Before we can do anything to 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 take a look at some of the methods we use if we cannot put the child on the scales.
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 that look at the reliability of parental estimation. It would be interesting then to breakdown the data between parents to find out if there is a difference between primary caregiver and the other parent – the assumption being that the primary caregiver would be more accurate. The parental estimation of weight 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 off, let’s take a look at some of the more widely used formulae…
The APLS formula
This is the formula that is burnt into my brain from my very 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 a number of aides 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 a large number of their colleagues were unable to recall the new ages or formulae when pressed. If Simon Carley finds this hard to remember what chance do us 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 estimation of weight with similar performance to the more complex Best-Guess method detailed below. I would argue with the conclusion of the paper, 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
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, the Best-Guess approach is possibly the most complex.
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 those of us that are 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 that we all have at our fingertips.
This method has been shown to be more accurate than the original APLS formula and as good as parental guess-work.
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. Children need to be between 46 and 145 cm in length and 3 to 34 kg in weight to fit on the original tape. It assumes a mesomorphic body habitus and so is likely to underestimate obese children and overestimate the 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. Selection of the appropriate endotracheal tube using the Broselow tape was based on 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 8o% of children, being more accurate in younger kids. Whilst it is accurate in children under one year of age it may be a challenge to hold down a wriggling, combative child in order to measure them. It is much less accurate in children in the over 6 age group, 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. In order 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 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 of its criticisms 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 measures the length of the child to get an approximation of weight. They then adjust, either up or down, according to body habitus. By providing options for children that appear very thin (5th centile), thin (25th centile), average (50th centile), heavy (75th centile) and obese (95th centile) there should be less inaccuracy. It is my understanding that it has yet to be validated outside of South Africa but it would be an interesting area of study.
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 gets older and thus become less accurate.We also know that an accurate weight is needed when calculating weight-based dosing requirements. We just have very little idea what the margin of error is.
Most of these techniques have been developed to work out a child’s actual body weight rather than their ideal body weight. Perhaps those drugs that are lipophilic (hydrophobic) are more accurately dosed to actual weight? There is little paediatric data regarding this in the literature. A 15% discrepancy is also unlikely to make much of a difference when it comes to the energy required for direct current counter shocks or fluid boluses.
So, you can see that all of the methods used to obtain a child’s weight can be inaccurate. If at all possible you should weight 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 that is adjusted to the patients body habitus.
There is a great review of this in a recent edition of Annals of Emergency Medicine.
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