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      The Limitations of Transforming Very High Body Mass Indexes into z -Scores among 8.7 Million 2- to 4-Year-Old Children

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      The Journal of Pediatrics
      Elsevier BV

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          Abstract

          <div class="section"> <a class="named-anchor" id="S1"> <!-- named anchor --> </a> <h5 class="section-title" id="d6762004e166">Objective</h5> <p id="P2">To examine the associations among several BMI metrics (z-scores, percent of the 95 <sup>th</sup> percentile (%BMI <sub>p95</sub>) and ΔBMIp95 (BMI minus 95 <sup>th</sup> percentile) as calculated in the CDC growth charts. It is known that the widely-used BMI z-scores and percentiles calculated from the growth charts can differ substantially from those that directly observed in the data for BMIs above the 97 <sup>th</sup> percentile (z = 1.88). </p> <p id="P3">Study design Cross-sectional analyses of 8.7 million 2- to 4-year-olds who were examined from 2008 through 2011 in the CDC’s Pediatric Nutrition Surveillance System. </p> </div><div class="section"> <a class="named-anchor" id="S2"> <!-- named anchor --> </a> <h5 class="section-title" id="d6762004e185">Results</h5> <p id="P4">Because of the transformation used to calculate z-scores, the theoretical maximum BMIz varied by more than 3-fold across ages. This results in the conversion of very high BMIs into a narrow range of z-scores that varied by sex and age. Among children with severe obesity, levels of BMIz were only moderately correlated (r ~ 0.5) with %BMI <sub>p95</sub> and ΔBMIp95. Among these children with severe obesity, BMIz levels could differ by more than 1 SD among children who had very similar levels of BMI, %BMI <sub>p95</sub> and ΔBMIp95 due to differences in age or sex. </p> </div><div class="section"> <a class="named-anchor" id="S3"> <!-- named anchor --> </a> <h5 class="section-title" id="d6762004e196">Conclusions</h5> <p id="P5">The effective upper limit of BMIz values calculated from the CDC growth charts, which varies by sex and age, strongly influences the calculation of z-scores for children with severe obesity. Expressing these very high BMIs relative to the CDC 95th percentile, either as a difference or percentage, would be preferable to using BMI-for-age, particularly when assessing the effectiveness of interventions. </p> </div>

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          Author and article information

          Journal
          The Journal of Pediatrics
          The Journal of Pediatrics
          Elsevier BV
          00223476
          September 2017
          September 2017
          : 188
          :
          : 50-56.e1
          Article
          10.1016/j.jpeds.2017.03.039
          5572545
          28433203
          081b796b-43f6-464f-93a5-70ecd9459dc4
          © 2017

          http://www.elsevier.com/tdm/userlicense/1.0/

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