Pharmaceuitcal Calculations

Question 1

What is the importance of pharmaceutical calculations?

Answer 1

• Slight miscalculations in dosage could have serious adverse effects for the patient- can potentially be fatal.

Question 2

Most common methods of calculation

Answer 2

• Ratio-Proportion C1/V1=C2/V2
  -Formulae
  Both methods work

Question 3

UNITS

Answer 3

• Kilogram = kg = 1000g
• Gram = g = 1000mg
• Milligram =mg = 1000mcg
• Microgram = mcg= 1000 nanogram
• Litre = L= 1000ml (millilitre)
  Incorrect conversions of units can lead to 10/100 fold errors.

Question 4

Good Practice

Answer 4

Avoid fractional doses (unless otherwise specified)
- 0.5g should be written as 500mg
Avoid unnecessary decimal points (unless otherwise specified)
- 5g not 5.00g
Avoid shorthand e.g Xmas/ml ( unless otherwise specified)
- Should be written as X mg in 1ml or X mg per ml

Question 5

% W/V = %Weight per Volume

Answer 5

• Percent of weight of solution in the total volume of solution. Percent here is the number of grams of solute in 100 ml of solution.
• e.g 5% w/v = 5g per 100ml

Question 6

%W/W = % Weight per Weight

Answer 6

• Weight concentration of a solution is expressed as %w/w, the volume in this case is disregarded and only the weight is used. Percent here is the number of g of one constituent within 100g of substance.
• e.g 20% W/W = 20g per 100g

Question 7

%V/V = % Volume per Volume

Answer 7

• Volume concentration of a solution is expressed as % v/v. This is used when both chemicals within a solution are liquid. Percent here is the number of ml of one chemical within 100ml of solution.
• e.g 50% V/V = 50 ml per 100ml

Question 8

Ratios

Answer 8

• strengths can be expressed as ratios
• i.e 1:1000 solution = 1g in 1000ml
  Another way to express this;
• i.e 1000mg in 1000ml
  Therefore 1mg in 1ml
• I.e 1:10,000 solution = 1g in 10,000ml
  Another way to express this;
• I.e 1000mg in 10,000ml
  Therefore 1mg in 10ml

Question 9

Worked example

Answer 9

Mannitol is a 50% w/v solution, how many ml contain 25g Mannitol?
50g: 100ml
Use C1/V1=C2/V2
=
Plug in the numbers and transpose the formula for V in this instance
=
50 = V
Therefore 25g of Mannitol in 50 ml

Question 10

Worked example

Answer 10

What volume would you use to inject 10mg of adrenaline intravenously using a 1:1000 injection?
- 1:1000= 1g in 1000ml or 1000mg in 1000ml or 1mg in 1ml
Use C1/V1=C2/V2
=
Plug the numbers and transpose the formula for V in this instance
1= V1 V = 10
Therefore you would use 10ml

Question 11

Calculating doses in children

Answer 11

Q- A 4 month old child has a fever and needs oral paracetamol. The dose is 12mg per kg and the child weighs 8kg. The paracetamol suspension is provided at a concentration of 120mg in 5ml.
What dose in ml is required, and how would this be administered?
A- 12mg per kg patient = 8kg
12 x8 = 96mg
Paracetamol available as 120mg in 5ml
Use C1/V1=C2/V2
=
Plug the numbers and transpose the formula for V this instance
120 = 5ml = V1 V2 =4ml (96/24)
Therefore, you would administer 4ml to the patient using an oral syringe.

Question 12

Calculating infusion rates

Answer 12

Rates might be prescribed as:
- Volume (e.g ml per hour, units per hour)
- Amount of drug (e.g mg per min, units per hour)
- e,g A 5 month old child, who weighs 10kg s prescribed ibuprofen, the dose is 10mg per kg per hour. Ibuprofen is available as a 100mg per 5ml solution. How many ml will the patient need an hour? = 5ml per hour.

Question 13

Tips for making calculations simple

Answer 13

• Highlight important info
• Eliminate irrelevant info