Foster, M. Weight estimation guidelines – Part 1, Don't Forget the Bubbles, 2019. Available at:
When a child is picked up by paramedics or brought into an emergency department, their weight is not always known and cannot always be formally measured. Many research teams across the globe are trying to find the best method to estimate a child’s weight, so medication can be dosed and equipment sized appropriately. Traditionally, age-based formulae have been used, but these are known to be very inaccurate. More reliable methods are available, however all require input of more information than just age, whether that be height, mid-arm circumference, a parent estimate or a smartphone image. You can find a summary of weight estimation techniques in this post from Andy Tagg. The question is, what method is sufficiently accurate and will work best in practice?
At the moment, Australian guidelines still use age-based formulae (namely the original APLS formula, weight = 2 x (age+4)). Even though they are very inaccurate, they have a number of advantages:
They are very quick. Most prescribers use these formulae in conjunction with resuscitation aids, emergency manuals or clinical practice guidelines which mean they do not need to remember the formula or do the calculation themselves as they are given a table with corresponding weight to age. Given age-to-weight conversions are often provided, staff do not need to be trained on how to gather the estimate. They do not require any additional equipment, which may be hard to find if an ambulance or emergency department rarely sees paediatric critical cases. An emergency department can predict the weight of the child that is about to arrive by ambulance if they have the child’s age, and can therefore start drawing up medications in advance.
Stress and cognitive load have been shown to be the key precipitating factors of human error in paediatric critical events. Human errors in these scenarios include significant medication errors, such as ten-fold errors (where 10x the medication is prescribed or administered because the decimal point is moved or the concentration incorrectly calculated). These have been shown to cause significant patient morbidity and mortality. Efforts to gather the further information needed to make the weight estimate more accurate (e.g. measuring the child, taking a sufficient quality image, finding a parent) increase the complexity of the weight estimation phase. Increased complexity is likely to increase cognitive load, and thus increase the risk of human error at all phases in the dosing process.
We need to find a weight estimation tool that can be used by anyone who might need to manage a paediatric critical event. This includes paramedics, junior medical staff and adult emergency department personnel that may need to manage patients before they reach a tertiary children’s hospital or paediatric emergency department. This means we need clear, easy-to-follow guidelines and associated training that can be rolled out broadly. It also emphasises the need to ensure we keep the cognitive burden as low as possible, as many prescribers will be in an unfamiliar, stressful situation, both of which further precipitate human error. Future protocols may also differ based on the paediatric emergency expertise and training available in that setting, for example, a paediatric emergency department may choose a more accurate method with higher cognitive load than an ambulance service.
Another important consideration is the time delay involved in each weight estimation strategy. Most events requiring weight estimation are time-critical in nature. It is important to not only consider the time involved in getting the estimate, but also the time needed to find the appropriate equipment, make subsequent dose calculations and prepare the dose for administration. This highlights the significant advantage of emergency departments being able to draw up medications prior to the child’s arrival, as having doses pre-calculated and pre-prepared would significantly reduce the time delay in drug administration.
Given rising rates of childhood obesity, we need to find a weight estimation strategy that will work for all body types and medication types. Some drugs should be dosed based on ideal body weight (IBW) whilst others should be dosed based on total body weight (TBW), depending on their pharmacokinetic properties. Similarly, dosing medication by TBW in obese children can lead to overdose. Sydney Children’s Hospital has given a nice overview to some of the adjustments which should be made for specific medications. However, adjusting weights for specific medications in a paediatric emergency may further add to the cognitive load.
Overall, the pros and cons of each group of techniques can be summarised in a table:
So, how important is it that we have an accurate weight estimate? And how important are other considerations such as reducing cognitive load and practicality (eg. speed, equipment and staff training requirements)?
Unfortunately, there is very limited data on patient outcomes available to help guide us. The small number of studies into the impact of weight errors look at incorrectly documented weights, such as where the wrong weight unit was recorded (pounds instead of kilograms) or where a decimal point was moved (6). No study has looked specifically at the harms caused by weight estimation error in paediatric emergencies. There is no suggestion that using the original APLS formula in Australia is currently causing harm to patients, but there is also no evidence that proves that it is not. Reducing error should always be the goal, however increasing the complexity of generating a weight estimate could increase the cognitive load, and thus increase the risk of more significant errors. When deciding on which weight estimation technique to use, we need to find a middle ground between accuracy and practicality with an emphasis on reducing overall cognitive load.
Wells M, Goldstein LN, Bentley A. The accuracy of emergency weight estimation systems in children – a systematic review and meta-analysis. Int J Emerg Med. 2017;1:1. Available from: https://intjem.biomedcentral.com/articles/10.1186/s12245-017-0156-5
Sutherland A, Ashcroft DM, Phipps DL. Exploring the human factors of prescribing errors in paediatric intensive care units. Arch Dis Child. 2019;0:1-8. Available from: https://adc.bmj.com/content/104/6/588.long
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Doherty C, McDonnell C. Tenfold medication errors: 5 years’ experience at a university-affiliated pediatric hospital. Pediatrics. 2012;129(5):916-24. Available from: https://pediatrics.aappublications.org/content/129/5/916.long
Foster M, Tagg A, Klim S, Kelly AM. Accuracy of parental estimate of child’s weight in a paediatric emergency department. Emerg Med Australas. 2019; in press.
Shaw KN, Lillis KA, Ruddy RM, Mahajan PV, Lichenstein R, Olsen CS, et al. Reported medication events in a paediatric emergency research network: sharing to improve patient safety. Emerg Med J. 2013;30(10):815-9. Available from: https://emj.bmj.com/content/30/10/815.long