4. Proportion – formed by using two ratios
that are equal
1 = 5 = 2 x 5 = 10 x 1
2 10
Cross products are equal to
10 = 10
½ is equal to 5/10
Interpretation/Calculating
Medication Dosages
6. Example:
The Md orders 20 mg IM of a drug for a
patient. The drug is available in 10 ml vial
that contains 50 mg of drug. How many ml
will be needed to supply the dose of 20 mg?
Given:
1. 10 ml vial
2. 50 mg of drug in 10 ml vial
3. 20 mg is the desired
7. Cont.
Ratio 10ml = 1ml
50mg 5mg
Ratio for the required dose x ml
20 mg
Proportion: 1ml = x ml
5mg 20 mg
5 x x = 1 x 20
5x = 20
x = 4ml ( 20 divided by 5 )
8. Example/s:.
1. Ampicillin oral susp. 250mg/5ml . How
many ml would be measured into a
medication syringe to obtain a dose of
75mg of ampicillin?
9. Example/s
2. Paracetamol 125mg/ 5ml. How many
ml would be measured into a
medication syringe to obtain 15 mg of
Paracetamol?
10. Example/s
3. How many ml of Paracetamol IV
should be given to obtain a dose of
15mg in a 15kg-man, if a vial contains
300mg/2ml of Paracetamol?
11. Example/s
4. A vial contains 250mg of tetracycline
HCl in a total of 2 ml of solution. How
many mg of tetracycline HCl are
contained in 0.6 ml of this solution?
12. Conversion Between
Systems of measurement
1000 g = 1 kg
1000 mg = 1 g
1000 ng = 1 mcg
1000 ml = 1 L
100 ml = 1 dL
13. Conversion Between
Systems of measurement
Rule: 1
To convert a quantity in the metric
system to a large metric unit (mg to g)
move the decimal point to the left
Smaller to larger =right to left
Example: 2mg = 0.002g
14. Conversion Between
Systems of measurement
Rule: 2
To convert a quantity to a smaller metric
unit, move the decimal point to the right
Example: 2.4 l = 2400 ml
16. Apothecary System of
weights
based upon the grain, which is the
smallest unit in the system
Nitroglycerin 1/100gr, 1/150 gr
Atropine sulfate 1/200gr. 1/150 gr
Approximate equivalent
15 grains = 1 gram
1grain = 60 mg
17. Apothecary System of
weights
Convert 15 milligrams to grains
1 grain = x grain
60 mg 15 mg
60 x = 15
x = 15/60
x = .25 grain
18. Example:
The physician orders 7 ½ grains of
aminophylline po for a client. On hand are
aminophylline tablets 500 mg. How many
tablets are required for one dose?
GIVEN:
1 gram = x gram
15 grain 7.5 grain
15 x = 7.5
x = 0.5 g (500mg)
19. Example:
How many mg of nitroglycerin are in
one 1/150 grain tablet of the drug?
Given:
1 gram = x gram
15 grains 1/150 grains
15 x = 1/150 = .0067
x = 0.0004g = 0.4 mg
21. Example:
The Md orders 1,000,000 units of Pen G for a
patient. The pen G on hand is available as a
solution containing 250,000 units/ml
250,000 units = 1, 000,000 units
1 ml x ml
250,000x = 1,000,000
x = 4 ml
22. Example 1:
The physician orders 250 mcg of
Vitamin B12 IM daily. The vitamin on
hand is labeled 1,000mcg/ml. How
many ml should be given to the patient?
23. Example 2:
A patient is to be given 25 mg
Diphenhydramine po and is available as
12.5mg/5ml prep.. How many ml
should be given?
24. Example 3:
A medication order calls for 750 mg
calcium lactate to be given TID PO. On
hand are tablets of calcium lactate 0.5
g. How many tablets should be given
for each dose?
25. Calculation of Dosages
Based on Weight
The physician orders thiabendazole
chewable tablets for a 110 pound child.
The recommended dosage is 20
mg/kg/dose. How many 500 mg tablets
should be given to this patient for each
dose.
26. Calculation of Dosages
Based on Weight
Convert weight to kg
20 mg = x mg
1 kg 50 kg
x = 1000 mg
500 mg = 1000mg
1 tab x tab
x = 2 tablets per dose
27. Example:
The recommended dose of Meperidine
is 6mg/kg/day for pain. It is given every
4 to 6 hours. How many ml of Demerol
injection 50mg/ml should be
administered to a 33 pound child as a
single dose every 6 hours?
28. Cont.
Calculate the daily dose for a 33-pound
child
6mg/ 1 kg (2.2lb) = x mg / 33 lb
x = 90 mg Demerol / day
29. Cont.
Calculate the number of ml of demerol
injection 50 mg/ml needed for the daily
dose
50 mg/ 1 ml = 90 mg/ x ml
30. Cont.
Calculate the number of ml to be given
every 6 hours
1.8 ml/ 24 h = x ml/ 6 h
24 x = 10.8
x = 0.45 ml
31. Calculations Involving
Intravenous Administration
How long will it take to complete an IV
infusion of 1.5 L of D5 W being
administered at the rate of 45 drops
/min? The IV set is calibrated to deliver
15 drops/ml.
34. Calculations Involving
Intravenous Administration
Calculate the number of hours required to
administer the total volume of the
solution. If 180 ml are delivered each
hour, then how many hours are required
to administer 1,500 ml?
180ml/hr = 1,500 ml/hr
180 x = 1,500
x = 8.3 hours or 8 hours 20 min.
35. Calculations Involving Piggyback
IV Infusion
An IV piggyback of Cefazolin 500mg in
100ml/hour is ordered. The piggyback
IV set is calibrated to deliver 10 gtts.ml.
How many drops/min. should be
administered?
36. Calculations Involving
Piggyback IV Infusion
100 ml/ 60 min. = x ml/ min.
60 x = 100
x = 1.7 ml/ min.
Flow rate:
Drop rate
10 gtt/ min./ 1min. = x gtt/ 1.7 ml
X = 17 gtt/ min.
37. Calculations Involving
Piggyback IV Infusion
Daily fluid requirement
2000ml of electrolyte solution
administered 24 hours and that
Cefazolin sodium 500 mg in 100 ml/hr is
ordered QID. The number od ml/day
and the times of the piggybank infusion
must be subtracted from the daily fluid
requirement.