Validation of the Mercy TAPE
Condition: Weight
MeSH terms: Body Weight
Study type: Observational

Summary: 
      In 'real-world' health care settings there exist a number of circumstances where the weight
      of a child is desirable or even necessary but unavailable. Numerous weight estimation
      strategies have been described but each has limitations. Investigators at Children's Mercy
      Hospitals and Clinics recently developed a weight estimation method and tool that addresses
      the limitations of previously published methods. This study is intended to validate the
      device in a population of children 2 months to 16 years of age.
    

Detailed description: 
      In 'real-world' health care settings there exist a number of circumstances where the weight
      of a child is desirable or even necessary but unavailable. The most conspicuous of these
      settings can be found in developing countries where many medical clinics lack suitable scales
      to obtain accurate infant and child weights. Though resource restrictions are less of an
      issue in developed countries, scenarios still exist where weight assessment is problematic.
      For example, accurate estimates of a child's weight are rarely available during emergency or
      trauma situations, and in some in-patient settings (e.g. critical care units, orthopedic
      clinics) obtaining an accurate patient weight can be impaired by the presence of external
      hoses, tubing, casts, and/or other medical equipment. Irrespective of the environment, the
      challenge that each of these settings present is the same; namely, the provision of
      age-appropriate, weight-based interventions which remain the most accurate approach to
      delivering therapy in children. Thus, techniques which permit accurate weight estimation
      address a critical medical need in both developing and developed countries.

      Numerous weight estimation strategies have been described with each used to varying degrees
      in clinical practice. Many of the published techniques have distinct advantages. For example;
      simple age-based equations can be used without the need for reference materials, strategies
      that utilize preprinted tables or tools limit the risk of calculation errors. Other
      techniques present unnecessary complexities for the end-user including; the need for
      subjective assessments of habitus, the requirement to solve exponential equations, the call
      for multiple formulae delineated by age bracket, or the reliance on one or more reference
      charts. Irrespective of their simplicity or complexity, almost all of the reported techniques
      have significant limitations. Relatively few methods have been evaluated in pediatric
      populations of varying races, ethnicities and nationalities and essentially no single
      previously described method provides accurate estimates of weight across broad age- and
      weight-bands.

      Apart from parental recall which can vary in accuracy, the most commonly used strategies for
      estimating weight rely on the child's age, length, or a combination of the two parameters.
      While simple and easy to integrate into a weight estimation technique, age based strategies
      fail to account for the extremes of body composition and stature that are observed in
      children of the same age. Similarly, length based strategies do not take into consideration
      that two children of the same height may demonstrate markedly discrepant weights based on
      underlying nutritional status (e.g. malnourished, underweight, overweight, obese).
      Consequently, many of the currently available weight estimation strategies perform well in
      only a small subset of children. As such, there remains a critical need for weight estimation
      methods that are accurate across a wide range of pediatric ages, weights, lengths,
      nationalities and body compositions despite the relative abundance of strategies that already
      exist.

      Investigators at Children's Mercy Hospitals and Clinics recently developed and validated a
      weight estimation method (the Mercy MethodTM) that addresses the principal limitations of
      previously published methods, requires no subjective assessment and performs robustly
      independently of age and length over a broad range of weights. As with other strategies, the
      Mercy Method incorporates growth velocity but uses humeral length as a surrogate for total
      body length. Total body length will be discrepant depending on whether the measurement is
      obtained with the child standing or lying down and can be difficult to obtain in a child who
      is uncooperative or obtunded. The Mercy Method also incorporates body habitus as a
      quantitative variable which improves the accuracy of the overall length-based weight estimate
      and removes the subjective nature of categorizing the child's body type into one of a few
      alternatives (e.g. "slim," "average," or "heavy"). By developing a model with these
      considerations in mind we were able to expand the age range to which our weight estimation
      method can be applied and remove length restrictions which are typically imposed because of
      the disproportionate increase in weight-for-height observed as children get older.

      In brief, demographic and anthropometric data on children 2 months to 16 years of age were
      extracted from the NHANES database and individual datasets were randomly assigned into a
      method development (n=17,328) or a method validation (n=1,938) set. Humeral length (HL) and
      mid-upper arm circumference (MUAC) were used to develop a weight estimation method by 1)
      collapsing length and habitus measurements into discrete bins, 2) examining the median
      population weight for each bin-pair, 3) statistically weighting the bin-pairs for age and
      sample size, and 4) calculating a fractional weight for each HL and MUAC. An individual
      weight estimate is generated by the simple addition of the MUAC and HL fractional bin value
      that corresponds to that individual child's measurements. The predictive performance this
      method was evaluated using the internal validation set and compared with the performance of
      13 previously published weight estimation methods applied to the same data.

      The Mercy Method outperformed the 13 other published methods when evaluated for
      goodness-of-fit, mean error, mean percentage error, root mean square error and percentage of
      children in agreement within 10% of actual weight. Most of the age-and length-based
      strategies examined overestimated weight in children classified, by BMI, as underweight and
      significantly underestimated weight in children classified as overweight or obese. The degree
      to which this occurred depended largely on the constants driving their mathematical
      equations, with some methods biased toward more accurate prediction in children of lower
      weight (e.g. Broselow) and others performing better among children in the higher weight
      brackets (e.g. Theron). Irrespective of directionality, the bias observed with some methods
      at the extremes of weight represented as much as a 3-fold error between predicted and actual
      weights. Discrepancies of this magnitude can be dangerous, and potentially life-threatening,
      depending on how 'forgiving' the intervention or treatment that is being administered.

      The singular habitus-based method (i.e. Cattermole) ranked among the best (after the Mercy
      Method) with respect to absolute bias; however, it performed only moderately well when
      precision and MPE were factored into the assessment. This method, which was developed in
      Chinese children consistently overestimated weight at lower absolute weights and
      underestimated weight at higher absolute weight irrespective of BMI percentile. This suggests
      that while the relationship between weight and MUAC tends to be linear within any given
      population, the mathematical constants that define the relationship differ between
      populations having different height-for-weight averages. Given the nature of the data used to
      develop and validate the Mercy Method, comparative performance of the Devised Weight
      Estimation Method (DWEM, the only other method to incorporate both body length and body
      habitus) could not be assessed. Notably, the DWEM involves a subjective rating of "slim,"
      "average," or "heavy". While DWEM has been shown to outperform other age-based methods, the
      categorical assignment of habitus coupled with inconsistencies in subjective assessment
      between and within observers [inter-rater agreement- 78% (range: 58-93%); intra-rater
      agreement- 86% (range: 81-94%)] contributed to bias and precision estimates that were larger
      than observed with strategies based solely on length.

      While the Mercy Method can be used as a reference table, a more practical application was the
      development of a simple and inexpensive device that can perform the two required measurements
      simultaneously and report the predicted weight directly from the device as opposed to
      consulting a separate table or chart. Consequently, the 3D Mercy TAPE was developed to
      perform both measurements simultaneously requiring no external references to arrive at the
      weight estimate for a given child. An alternative 2D Mercy TAPE was also designed . It
      requires two serial measurements with the same simple addition used with the 3D TAPE but does
      not require any folding or manipulation when removed from its packaging. Both devices are
      intended to be printed on any flexible, non-stretchable medium (e.g. paper, plastic coated
      paper, fiberglass) so as to be disposable or semi-permanent, inexpensive to mass produce and
      easy to store.

      In its numeric form, the Mercy TAPE would be expected have limited utility in settings where
      care providers are illiterate or do not use a written language. However, the tool can be
      easily revised with colors and/or symbols whose combination would correspond to a given dose,
      intervention strategy or weight target. While the Mercy Method is expected to perform well in
      U.S. children given its creation using data from a U.S. database, external validation of the
      in non-U.S. settings is currently ongoing with support of the World Health Organization to
      gauge its utility in children of varying ethnicity and geographic origin. The related 2D and
      3D Mercy TAPE still awaits prospective evaluation. The requisite study to satisfy the
      validation requirements are described herein under the hypothesis: The Mercy TAPE will
      demonstrate the same predictive performance as the Mercy method in an independent pediatric
      assessment.
    

Intervention name(s): Mercy TAPE
Intervention type(s): Device
Intervention description(s): 2D Mercy TAPE and 3D Mercy TAPE
Observational model: Cohort
Time perspective: Prospective
Sampling method: Probability Sample
Study population: 
        Normal healthy children
      
Keywords: children for whom there is no scale to determine weight