how to series
Free How to undertake oral medicines calculations
Jane Brindley Senior lecturer in adult nursing, School of Nursing, Faculty of Health and Wellbeing, Canterbury Christ Church University, Kent, England
• To enhance your knowledge of the procedures involved in oral medicines calculations
• To understand the role of the nurse in reducing medication errors
• To recognise the importance of local and national policies in protecting patients from harm
Rationale and key points
This article provides a step-wise, practical approach to undertaking oral medicines calculations for tablets, capsules and liquid preparations. It also explores the evidence base behind medication errors in relation to calculations.
• Medication errors have been identified as one of the most frequent types of patient safety incident that occur, and they are the most common, single preventable cause of adverse events.
• Undertaking a medicines calculation can reduce the risk of an incorrect medicines dose being administered to the patient, which would result in a suboptimal therapeutic benefit or possible harm to the patient.
• Nurses must ensure that they practise undertaking medicines calculations, while healthcare organisations have an important role in ensuring this skill is developed and maintained among their staff.
‘How to’ articles can help to update your practice and ensure it remains evidence-based. Apply this article to your practice. Reflect on and write a short account of:
1. How this article might improve your practice when undertaking oral medicines calculations.
2. How you could use this information to educate nursing students or your colleagues on the appropriate methods for undertaking oral medicines calculations.
Nursing Standard. doi: 10.7748/ns.2019.e11404Peer review
This article has been subject to external double-blind peer review and checked for plagiarism using automated softwareCorrespondence
Brindley J (2019) How to undertake oral medicines calculations. Nursing Standard. doi: 10.7748/ns.2019.e11404Disclaimer
Please note that information provided by Nursing Standard is not sufficient to make the reader competent to perform the task. All clinical skills should be formally assessed at the bedside by a nurse educator or mentor. It is the nurse’s responsibility to ensure their practice remains up to date and reflects the latest evidence
Published online: 28 May 2019
Preparation and equipment
• When undertaking medicines calculations and administration, the nurse must ensure that they follow national and local policies, procedures and protocols, such as checking a calculation result with another healthcare practitioner if this is required. For controlled medicines, the National Institute for Health and Care Excellence (NICE) (2016) recommends arranging for another healthcare professional to undertake a second check of dose calculations.
• The nurse should reflect on their personal competence and take steps to address any deficits or uncertainty in medicines calculations, knowledge and competence.
• Where possible, the nurse should find a quiet, undisturbed area to undertake medicines calculations. The nurse should inform their colleagues that this is necessary and that they should not be disturbed, to reduce interruptions and distractions as much as possible.
• There is minimal equipment required to undertake medicines calculations. The nurse will require the patient’s medicines chart to check the prescription information and dose. The use of a pen, paper and calculator may aid the mathematics of the calculation. Supportive devices such as a calculator can reduce mathematical calculation errors; however, they do not improve results in medicines calculation tests if the causes of errors are conceptual (Weeks et al 2013, Brindley 2017), as discussed later in this article.
• When undertaking medicines calculations and administration, the nurse must ensure that they follow national and local policies, procedures and protocols, such as checking a calculation result with another healthcare practitioner if this is required
• The term ‘medication error’ refers to any preventable error that occurs during the medicines administration process, including errors in medicines calculations
• Medicines calculations should be undertaken to reduce the risk of medication errors that can result in suboptimal treatment or harm to the patient
• If the nurse has any doubts about the accuracy of a medicines calculation result, or their competence in undertaking the calculation, they must not administer the medicine. Instead, they must seek immediate assistance and advice, for example by asking another nurse to undertake the calculation
1. Ensure the prescription on the patient’s medicines chart is correctly and legibly written and that it remains appropriate to administer the medicine. If there is any doubt about what is written, or whether the medicine is still required, do not administer the medicine and seek immediate assistance and advice from the prescriber or pharmacist.
2. Check the patient’s medicines chart to ensure that the medicine is due to be administered, to avoid spending unnecessary time working out the calculation.
3. Check the prescribed route of administration on the patient’s medicines chart, since the medicines calculation will vary between the oral and intravenous routes.
4. Ascertain if the oral medicine is in tablet or capsule form, or comprises a liquid preparation, because the medicines calculation formula used will vary between these forms.
5. Check that the prescribed dose of the medicine is in the same unit of measurement as the medicine stock you are going to use, for example that both are in milligrams (mg) or micrograms (mcg) (for the purpose of this article and ease of reading, the abbreviation mcg is used for micrograms. However, NICE (2019) guidelines state that micrograms and nanograms should not be abbreviated in clinical practice, to avoid medication errors during prescribing and administration). If the prescribed dose and stock are not in the same unit of measurement, convert the units of the stock dose to the units of the prescribed dose of the medicine to make the calculation easier to undertake. Information on the conversion of units of measurement is outlined in Box 1.
6. Select the appropriate medicines calculation formula to use, depending on whether the required oral medicine is in tablet or capsule form, or is a liquid preparation.
• For oral tablets or capsules – to calculate how many tablets or capsules to administer, consult the patient’s medicines chart to identify the dose required. Check the stock dose, which will be on the medicines packaging. Box 2 shows the formula for oral tablets or capsules calculations.
• For liquid preparations – to calculate the volume of liquid to administer, consult the patient’s medicines chart to identify the dose required. Check the stock strength, which will be on the medicines label, and whether the dose of the medicine is within a set volume, for example 100mg ibuprofen in 5mL oral suspension. Box 3 shows the formula for oral liquid preparations calculations.
7. Check the calculation result carefully. If necessary, undertake the calculation again to ensure it is correct. Consider if the result looks illogical or incorrect, for example if it would require a large number of tablets or volume of liquid to be administered. Check the result with a colleague if this is the local policy or if you are in any doubt regarding the accuracy of the result. If in any doubt, do not administer the medicine and seek expert advice, for example from the nurse in charge or the pharmacist. Follow local policy in documenting any medicine that is not administered.
Conversion of units of measurement
The order of unit sizes is as follows:
To convert between the measurements, multiply by 1,000, or divide by 1,000 as follows:
• mcg to mg – divide by 1,000
• mg to g – divide by 1,000
• g to mg – multiply by 1,000
• mg to mcg – multiply by 1,000
Converting micrograms (mcg) to milligrams (mg)
To convert mcg to mg, divide by 1,000. For example, the nurse would convert 1,500mcg to mg as follows:
1,500 / 1,000 = 1.5mg
Converting milligrams (mg) to grams (g)
To convert mg to g, divide by 1,000. For example, the nurse would convert 750mg to g as follows:
750 / 1,000 = 0.75g
Converting grams (g) to milligrams (mg)
To convert g to mg, multiply by 1,000. For example, the nurse would convert 0.5g to mg as follows:
0.5 × 1,000 = 500mg
Converting milligrams (mg) to micrograms (mcg)
To convert mg to mcg, multiply by 1,000. For example, the nurse would convert 2mg to mcg as follows:
2 × 1000 = 2,000mcg
Formula for oral tablets or capsules calculations
The formula for calculating oral tablets or capsules is as follows:
Dose required / Stock dose = number of tablets or capsules to be administered
This formula could also be written as:
What you want / What you have = what you give
For example, a patient has been prescribed 30mg prednisolone as an oral tablet. You have 10mg prednisolone tablets in stock.
30mg / 10mg = administer three tablets
Formula for oral liquid preparations calculations
The formula for calculating oral liquid preparations is as follows:
Dose required / Stock strength × volume of the solution = volume of liquid to be administered
This formula could also be written as:
What you want / What you have × what it is in = what you give
For example, a patient has been prescribed 250mg erythromycin oral dose, and is unable to take tablets or capsules. You have an oral suspension of 125mg erythromycin in 5mL.
250mg / 125mg × 5mL = administer 10mL
Medicines calculations are an essential aspect of the wider process of safe medicines administration. Medicines calculations should be undertaken to reduce the risk of medication errors, which can result in suboptimal treatment or harm to the patient. However, preventing all medication errors is not possible, because experience and competency cannot negate the possibility of human error (Brindley 2018).
The term ‘medication error’ refers to any preventable error that occurs during the medicines administration process, including errors in medicines calculations. Medication errors have been identified as one of the most common types of medical errors (Fleming et al 2014, Elden and Ismail 2016, Brindley 2017), and may be related to professional practice, healthcare products, procedures, and systems, including prescribing, order communication, product labelling and packaging (World Health Organization (WHO) 2016, Department of Health and Social Care (DHSC) 2018).
There are several types of medication errors, including (Brindley 2017):
• Mathematical calculation errors – those resulting from suboptimal arithmetic skills.
• Procedural errors – those resulting from placing numbers inappropriately within medicines calculation formulas, or inaccurately converting units of measurement.
• Conceptual errors – those resulting from reading a patient’s medicines chart incorrectly, measuring an incorrect dose of a medicine after undertaking an accurate calculation, or failing to recognise that an implausible medicines calculation result is inaccurate (Eastwood et al 2011, Sulosaari et al 2015, Brindley 2017).
Gluyas (2018) took a wide view of medication errors and explored the role of individual healthcare practitioners and healthcare organisations in implementing systems that can reduce these errors. A punitive environment can result in a ‘blame and shame’ culture, in which healthcare professionals develop a fear of retribution or disciplinary proceedings arising from any mistakes they make. Furthermore, this approach does not recognise that both individual and systemic factors can lead to errors, and therefore denies the individual and the healthcare organisation the opportunity to learn from errors (Dekker 2011, Gluyas 2018).
Individual factors may include lack of knowledge or understanding of medicines calculations formulae, or inadequate mathematical skills. Systemic factors could include an organisational culture of assuming staff competency in medicines calculations, or an environment where it is considered acceptable to interrupt a colleague to ask a non-urgent question when they are preparing or administering medicines. Therefore, a culture of learning and support may be beneficial.
In some cases, calculating the correct dose of a medicine may appear relatively straightforward. However, the National Reporting and Learning System (2017) reported 188,110 patient safety incidents related to medication errors between April 2015 and March 2016 in the UK. These incidents included prescribing and dispensing errors, as well as errors in medicines administration. It has been identified that nurses can safeguard against these errors, thus intercepting a significant proportion of prescriber errors (Cloete 2015).
The WHO (2017) launched a Global Patient Safety Challenge entitled Medication Without Harm. Beyond the harm caused to patients, it is estimated that the annual cost associated with medication errors is around $42 billion worldwide. Therefore, reducing the effects and frequency of medication errors will improve patient safety, relieve economic burdens and develop healthcare systems that minimise the risk of errors (DHSC 2018). The WHO (2017) aims to reduce severe avoidable medicines-related harm globally by 50% in the next five years. This will be achieved by encouraging stakeholders – such as policymakers, healthcare professionals and patients – and countries to focus on early action priorities, as well as developmental programmes to improve practice and healthcare systems (DHSC 2018).
Importance of medicines calculations skills for nurses
Healthcare regulatory codes of practice are in place to protect patients and the public from harm through the actions or omissions of an individual healthcare practitioner. In the UK, The Code: Professional Standards of Practice and Behaviour for Nurses, Midwives and Nursing Associates (Nursing and Midwifery Council (NMC) (2018a) requires all nurses to advise on, prescribe, supply, dispense or administer medicines within the limits of their training and competence, the law, local and national guidance, and other relevant policies and regulations.
The updated NMC (2018b) standards of proficiency for nurses state that they must be able to ‘understand the principles of safe and effective administration and optimisation of medicines in accordance with local and national policies and demonstrate proficiency and accuracy when calculating dosages of prescribed medicines’ and ‘undertake accurate drug calculations for a range of medications’. Similarly, the NMC (2018c) standards for pre-registration nursing programmes state that approved education institutions must ‘ensure that all programmes include a health numeracy assessment related to nursing proficiencies and calculation of medicines which must be passed with a score of 100%’.
In the UK, the revalidation process requires nurses to reflect on their practice through various activities, such as obtaining practice-related feedback and writing reflective accounts (NMC 2019). Nurses can reflect on their practice in relation to medicines calculations to identify any deficits in their knowledge or skills. They could then address any issues identified through personal study, formal courses, or working with a competent colleague or mentor.
If the nurse has any doubts about the accuracy of a medicines calculation result or their competence in undertaking the calculation, they must not administer the medicine. Instead, they must seek immediate assistance and advice, for example by asking another nurse to undertake the calculation. The Royal Pharmaceutical Society and Royal College of Nursing (2019) guidelines state that any calculations required must be ‘double checked where practicable by a second person and uncertainties raised with the prescriber or a pharmacy professional’.
Healthcare organisations have a responsibility to ensure there are user-friendly systems in place in relation to medicines calculations and administration, and that their staff have the support and education to practise competently (Gluyas 2018).
Brindley J (2017) Undertaking drug calculations for oral medicines and suppositories. Nursing Standard. 32, 7, 56-63. doi: 10.7748/ns.2017.e10953.
Brindley J (2018) Undertaking drug calculations for intravenous medicines and infusions. Nursing Standard. 32, 20, 55-63. doi: 10.7748/ns.2018.e11029.
Cloete L (2015) Reducing medication errors in nursing practice. Cancer Nursing Practice. 14, 1, 29-36. doi: 10.7748/cnp.14.1.29.e1148.
Dekker S (2011) Drift into Failure: From Hunting Broken Components to Understanding Complex Systems. Ashgate Publishing, Farnham.
Department of Health and Social Care (2018) The Report of the Short Life Working Group on Reducing Medication-related Harm. The Stationery Office, London.
Eastwood KJ, Boyle MJ, Williams B et al (2011) Numeracy skills of nursing students. Nurse Education Today. 31, 8, 815-818. doi: 10.1016/j.nedt.2010.12.014.
Elden NMK, Ismail A (2016) The importance of medication errors reporting in improving the quality of clinical care services. Global Journal of Health Science. 8, 8, 243-251. doi: 10.5539/gjhs.v8n8p243.
Fleming S, Brady AM, Malone AM (2014) An evaluation of the drug calculation skills of registered nurses. Nurse Education in Practice. 14, 1, 55-61. doi: 10.1016/j.nepr.2013.06.002.
Gluyas H (2018) Understanding the human and system factors involved in medication errors. Nursing Standard. doi: 10.7748/ ns.2018.e11176.
National Institute for Health and Care Excellence (2016) Controlled Drugs: Safe Use and Management. NICE guideline No. 46. NICE, London.
National Institute for Health and Care Excellence (2019) Prescription Writing. http://bnf.nice.org.uk/guidance/prescription-writing.html (Last accessed: 10 May 2019.)
National Reporting and Learning System (2017) FOI National Reporting and Learning System (NRLS): Medication Errors. http://www.gov.uk/government/publications/foi-national-reporting-and-learning-system-nrls-medication-errors (Last accessed: 10 May 2019.)
Nursing and Midwifery Council (2018a) The Code: Professional Standards of Practice and Behaviour for Nurses, Midwives and Nursing Associates. NMC, London.
Nursing and Midwifery Council (2018b) Future Nurse: Standards of Proficiency for Registered Nurses. NMC, London.
Nursing and Midwifery Council (2018c) Realising Professionalism: Standards for Education and Training. Part 3: Standards for Pre-Registration Nursing Programmes. NMC, London.
Nursing and Midwifery Council (2019) Revalidation. How to Revalidate with the NMC: Requirements for Renewing your Registration. NMC, London.
Royal Pharmaceutical Society, Royal College of Nursing (2019) Professional Guidance on the Administration of Medicines in Healthcare Settings. http://www.rpharms.com/Portals/0/RPS%20document%20library/Open%20access/Professional%20standards/SSHM%20and%20Admin/Admin%20of%20Meds%20prof%20guidance.pdf?ver=2019-01-23-145026-567 (Last accessed: 10 May 2019.)
Sulosaari V, Huupponen R, Hupli M et al (2015) Factors associated with nursing students’ medication competence at the beginning and end of their education. BMC Medical Education. 15, 223. doi: 10.1186/s12909-015-0513-0.
Weeks KW, Clochesy JM, Hutton BM et al (2013) Safety in numbers 4: the relationship between exposure to authentic and didactic environments and nursing students’ learning of medication dosage calculation problem solving knowledge and skills. Nurse Education in Practice. 13, 2, e43-e54. doi: 10.1016/j.nepr.2012.10.010.
World Health Organization (2016) Medication Errors: Technical Series on Safer Primary Care. WHO, Geneva.
World Health Organization (2017) WHO Launches Global Effort to Halve Medication-related Errors in 5 Years. http://www.who.int/news-room/detail/29-03-2017-who-launches-global-effort-to-halve-medication-related-errors-in-5-years (Last accessed: 10 May 2019.)
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