Wes Mountain/The Pharmaceutical Journal
By the end of this article, you should be able to:
- Systematically work through kidney function calculations and be able to use the Cockroft and Gault formula to estimate creatine clearance;
- Understand whether ideal body weight or actual body weight should be used within the calculation;
- Understand the importance of units and rounding when solving calculations.
Introduction
Kidney function calculations provide vital information that can inform dosage. For example, the Medicines and Healthcare products Regulatory Agency (MHRA) advises that when prescribing in renal impairment, there is appropriate estimation of renal function to avoid the risk of adverse drug reactions1. Calculations should also be performed when patients are receiving direct-acting oral anticoagulants (DOACs) and other drugs with a narrow therapeutic index that are renally excreted (e.g. digoxin).
According to the General Pharmaceutical Council (GPhC) assessment framework, foundation trainees should expect at least one calculation question on kidney function to appear in the exam. As with other calculations, knowledge of conversions of units, definitions of concentrations and rounding is important, along with being able to correctly use a provided formula.
One of the most commonly used methods for estimating kidney function is to calculate an estimate for creatine clearance using the Cockcroft and Gault formula2. For the purposes of the exam, you do not need to memorise the formula as it is usually provided within the question but you do need to ensure you are fully competent at using it.
The Cockcroft and Gault formula uses the patient’s age, body weight and knowledge of serum creatine.
Age is in years
Weight is in kilograms. Ideal body weight (see Box) is most typically used although there are cases where actual body weight may be appropriate (e.g. for underweight patients)
Height is measured in centimetres.
Serum creatinine in micromole/L
Constant = 1.23 for men; 1.04 for women
The examiners are most likely to ask you to calculate a dose for a drug in a patient with renal impairment by referring to a resource (e.g. the SmPC for the drug).
Let us illustrate how this formula could be used by working through the example below.
Box: How to calculate ideal body weight
Ideal body weight can be calculated using the following formula[2].
Where:
Constant = 50 for men; 45.5 for women
Example question
A 75-year-old woman weighing 75kg and height 1.79 metres is due to commence treatment with an IV antibiotic A, which has a narrow therapeutic range. She has a stable serum creatinine of 105 micromole/L.
Calculate a suitable dose in (mg) given every 24 hours. Give your answer rounded to the nearest 15mg dose.
Information provided by the resource (e.g. the SmPC for the drug):
Antibiotic A dosing based on CrCl (mL/min):
- Greater than 80ml/min = 15mg/kg/24hours;
- 60-80ml/min = 12mg/kg/24 hours;
- 40-60ml/min = 7.5mg/kg/24hours;
- 30-40ml/min = 4mg/kg/24hours;
- 20-30ml/min = 7.5mg/kg/48hours;
- 10-20ml/min = 4mg/kg/72hours.
Formula and further information:
(Where the patient’s actual body weight is less than their ideal body weight, actual body weight should be used instead).
*Ht = height in cm.
Comments and explanations
The above question requires the correct use of the provided formula for CrCl and selecting the dose that corresponds to the CrCl calculated.
To answer the question correctly we proceeded using the following steps. Other approaches or an alternative sequence of steps are also valid.
Step one
Gather the pertinent patient information from the question.
Age is 75 years, weight is 75kg and height is 1.79m.
Convert 1.79m to cm and this becomes 179cm.
Step two
Calculate the ideal body weight (IBW).
Since the patient’s IBW is less than the actual body weight, IBW is used in this case for calculating the dose. With some drugs, the formula may specify whether actual body weight or ideal body weight should be used.
Using the information gathered in step 1, ideal body weight can next be calculated.
Step three
Calculate CrCl using the provided formula.
Step four
Read off the recommended dose for this rate of CrCl provided in the resource.
In this case, the dose for a CrCl of 44.93mL/min is 7.5mg/kg/24 hours.
Step five
Note that the question asks you to calculate a suitable dose in mg given every 24 hours so we now need to convert the daily dose from mg/kg to mg.
Step six
Round to the nearest 15mg
Dose = 525mg
Disclaimer
The views in this article are those of the authors and do not represent the views of any organisations they are associated with. The questions and explanations presented here are for educational purposes only and do not replace your training, knowledge and application of professional judgement as a pharmacist or trainee pharmacist. The example questions used in this article and the answers provided are for educational purposes and should not be translated to represent what would happen in real practice.
Acknowledgements:
All practice questions were reproduced with kind permission provided by ‘Focus Pre Reg Revision’.
- 1.Prescribing medicines in renal impairment: using the appropriate estimate of renal function to avoid the risk of adverse drug reactions. Medicines and Healthcare products Regulatory Agency. Accessed October 2024. https://www.gov.uk/drug-safety-update/prescribing-medicines-in-renal-impairment-using-the-appropriate-estimate-of-renal-function-to-avoid-the-risk-of-adverse-drug-reactions
- 2.Prescribing in renal impairment. National Institute of Health and Care Excellence. Accessed October 2024. https://bnf.nice.org.uk/medicines-guidance/prescribing-in-renal-impairment/
