Equipment As with any routine clinical assessment, using the correct equipment is important. This is certainly true for the assessment of growth where one of the biggest sources of error is broken, incorrectly calibrated, or out of date equipment. The other source of errors is poor measuring technique.
Although in specialist clinics very accurate equipment such as the Harpenden stadiometer is used, it requires regular calibration and maintenance. There are other types of stadiometer, including electronic ones which once installed accurately do not require recalibration and are suitable for routine use in clinics, wards, and surgeries. There is also relatively cheap portable equipment such as the Leicester height measure which, when used with the correct measurement technique can give an accurate height reading.
Weighing scales are often poorly maintained and calibrated and do not give an accurate readout across the whole range. Children’s weight is best measured with correctly calibrated and zeroed class III electronic scales. Head circumference is best measured with a specially designed measure, such as a metal anthropometric tape or the plastic Lasso- o (Figure 1). If that is not available then a paper tape folded lengthways to reduce its width can be used.
Fig1. Recommended approach to measurement of supine length, weight, and occipito- frontal head circumference. Reproduced with kind permission from RCPCH and Harlow Printing Limited. Copyright 2019 © Royal College of Paediatrics and Child Health.
Measuring Technique— Length and Height
It is now recommended that length should be measured in children up to the age of two years. The correct technique requires two people— one to hold the infant’s head against the headboard with the eyes facing forwards and in the Frankfurt plane (the outer canthus of the eye in the same plane as the upper margin of the pinna). The other observer straightens the legs having removed the nappy beforehand and holds the legs extended and brings the board against both heels and the length is read to the nearest 0.1 cm (Figure 1). Height should be measured in children over 2 years of age, using the appropriate equipment with the child standing as tall as possible, back touching the wall or backplate of the stadiometer, feet together, ankles together, and heels placed against the foot restraint. The head is placed in the Frankfurt plane and the measuring board is brought down on the top of the head (Figure 2). Shoes need to be removed and hair arrangements that prevent the measuring arm resting on the top of the head should be undone. The practice of stretching, putting upward pressure under the mastoid processes while the child takes a deep breath and then exhales is sometimes recommended to attempt to compensate for the up to 2 cm loss in height that occurs gradually throughout the day on account of spinal compression (diurnal variation). The difficulty here is that interobserver variations in stretching are difficult to control for, but the key to accuracy is that consistency of technique is applied, whether stretching is used or not. Instruction in measurement techniques and videos are available on the UK Royal College of Paediatrics and Child Health website.
Fig2. Recommended approach to measuring standing height indicating the Frankfurt plane, and weight. Reproduced with kind permission from RCPCH and Harlow Printing Limited. Copyright 2019 © Royal College of Paediatrics and Child Health
Reliability and Reproducibility
With appropriately calibrated equipment the reproducibility of the measurement of length or height is usually within 0.5 cm. This variability is known as the measurement error. With a trained auxologist using a calibrated stadiometer this can be lower than 0.2 cm. Training is therefore vital and the recognition of measurement error is important as this is magnified when height velocity is calculated from two height values. For this reason, height velocity measurements at intervals of less than one year are likely to contain a greater proportion of measurement error. Furthermore, height velocity reference charts are only valid for calculation of the growth increment over a whole year. This also has the benefit that it cancels out any seasonal variation in growth which is well described, the timing of peaks and troughs in velocity varying between individuals.
Timing of Measurements
There is considerable debate as to what are the optimal ages for children to be measured. Weight, head circumference, and ideally length should be measured at birth. The infant should be weighed again within the first two weeks to assess postnatal weight loss. After that, it is recommended that infants are weighed at 2, 3, and 4 months coinciding with the UK vaccination schedule and subsequently at all other points of interaction with healthcare services. It is recommended that length is measured when required and certainly whenever there is concern about weight gain. However, it is good practice for an infant to have their length measured within the first year, then height in the toddler years and then also preschool to assess for any growth deviation. The UK National Child Measuring Programme includes height and weight estimation at school entry at age 4– 5 years and on primary school exit at age 10– 11 years. This was initiated as a childhood obesity surveillance scheme, and not primarily for screening for individual growth abnormalities.
