= no. of fluids (cc) X drop factor
no. of drops/min X 60

44. • Your drug order is for 1000mL of D5/0.5NSS to run for 8
hours.
– Would you used macrodrip or microdrip IV set?
– Calculate the drops per minute (gtt/min) using the 2-step method

45. Example:
No. of drops/min= no. of fluids (cc) X drop factor
no. of hours X 60
1. D5LR 1L for 12 hours. The drop factor is 15gtts/ml. Compute for
the no. of drops/min.
gtts/min= 1000cc X 15
12 hour X 60
20gtts/min.

46. 2.No. of Hour
No of hours= no. of fluids (cc) X drop factor
no.of drops/min X 60
D5LR 1L at 2ogtts/min. The IV set delivers at 15gtts/ml. Compute for the no. of
hours consumed.
No. of hour= 1000cc X 15
20gtts/min X 60
12 hours = answer

47. Fahrenheit And Centigrade
conversions
C= F- 32 x 5
9
F= C X 9 +32
5

48. Reference Information:
Conversion Factors And Equivalents:
• 1cc = 1ml
• 1gm = 1000mg.
• 1mg = 1000mcg.
• 1kg = 1000gm
• 1000cc = 1 liter = 1 quart
• 1 oz = 30 cc
• 1 teaspoon = 5 ml
• 1 tablespoon = 15 ml
• 1kg. = 2.2 lbs.
• 1 grain = 60mg.
• ½ grain = 30mg

49. Route:
• p.o = per orem
• I.M. = intramuscular ( 90 degree)
• S.Q/ S.C = subcutaneous ( 45 degree)
• I.V= intravenous
• I.D = intradermal
• Direct I.V
• Nebulization
• Intrathecal = spinal
• p.c. =post cebum (after meal)
• a.c = ante-cebum ( before meal )
• O.D = oculo-dexter (right eye)
• O.S = oculo-sinester ( Left eye)
• O.U = oculo-urique (Both eyes )
• E.T = endotracheal
• Per rectum- anus/anal
• Intra-vaginal- vagina
• I.J = Intra-jugular
• S.L = Sublingual

50. Frequency
• OD = once a day • PRN = as needed
• BID = twice a day • H.S = hour of Sleep
• TID = Thrice a day • STAT = immediately
• QID = Four times a day • ASAP = as soon as possible
• Q4hr = every 4 hours • QD = everyday
• Q6hr = Every 6 hours • QOD = every other day
• Q8hr – Every 8 hours • D
• Q12hr = every 12 hour