ACCURACY OF NELSON AND BEST GUESS FORMULAE IN ESTIMATION OF WEIGHTS IN NIGERIAN CHILDREN POPULATION

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A.O. Omisanjo¹, A.E. Orimadegun² and F.O. Akinbami³

1. Department of Paediatrics, University College Hospital, Ibadan, Nigeria.
2. Institute of Child Health, College of Medicine, University of Ibadan, Ibadan, Nigeria.
3. Dept. of Paediatrics and Child Health, Niger Delta University Teaching hospital, Bayelsa State, Nigeria.

Abstract

Background: An alternative method of estimating children’s weights, when direct weighing is impracticable is the use of age-based formulae but these formulae have not been validated in Nigeria. This study compares estimated weights from two commonly used formulae against actual weights of healthy children.

Methods: Children aged 1 month to 11 years (n= 2754) were randomly selected in Ibadan, Nigeria using a two-stage sampling procedure. Weight of each child, measured using a standard calibrated scale and determined using Nelson and Best Guess formulae, were compared. Demographic characteristics were also obtained. Mean percentage error (MPE) was calculated and stratified by gender and age. Bland-Altman graphs were used for visual assessment of the agreement between estimated and measured weights. Clinically acceptable MPE was defined as ±5%. Descriptive statistics and paired t test were used to examine the data. Statistical level of significance was set at p = 0.05.

Results: There were 1349 males and 1405 females. Nelson and Best Guess formulae overestimated weight by 10.11% (95% CI: -20.44, 40.65) in infants. For 1-5 years group, Nelson formula marginally underestimated weight by -0.59% (95% CI: -5.16, 3.96) while it overestimated weight by 9.87% (95% CI: 24.89, 44.63) in 6-11 years. Best Guess formulae consistently overestimated weight in all age groups with the MPE ranging from 10.11 to 30.67%.

Conclusion: Nelson and Best Guess formulae are inaccurate for weight estimations in infants and children aged 6-11 years. Development of new formulae or modifications should be considered for use in the Nigerian children population.

Keywords: Measured weight, Best Guess formula, Nelson formula, Mean percentage error

Introduction

Determination of the weight of a child is an essential part of paediatric practice whether in the emergency unit, ward or clinic setting. The weight is an important element in making a number of diagnostic and treatment decisions including nutritional status assessment, drug doses, sizes of equipment, use of treatment normogram, fluid therapy and energy levels for defibrillation. Also, weight determination is a major component of growth monitoring and it is critical to the institution of most preventive child health interventions included in the child survival strategies.^1,2 The most accurate method of determining a child’s weight is to weigh the child on a standard machine with calibrated scales. However, this may not sometimes be practicable. For instance, when resuscitating a critically ill child. In such situations, the child healthcare providers instead may ask the caregivers for the child’s weight or estimate the weights based on their experience or use other means like employing age-based formulae. Asking parents for the weight of their child may seem more feasible but it is less reliable in emergency setting than trying to weigh a critically ill child.^3-5 A previous study showed that weight estimation by parents, physicians and nurses were similarly unreliable even in the United States of America where literacy rate is higher than in Africa.⁵ Conversely, some authors showed that Australian and Israeli parents’ estimate of a child’s weight were quite accurate.^3,4

In Nigeria, anecdotal observations showed that, healthcare providers in most clinics and hospitals use formulae for quick estimation of children’s weight whenever weighing is considered time-wasting or child is too ill to be moved around for such a procedure. Sometimes a weighing scale may not even be available in rural areas and estimating the child’s weight remains the only feasible option for getting the weight. Where weighing is impracticable or there is no weighing scale, the relevance of knowing a child’s expected weight and the urgency with which paediatricians and other child healthcare providers estimate weight in their practice underscore the need to get it done accurately and as quickly as possible. Then, a proven alternative to use of weighing scale is the use of formulae for estimation of weights in paediatric practice.⁶

Some of the commonly used age-based formulae include: the Nelson formulae,^7,8 Advanced Paediatric Life Support (APLS) formula,⁹ Best Guess formulae,¹⁰ Argall formula ¹¹ and Luscombe formulae.¹² Many studies have shown that the accuracy of different methods of weight estimation vary amongst different populations.^11,13-16 Many of the formulae for weight estimation were not only derived in the western paediatric populations, but thereafter they were subjected to validation^12,16-19 locally before their use in those countries. Despite the wide use of Nelson and Best Guess formulae for estimation of weight in Nigerian children, data on their validations are sparse in the African population. Only in a few African countries, namely; South Africa,²⁰ Malawi²¹ and Kenya²² were studies carried out to evaluate the accuracy of formulae used in children.

Accurate and reliable means of weight estimation in children is vital. For instance, inaccurate weight estimation may increase the likelihood of drug adverse events and toxicity.²³ This underscores the need to evaluate the methods by which weight estimation is performed with or without modifications to the existing formulae in Nigerian children. This study was carried out to assess the accuracy of Nelson and Best Guess formulae in use for weight estimation. The main question to be answered was “do age-based Nelson and Best Guess formulae accurately estimate the weight of Nigerian children?”