Medication maths: how to solve infusion calculations (video)

Step-by-step video tutorials for solving pharmacy registration assessment calculations.
Medication maths: how to solve infusion calculations

At the end of this article, individuals should be able to:

  • Systematically work through infusion related calculations to reach the correct answer;
  • Ensure the dose, duration and rate of infusion prescriptions are correct;
  • Understand the importance of units and rounding when solving calculations.

This article forms part of a series of pharmacy calculation video tutorials brought to you by The Pharmaceutical Journal. Other tutorials in the series include:

Calculating the dose, duration and rate of drug infusions is a necessary skill for pharmacists. The ability to do so correctly is tested as part of the General Pharmaceutical Council registration assessment.

In the following videos, Simon Harris, head of education at Greenlight Campus, provides trainees with a step-by-step walk through of three example infusion calculations. The following questions and answers are provided as examples only and are included to give you an idea of the types of question that will be asked during the registration assessment. Please note that these exact questions will not be included.

Example 1: Infusions — dose

Question:

Miss A is currently on a heparin infusion in hospital. At 14:00, during your ward round, you notice her pump is beeping and showing an occlusion. You alert the nurse who presses the ‘Stop’ button. The senior house officer comes into the ward and asks you how much heparin Miss A has had so far today.

The last rate set on the pump was 2mL/hour and it has not been altered since it was set up at 08:00. Her prescription reads 25,000 units in 50mL sodium chloride 0.9%.

How many units of heparin has Miss A received so far?

Solution:

Example 2: Infusions — duration

Question:

The following hospital prescription is written for a child aged 9 years weighing 31kg.

DateInfusionInfusion ratePrescriber signature
Today’s dateImmunoglobulin 10% solution for infusion,
0.5g/kg
0.3mL/kg/hr for 15 minutes then
1.8mL/kg/hr for 30 minutes then
2.2mL/kg/hr for 45 minutes then
6mL/kg/hr for the remainder of the infusion
I. Prescriber
Today’s date

Using the information in the resource pack provided, what is the total infusion duration of the immunoglobulin if it is infused at the prescribed rate? Round your answer up to the nearest whole minute.

Solution

Example 3: Infusions — rate

Question:

A woman aged 78 years has a percutaneous endoscopic gastrostomy tube in situ. She requires 2500 kcal per 24 hours from her enteral feed and has been instructed to use Jevity®, which contains 154 kcal per 100mL. She would like to feed for 8 hours overnight. 

At what rate in mL/hour should the feed be delivered? Give your answer to the nearest 5mL.

Solution:

For thousands of practice questions to help you prepare for the GPhC registration assessment, including extended matching answer, single best answer and calculations, visit ONtrack.

Last updated
Citation
The Pharmaceutical Journal, PJ, July 2021, Vol 307, No 7951;307(7951)::DOI:10.1211/PJ.2021.1.93449