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I can guess your weight

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Before we can treat a child in the emergency department, we need to know how much they weigh. We have weight-based formulas for everything from the dose of antibiotic we prescribe to the size and length of the endotracheal tube we insert. In another post examining one of the basic premises of what we do, I want to examine some of the methods we use if we cannot put the child on the scales.

We don’t know a child’s weight when they are picked up by paramedics or brought into an emergency department.

Many research teams have been trying to find the best way to estimate a child’s weight so medication can be dosed safely and equipment is sized appropriately.

So, what weight estimation guidelines are we supposed to use?

Traditionally, we’ve used age-based formulae, but these are inaccurate. More reliable methods are available. However, all require more information than age: height, mid-arm circumference, parent estimate, or smartphone image.

We still use age-based formulae in Australia, and although inaccurate, they have some advantages.

Why use age-based formulae?

They are very quick. Most prescribers use these formulae with resuscitation aids, emergency manuals or clinical practice guidelines. This means they do not need to remember the formula or do the calculation themselves as they are given a table with corresponding weight to age.

Since age-to-weight conversions are often provided, staff do not need to be trained to gather the estimate.

They do not require additional equipment, which may be hard to find if an ambulance or emergency department rarely sees paediatric critical cases.

You can predict the weight of the child that is about to arrive by ambulance if you have their age. This means you can draw up any critical medications in advance.

The impact of cognitive load on medication errors

Extrinsic cognitive load is a key risk factor for committing a human error in paediatric critical events. Mistakes include significant medication errors like ten-fold errors (where 10x the medication is prescribed or administered because the decimal point is moved or the concentration is incorrectly calculated). These cause significant patient morbidity and mortality.

Gathering more information to make the weight estimate more accurate (e.g., measuring the child, taking a sufficient-quality image, or finding a parent) increases the complexity of the weight estimation phase. This increased complexity increases cognitive load and the risk of human error at all phases of the dosing process.

What is the ideal weight estimation tool?

We need to find something that can be used by anyone who might need to manage a critical paediatric event. This includes paramedics, junior medical staff and adult emergency department personnel who may need to manage patients before they reach a tertiary children’s hospital or paediatric emergency department.

We need clear, easy-to-follow guidelines and associated training that can be rolled out broadly. We must keep the cognitive burden as low as possible, as many prescribers will be in an unfamiliar, stressful situation.

Future protocols may also differ based on the paediatric emergency expertise and training available in that setting, so this needs to be considered. For example, a paediatric emergency department may choose a more accurate method with a higher cognitive load than an ambulance service.

Another crucial factor is the time delay associated with each weight estimation strategy. When it comes to events that require weight estimation, time is of the essence. It is essential to consider the time required to obtain the estimate and the time needed to locate the appropriate equipment, perform subsequent dose calculations, and prepare the medication for administration. There is a significant advantage if emergency departments can prepare medications in advance, even before the child arrives. Pre-calculating and pre-preparing doses would significantly reduce the time delay in administering the drugs, which is paramount in emergencies.

In light of the increasing prevalence of childhood obesity, finding a weight estimation strategy that can accommodate various body types and medication requirements is crucial. Different drugs necessitate dosing based on different factors, such as ideal body weight (IBW) or total body weight (TBW), depending on their pharmacokinetic characteristics. Dosing medication based on TBW in obese children can potentially result in overdosing.

Sydney Children’s Hospital has provided a comprehensive overview of the necessary adjustments for specific medications, which is highly valuable. However, in pediatric emergencies, weight adjustments for each medication can further burden healthcare providers with additional cognitive load.

Regrettably, available data on patient outcomes is scarce to provide clear guidance. Existing studies examining the impact of weight errors focus on incorrectly recorded weights, such as instances where the wrong unit (e.g., pounds instead of kilograms) or decimal point placement was used. None of these studies specifically investigate the harms resulting from weight estimation errors.

There is no suggestion that using the original APLS formula in Australia is causing harm to patients. However, there is also a lack of evidence to definitively prove otherwise. Therefore, the overarching goal should always be to minimize errors.

Nonetheless, it is important to consider that increasing the complexity of weight estimation methods could heighten the cognitive load on healthcare providers, increasing the risk of more significant errors. Hence, it is crucial to strike a balance between accuracy and practicality.

Getting an accurately dosed drug into a critically ill or injured child is complex.

Unfortunately, there is a lack of high-quality evidence to guide our practice regarding weight estimation and drug dosing. We have limited knowledge about the consequences of dosing errors and the appropriate doses for many medications.

We can take two perspectives.

One is to argue that accurate weight estimation doesn’t matter since no evidence shows its impact on outcomes. The other is to advocate for maximum accuracy to minimize potential drug dosing errors, which is the more ethical option.

Fortunately, recent evidence supports the idea that accurate weight estimation is crucial. A significant medication error is likely in at least one-third of clinically stable children, and the situation becomes even more concerning for children needing resuscitation.

We should focus on weight estimation errors and consider the cumulative impact of other errors in weight-based drug dosing, known as compounded errors. These can occur during medication preparation and administration. Medication preparation and administration errors may match or surpass the weight estimation error. Moreover, drug concentration errors can contribute an additional 10 to 15% error. Therefore, a 20% weight estimation error, combined with a 20% administration error and a 10% concentration error, quickly becomes a potential 50% error.

Let’s use adrenaline as an example. The “low” indicates the maximum potential underdosing and the “high” the maximum potential overdosing at each step of the compounded error.

*An adrenaline/epinephrine solution must contain 90% to 115% of the labelled amount to meet United States Pharmacopeia standards (Epinephrine injection. The United States Pharmacopeia: The National Formulary. United States Pharmacopeial Convention, Rockville, MD; 2013).

So, what is the solution? We must use accurate weight estimation systems and ensure appropriate, goal-directed training in preparing and administering emergency medications. Errors should be minimised at every step of the drug dosing-delivery process.

There are three important considerations when evaluating weight estimation systems: their accuracy, usability, and ability to integrate with a drug dosing system.

Accuracy of weight estimation guidelines

Our goal regarding accuracy in weight estimation is straightforward: We want the most precise system available as long as it’s not excessively costly. However, given the increasing prevalence of childhood obesity, we now face the challenge of needing a system that can estimate both total and ideal body weights. This flexibility allows us to optimize drug dosing for each specific medication and each patient.

While there is limited data on the outcomes of incorrect dosing in obese children, there is enough evidence to raise concern. Therefore, it is crucial to establish a minimum accuracy target. 95% of weight estimates should lie within 20% of the actual weight for any system we use. The PAWPER tape, the Mercy method, and potentially parental estimates are the approaches that come closest to meeting this standard.

Usability of weight estimation guidelines

In addition to accuracy, the usability of weight estimation systems plays a crucial role. This encompasses two important aspects: ease of use and vulnerability to human and patient factor errors. A system’s usability not only affects its accuracy but also impacts its practicality. Striking a balance between usability and accuracy is essential to minimize healthcare providers’ stress during emergencies.

The ideal weight estimation system should be accurate and help reduce cognitive burden. We carefully considered the usability factor when designing the PAWPER tape. This system is designed to be quick and easy to use without adding to healthcare providers’ cognitive load. On the other hand, the Mercy method is a bit more complex and is more vulnerable to human and patient factor errors.

Finding a weight estimation system that is both accurate and user-friendly is vital in ensuring effective and efficient weight estimation, especially in time-sensitive situations. By prioritizing usability, we can enhance the usability-accuracy balance and better support healthcare providers in delivering optimal care during pediatric emergencies.

The ability of a weight estimation system to contribute to the accuracy of downstream processes needs to be considered. This refers to how it can improve the accuracy of drug dosing by how well the system integrates with a drug dosing guide. The best example (in a good way) is an App that can generate an accurate weight estimate automatically used for drug dosing calculations with limited further user input. The worst system has no integration, such as parental weight estimates or age-based formulas. Colour-coded systems and other length-based tapes with precalculated drug doses fall in the middle.

Having identified the most appropriate weight estimation system for your setting, the next step is to use it optimally. There are valid arguments about ensuring the system is maximally usable with a low cognitive load, but no system is completely cognitively neutral. However, complexity during emergencies is the reason that emergency medicine specialists exist. Our training and learning need to prepare us to practise effectively during emergencies, and the fact that treatment occurs during an emergency should not excuse a diminished quality of care. These circumstances should not excuse potentially harmful practices. 

How good are parents at guessing the weight of their child?

One study comparing a number of the traditional formulae to parental best guess found that parents were accurate, with 78% of their guesstimates being within 10% of the measured weight (COI: This study took place in my institution). It ties in with other studies examining the reliability of parental estimation. It would be interesting then to break down the data between parents to find out if there is a difference between the primary caregiver and the other parent – assuming that the primary caregiver would be more accurate. The parental weight estimation has also only been studied in low-stress, low-stakes environments. When dealing with their critically ill child, it may be hard for a parent to answer such a simple question.

There are two basic approaches. There is the age-based approach, and there is the length-based approach.

First, let’s look at some of the more widely used formulae…

The APLS formula

This formula was burnt into my brain from my first APLS course in Guildford many years ago.

Weight (kg) = (Age +4) × 2

It is difficult to confirm where this formula originates.  It has been suggested that it comes from Fanconi’s (Yes, that Fanconi) 1953 Textbook of Paediatrics. Mike Wells kindly pointed me to a 1954 paper by A.A. Weech that also visits similar ground. In his Signposts on the Highway of Growth, he suggests several aide memoires to help the student determine the average weight for a given age. The classic APLS formula was then updated in 2011 to something more complex.

For infants < 12 months: Weight (kg) = (0.5 × age in months) + 4

For children aged 1 to 5 years: Weight (kg) = (2 × age in years) + 8

For children aged 6 to 12 years: Weight (kg) = (3 × age in years) + 7

Dilshad Marikar et al. found that many of their colleagues could not recall the new ages or formulae when pressed. If Simon Carley finds this hard to remember, what chance do we mere mortals have?

The Argall Formula

Weight (kg) = (Age +2) × 3

This formula was derived from a cohort of 300 children (aged 14 months to 10 years 10 months) from Manchester, England. Validation studies have found that it was accurate (within 10% of the actual measured weight) only 37% of the time. It has been relegated to the wastebasket of time.

The Luscombe Formula

This formula was based on data from 17,244 children from Sheffield and the surrounding environs.

Weight (kg) = (3 × age) + 7

Kelly et al. declared it to be one of the more accurate methods for estimating weight, with similar performance to the more complex Best-Guess method detailed below. I would argue with the paper’s conclusion, however.

The Luscombe formula is among the more accurate age-based weight estimation formulae. When more accurate methods (e.g. parental estimation or the Broselow tape) are not available, it is an acceptable option for estimating children’s weight.

The Broselow tape is only more accurate in children under a year of age.

The Best-Guess Formula

The best guess approach is possibly the most complex, derived from the largest data set of 70,181 children who presented to the Royal Children’s Hospital in Brisbane, Australia, over a three-year period.

For infants < 12 months: Weight (kg) = (age in months +9) / 2

For children aged 1 to 5 years: Weight (kg) = 2 × (age in years + 5)

For children aged 5 to 14 years: Weight (kg) = 4 × age in years

One of the problems of all of these age-based formulae is that they rely on the reference population to derive a median. What might work for children in the UK may not work for children in Hong Kong. There is also the concern of the increasing obesity epidemic. Are the children of Manchester 20 years ago the same as those today?

They need to be easy to use, even for the mathematically challenged. If you need to access the calculator function on your phone, then the formula is useless. Perhaps it would be better to use the calculator at our fingertips.

This method is more accurate than the original APLS formula and as good as parental guesswork.

Now, let’s take a look at some of the length-based techniques

The Broselow tape

The Broselow tape was developed in 1998 from a cohort of more than 20,000 children measured during the 1979 National Centre for Health Statistics survey. To fit on the original tape, children must be between 46 and 145 cm long and 3 to 34 kg in weight. The tape assumes a mesomorphic body habitus, so it is likely to underestimate obese children and overestimate thin ones. One Hong Kong-based study found that 40% of 10-year-olds were too tall for the tape.

We use the Broselow tape to select a size band and grab the appropriately coloured kit bag. The appropriate endotracheal tube was selected using the Broselow tape based on an analysis of 205 children undergoing elective surgery.

So, how did they know if the tube was the right size? They looked at the cuff leak pressures. The tape selected the appropriate tube 77% of the time and was accurate to within 0.5mm of the ‘correct’ size 99% of the time. The traditional age-based formula of age/4 + 4 calculated the ‘correct’ tube size 47% of the time (increasing to 86% if ±0.5mm).

External validation has shown that the Broselow tape has a 15% error rate for around 80% of children, being more accurate in younger kids. While it is accurate for children under one year of age, it may be a challenge to hold down a wriggling, combative child to measure them. It is much less accurate in children over six, being able to correctly colour code the children only 48.9% of the time.

Who is Broselow?

James Broselow was an emergency physician in North Carolina. To quell the potential anxiety of looking after sick children, he came up with the idea of ‘the tape’. Using local paediatricians and aided by a nearby college that took up the cry for statistical help, he soon had raw data to group children.

With the help of Bob Luten, they created a colour-coded system so that one just had to read off the colour that the child fell on, grab the appropriate bag, and you were set.

Regional variations of the tape have been developed for specific populations, such as the Malawi tape. One criticism is that it underestimates weight, especially in obese kids, and overestimates those from low/middle-income countries.

The PAWPER tape

The Paediatric Advanced Weight-Prediction in the Emergency Room tape uses WHO data to provide weight-for-length data for a selection of body types.

Developed in South Africa, it uses a two-step approach. First, the clinicians measure the child’s length to approximate weight. They then adjust, either up or down, according to body habitus.

There should be less inaccuracy by providing options for children who appear very thin (5th centile), thin (25th centile), average (50th centile), heavy (75th centile), and obese (95th centile).

I understand that it has yet to be validated outside of South Africa, but it would be an interesting study area.

An important challenge of these length-based measures is that the child has to be physically present.  When an emergency department is notified of the imminent arrival of a critically ill child, it is much better if the receiving team have the appropriately sized equipment and appropriately dosed medication ready.

Does any of this actually matter?

Most methods of estimating a child’s weight tend to underestimate as the child ages and thus become less accurate. We also know an accurate weight is needed when calculating weight-based dosing requirements. We have very little idea what the margin of error is.

Most of these techniques have been developed to calculate a child’s actual body weight rather than their ideal body weight. Perhaps drugs that are lipophilic (hydrophobic) are more accurately dosed to actual weight? The literature has little paediatric data regarding this. A 15% discrepancy is unlikely to significantly affect the energy required for direct current counter shocks or fluid boluses.

So, you can see that all the methods used to obtain a child’s weight can be inaccurate. If possible, you should weigh the child and, if that is not possible, listen to what the parents say. The next most accurate method is to use a length-based tape adjusted to the patient’s body habitus.

Selected References

Krieser D, Nguyen K, Kerr D, Jolley D, Clooney M, Kelly AM. Parental weight estimation of their child’s weight is more accurate than other weight estimation methods for determining children’s weight in an emergency department?. Emergency Medicine Journal. 2007 Nov 1;24(11):756-9.

Lubitz DS, Seidel JS, Chameides L, Luten RC, Zaritsky AL, Campbell FW. A rapid method for estimating weight and resuscitation drug dosages from length in the pediatric age group. Annals of emergency medicine. 1988 Jun 30;17(6):576-81.

Argall JA, Wright N, Mackway-Jones K, Jackson R. A comparison of two commonly used methods of weight estimation. Archives of disease in childhood. 2003 Sep 1;88(9):789-90.

Mackway-Jones, K.Advanced paediatric life support. London: BMJ Publishing Group,1994

M. Samuels, Ed., Advanced Paediatric Life Support, the Practical Approach Wiley-Blackwell, 5th edition, 2011

Luten RC, Wears RL, Broselow J, Zaritsky A, Barnett TM, Lee T, Bailey A, Vally R, Brown R, Rosenthal B. Length-based endotracheal tube and emergency equipment in pediatrics. Annals of emergency medicine. 1992 Aug 31;21(8):900-4.

Abdel-Rahman SM, Ridge A, Kearns GL. Estimation of body weight in children in the absence of scales: a necessary measurement to insure accurate drug dosing. Archives of disease in childhood. 2014 Jun 1;99(6):570-4.

Luscombe M, Owens B. Weight estimation in resuscitation: is the current formula still valid?. Archives of disease in childhood. 2007 May 1;92(5):412-5.

Tinning K, Acworth J. Make your Best Guess: an updated method for paediatric weight estimation in emergencies. Emergency Medicine Australasia. 2007 Dec 1;19(6):528-34.

Marikar D, Varshneya K, Wahid A, Apakama O. Just too many things to remember? A survey of paediatric trainees’ recall of Advanced Paediatric Life Support (APLS) weight estimation formulae. Archives of disease in childhood. 2013 Nov 1;98(11):921

Young TP, Chen BG, Kim TY, Thorp AW, Brown L. Finger counting: an alternative method for estimating pediatric weights. The American journal of emergency medicine. 2014 Mar 31;32(3):243-7.

Cattermole GN, Leung PY, Mak PS, Graham CA, Rainer TH. Mid-arm circumference can be used to estimate children’s weights. Resuscitation. 2010 Sep 30;81(9):1105-10.

Young KD, Korotzer NC. Weight Estimation Methods in Children: A Systematic Review. Annals of emergency medicine. 2016 Apr 19.

Wells M, Coovadia A, Kramer E, Goldstein L. The PAWPER tape: a new concept tape-based device that increases the accuracy of weight estimation in children through the inclusion of a modifier based on body habitus. Resuscitation. 2013 Feb 28;84(2):227-32.

King BR, Baker MD, Braitman LE, Seidl-Friedman J, Schreiner MS. Endotracheal tube selection in children: a comparison of four methods. Annals of emergency medicine. 1993 Mar 31;22(3):530-4.

Fanconi G, Wallgren A, Collis WRF. Textbook of Paediatrics. William Heinemann Medical Books Ltd, London 1952

Georgoulas VG, Wells M. The PAWPER tape and the Mercy method outperform other methods of weight estimation in children at a public hospital in South Africa. SAMJ: South African Medical Journal. 2016 Sep;106(9):933-9.

Weech AA. Signposts on the highway of growth. AMA American journal of diseases of children. 1954 Oct 1;88(4):452-7

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