Medication Calculations: Preventing Errors
Published: 22 January 2024
Published: 22 January 2024
Maths can be intimidating to some but being able to make quick calculations is an essential skill for healthcare practitioners.
Calculating the correct dose of any medicine you administer is crucial in avoiding medication errors. Administering the wrong dose can have severe consequences for the patient, such as preventable morbidity and even death (Tariq et al. 2023).
While miscalculation is a serious and entirely preventable adverse event, healthcare professionals are human, and errors can still occur.
We, therefore, have a responsibility to our patients to find ways of minimising instances of medication errors from occurring.
We can’t rely solely on technology. Even though devices like calculators and programmable IV pumps are convenient and helpful, we need to be able to verify calculations and have a sense of what seems right mathematically (Wilson & Miller 2013; Schnur 2021).
While the exact rates of medicine miscalculation in Australia are unknown, medicine-related problems account for 250,000 hospital admissions every year (DoHaAC 2022).
Furthermore:
(DoHaAC 2022; PSA 2019)
One study, which examined medication errors made in an Australian pediatric hospital over a five-year period, found that medicine miscalculation was a factor in 341 or 5.1% of cases (Manias et al. 2018).
Numeracy skills are crucial in ensuring patient safety. However, literature suggests that undergraduate nursing students and registered nurses alike lack competency in this area (Minty-Walker et al. 2021).
This article will provide a refresher on the basics of medicine calculation.
Common causes of medication errors and miscalculations include:
(Sayadi et al. 2021; Tariq et al. 2023)
Medicines use a range of units of measurement. Knowing the differences between each unit of measurement and being able to convert one to another are essential competencies, as conversion mistakes can cause patients to receive much more or less medicine than they need, with potentially severe consequences (Wright & Shepherd 2017).
Medicines are generally measured using either:
(Wright & Shepherd 2017)
The main units of measurement that healthcare practitioners will need to convert are metric weights.
An important rule to remember is that metric weights use a factor of 1,000, meaning that:
(Wright & Shepherd 2017; Tweedy & Mason 2000)
Note: Metric volumes also use a factor of 1,000, i.e. 1 L = 1,000 mL and 1 mL = 0.001 L.
Kilograms | Grams | Milligrams | Micrograms | Nanograms | |
---|---|---|---|---|---|
Kilograms | 1 kg = 1,000 g | 1 kg = 1,000,000 mg | 1 kg = 1,000,000,000 micrograms | 1 kg = 1,000,000,000,000 nanograms | |
Grams | 1 g = 0.001 kg | 1 g = 1,000 mg | 1 g = 1,000,000 micrograms | 1 g = 1,000,000,000 nanograms | |
Milligrams | 1 mg = 0.000001 kg | 1 mg = 0.001 g | 1 mg = 1,000 micrograms | 1 mg = 1,000,000 nanograms | |
Micrograms | 1 microgram = 0.000000001 kg | 1 microgram = 0.000001 g | 1 microgram = 0.001 mg | 1 microgram = 1,000 nanograms | |
Nanograms | 1 nanogram = 0.000000000001 kg | 1 nanogram = 0.000000001 g | 1 nanogram = 0.000001 mg | 1 nanogram = 0.001 micrograms |
These conversions must be checked thoroughly to ensure the patient doesn’t receive too much or too little of their medicine. Perform the calculation twice in order to verify the result and always ask a colleague to perform the calculation themselves in order to double-check (Wright & Shepherd 2017).
Calculating Tablet Dose Forms
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Calculating Mixtures and Solutions
*This formula is not only used for intravenous medicines, but for oral medicines as well.
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Calculating Intravenous Fluid Rate
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Calculating Millilitres Per Minute
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Calculating Drops Per Minute
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(SLSS 2022)
Sometimes a calculation will result in a number with decimals. Depending on the situation, you might need to round these numbers off to a whole number or round off to a certain number of decimal places.
When rounding off, check the first number out of all the ones you want to drop:
(Centennial College 2024)
For example, say you want to round off the number 5.762.
(SLSS 2022; Wright & Shepherd 2017)
You can check your competency in arithmetic, conversion and calculation by taking interactive medicine calculation quizzes on the Flinders University website.
Medicine miscalculation is an issue for new and experienced health practitioners alike.
Literature suggests that a two-fold approach is important for reducing the rates of miscalculation: focusing on undergraduate mathematical skills as well as testing graduates for ongoing competence. Suggestions for undergraduates include practical sessions and assessments in safe environments that can aid the hands-on learner, and assisting them to gain confidence as practitioners. Remedial help should be available for those who require it.
Despite numerical competency tests being perceived as 'highly stressful and exceptionally challenging' by healthcare professionals (Bayne & Bindler 1997), it’s been argued that regular testing using a recognised, validated and reliable assessment tool be used as a means of maintaining numeracy skills (Warburton 2010).
Furthermore, a 2021 study found that a medicine calculation training program was beneficial in reducing mathematical anxiety and improving calculation skills among students. It suggested that introducing similar programs in nursing schools would be beneficial in reducing the risk of miscalculation (Sayadi et al. 2021).
Raising awareness of medicine miscalculations within the healthcare population can be achieved by regularly monitoring and reporting these errors. Practitioners can be involved in assessing the cause of the problem and can make suggestions on how to avoid future errors.
Using calculators and conversion charts may be helpful, but should never be used as a substitute for taking into consideration the logical or expected answer. Unfortunately, today's busy healthcare environments mean that all too often, we’re rushing to complete tasks efficiently.
It’s important to nurture an environment where you and your colleagues feel supported in medical calculations, even if you occasionally make errors. Discussing the error in a supportive manner will go a long way in encouraging struggling practitioners to persevere and achieve better calculation results in the long term.
Question 1 of 5
Your patient requires a dosage of 750 mg of cephalexin. The stock solution contains 0.25 g per 5 mL. How many mL should be administered?
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