Drug calculation\Mohammad Alme`ani

DRUG CALCULATION AND FLUID PREPARATIONS

1. Mohammad Al-me`ani. , MSN, RN.

2. Introduction
Base units
Units and Equivalences
Converting Units
Examples
 Problems
Dosage calculation
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3. Many different units are used in medicine, for
example:
 drug strengths, e.g. digoxin injection 500
micrograms in 1mL;
 dosages, e.g. dobutamine 3mcg/kg/min;
 patient electrolyte levels, e.g. sodium 137mmol/L.
 It is important to have a basic knowledge of the units
used in medicine and how they are derived.
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4. It is particularly important to have an
understanding of the units in which drugs can
be prescribed, and how to convert from one
unit to another
It is very important in drug calculations.
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5. The main units are those used to measure
weight, volume and amount of substance:
1. Weight: kilogram (kg)
2. Volume: litre (l or L)
3. Amount of substance: mole (mol)
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7. It is best to work with the smaller unit to avoid the use of
decimals.
 When converting, the amount remains the same, only the unit
changes
Remember to look at the units carefully; converting from one unit
to another may involve several steps.

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8. To convert from a larger unit to the next smaller unit, multiply
by 1,000.
 To convert from a smaller unit to the next larger unit, divide
by 1,000.

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10. 1.Convert 0.5 g to milligrams.
You are going from a larger unit to a smaller unit, so you have to multiply by
1,000, i.e. 0.5 × 1,000 = 500 milligrams .
2.Convert 2,000 g to kilograms.
You are going from a smaller unit to a larger unit, so you have to divide by 1,000,
i.e. 2,000 g =(2,000\1,000 )kg = 2 kg
3.Convert 0.005kg to milligrams.
 first convert to grams: 0.005 kg = (0.005 × 1,000)g = 5 g Next, convert grams to
milligrams: 5 g = (5 × 1,000)mg = 5,000 mg
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11. Convert 0.0125 kilograms to grams. (12.5 grams)
Convert 3.2 litres to millilitres.(3,200 millilitres)
Convert 3,750 grams to kilograms.(3.75 kilograms)

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12. Calculating the Number of Tablets or Capsules Required

Amount to give = Desired Dose (D) * Volume (V)
Dose on Hand (H)

Q1 :A patient is prescribed 75 mcg of thyroxine sodium but the
tablets available is 25 mcg. How many tablets are required?
75
= 3 tablets
25
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14. Q2 : A patient is prescribed 2 g of flucloxacillin to be given orally
but it is available in 500 mg capsules. How many capsules
should you give?
To convert grams to milligrams, multiply by 1,000: 2 g = (2 ×
1,000) mg = 2,000 mg
2,000 \500 = 4 capsules
Q3 : 500 mg is prescribed and the tablets are 250 mg each. How
many tablets will you give?(Two 250 mg tablets)
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15. Q4 :order-dilantin 50 mg p.o drug available 125mg\5ml ?
D=50mg , H = 125 mg , V = 5 ml
50\125*5 = 250\125=2 ml .
Q5: order 1g p.o , drug available 250 mg capsule ?
D=1g =1000mg
H = 250 mg
1000\250= 4 capsules
Q6: order – digoxin , 25 mg p.o ,drug available ,125 mg per tab?
, 25 mg(D)\ ,125 mg(H) = 2 TAB .

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16. Q7 :dexamethasone 1 mg, drug available ,5mg per tablet?
D=1mg , H = ,5 mg , V = 1 tab
1mg\,5mg * 1 = 2 tablet.
Q8: order tagamet ,6g , drug available 300 mg\tablet ?
D=,6 g *1000= 600mg
H = 300 mg
600\300 *1 = 2 tablet

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17. Q9: ampicillin , 5 g p.o ,drug available , 250 mg \5 ml ?
D=,5 g *1000= 500mg
H = 250 mg
V=5ml
500mg\250mg *5ml =10ml.


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Ex1 : 1 litre of sodium chloride 0.9% (‘normal saline’) is to be
given over 8 hours: what drip rate is required using a standard
giving set (SGS), 20 drops/mL?
STEP ONE
First convert the volume to a number of drops. To do this, multiply the volume of the
infusion by the number of ‘drops per mL’ for the giving set, i.e.
1 litre=1,000 mL, so it will be
1,000 × 20 = 20,000 drops
You have just calculated the number of drops to be infused

26. STEP TWO
Next convert hours to minutes by multiplying the number of hours over
which the infusion is to be given by 60 (60 minutes = 1 hour).
 8 hours = 8 × 60 = 480 minutes
STEP THREE
 Write down what you have just calculated, i.e. the total number of drops to
be given over how many minutes.
 20,000 drops to be given over 480 minutes
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27. STEP FOUR
Calculate the number of drops per minute by dividing
the number of drops by the number of minutes:
20 000\ 480 = 41.67 drops/min
Since it is impossible to have part of a drop, round up
or down to the nearest whole number: 41.67 ≈ 42
drops/min
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31. Question 1 : 500 mL of sodium chloride 0.9%
over 6 hours (28 drops/min, approx.)
Question 2 : 1 litre of glucose 4% and sodium
chloride 0.18% over 12 hours (28 drops/min.)
 Question 3 : 1 unit of blood (500 mL) over 6
hours (21 drops/min.)
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33. Dosages can be expressed in various ways: mg/min
or mcg/min and mg/kg/min or mcg/kg/min; and it
may be necessary to convert to mL/hour (when using
infusion pumps).
EXAMPLE
You have an infusion of dopamine 800 mg in 500
mL. The dose required is 2 mcg/kg/min for a patient
weighing 68 kg. What is the rate in mL/hour?
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34. STEP ONE
When doing this type of calculation, you need to
convert the dose required to a volume in mL and
minutes to hours.
STEP TWO
First calculate the dose required:
dose required = patient’s weight × dose prescribed
 = 68 × 2 = 136 mcg/min
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35. STEP THREE
The dose is 136 mcg/min. The final answer
needs to be in terms of hours, so multiply by
60 to convert minutes into hours:
 136 × 60 = 8,160 mcg/hour
Convert mcg to mg by dividing by 1,000:
 = 8.16 mg/hour
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36. STEP FOUR
The next step is to calculate the volume for the dose
required. Calculate the volume for 1 mg of drug:
 You have: 800 mg in 500 mL:
 1 mg = = 0.625 Ml
STEP FIVE
 Thus for the dose of 8.16 mg, the volume is equal to:
 8.16 mg = 0.625 × 8.16 = 5.1 mL/hour
 ANSWER: The rate required is 5.1 mL/hour
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37. EXAMPLE
An infusion pump containing 250 mg dobutamine in 50 mL is
running at a rate of 3.5 mL/hour. Convert this to mcg/kg/min
to check that the pump is set correctly. (Patient’s weight = 73
kg).
rate = 3.5 mL/hour
 amount of drug (mg) = 250 mg
 weight (kg) = 73 kg
volume (mL) = 50 mL
 60 converts minutes to hours
 1,000 converts mg to mcg
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38. STEP ONE
In this type of calculation, convert the volume
being given to the amount of drug, and then
work out the amount of drug being given per
minute or per kilogram of the patient’s weight.

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39. STEP TWO
You have 250 mg of dobutamine in 50 mL.
 First it is necessary to work out the amount in 1 mL:
250 mg in 50 mL
1 mL = mg = 5 mg
STEP THREE
The rate at which the pump is running is 3.5 mL/hour. You
have just worked out the amount in 1 mL (Step Two)
 3.5 mL/hour = 5 × 3.5 = 17.5 mg/hour
Now convert the rate (mL/hour) to the amount of drug being
given over an hour.
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40. STEP FOUR
The question asks for the dose in mcg/kg/m so convert
milligrams to micrograms by multiplying by 1,000:
17.5 × 1,000 = 17,500 mcg/hour
STEP FIVE
 Now calculate the rate per minute by dividing by 60 (converts
hours to minutes):
= 291.67 mcg/min
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41. STEP SIX
 The final step in the calculation is to work out
the rate according to the patient’s weight (73
kg). (If the dose is not given in terms of the
patient’s weight, then miss out this final step.)
 = 3.99 mcg/kg/min
This can be rounded up to 4 mcg/kg/min.
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46. Question 1 :You have dopamine 200 mg in 50 mL and the
rate at which the pump is running is 4 mL/hour. The
prescribed dose is 3 mcg/kg/min. What dose is the pump
delivering? (Patient’s weight = 89 kg) If the dose is wrong, at
which rate should the pump be set?
Question 2 You have dobutamine 250 mg in 50 mL and the
rate at which the pump is running is 5.4 mL/hour. The
prescribed dose is 6 mcg/kg/min. What dose is the pump
delivering? (Patient’s weight = 64 kg) If the dose is wrong, at
which rate should the pump be set?
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47. rate = 4 mL/hour
 amount of drug (mg) = 200 mg
 weight (kg) = 89 kg
volume (mL) = 50 mL
 60 converts minutes to hours
 1,000 converts mg to mcg
mcg/kg/min = rate (mL/hour) × amount of drug ×
1,000\weight(kg) × volume (mL) ×60
4 ×200 × 1. 000\ 89 × 50 × 60 = 2.99 mcg/kg/min ≈ 3
mcg/kg/min


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48. rate = 5.4 mL/hour
amount of drug (mg) = 250 mg
 weight (kg) = 64 kg
volume (mL) = 50 mL
60 converts minutes to hours
1,000 converts mg to mcg
mcg/kg/min = rate (mL/hour) × amount of drug ×
1,000\weight(kg) × volume (mL) ×60
5 4 × 250 × 1 000\ 64 × 50 × 60 = 7.03 mcg/kg/min (7
mcg/kg/min)

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