Question

Choose the correct dosage.

Ordered: 75,000 U IM p.r.n.

diluent             Resulting concentration

7.8 ml            50,000 U/ml

5.6 ml 100,000 U/ml

3.4 ml 200,000 U/ml

Answer 1

The dose ordered - 75,000 U IM p.r.n

p.r.n = pro re nata (latin) = When necessary ( english)

Step 1: Choose the correct solution

• the solution is the mixture of solute and solvent
• From the above problem provides the diluent used and the resulting concentration (u/ml)
• In other words, when 7.8 ml of diluent was used, the resulting solution contains 50,000 U per ml of the drug.
• It is quite evident that by increasing the amount of diluent, the resulting drug concentration reduces. Example, when 7.8ml of diluent was added, a more dilute solution of 50,000 U/ ml of the drug was produced
• On the contrary, When less amount of diluent was used a more concentrated solution of the drug was produced
• Now, for us to achieve a dose of 75,000 U, the solution containing 50,000 U/ml seems to be the best.

Step 2: Calculate the amount ( in ml) needed to provide a dose of 75,000 U

• If 1 ml contains 50,000 ml of drug
• The 'x' ml will contain 75,000 of drug.
• Therefore x = 75,000 ml / 50,000 ml
• x = 75,000 / 50,000 = 1.5 ml

Final statement - 1.5 ml of 50,000 U/ ml solution will provide dose of 75,000 U