Research Article: Z-score differences based on cross-sectional growth charts do not reflect the growth rate of very low birth weight infants

Date Published: May 7, 2019

Publisher: Public Library of Science

Author(s): Niels Rochow, Erin Landau-Crangle, Hon Yiu So, Anna Pelc, Gerhard Fusch, Jan Däbritz, Wolfgang Göpel, Christoph Fusch, Harald Ehrhardt.


To test whether the assessment of growth in very low birth weight infants during the hospital stay using z-score differences (Zdiff) is confounded by gestational age (GA), birth weight percentiles (BW%ile), and length of the observation period (LOP). We hypothesize that Zdiff calculated from growth charts based on birth weight data introduces a systematic statistical error leading to falsely classified growth as restricted in infants growing similarly to the 50th percentile.

This observational study included 6,926 VLBW infants from the German Neonatal Network (2009 to 2015). Inclusion criterion was discharge between 37 and 41 weeks postmenstrual age. For each infant, Zdiff, weight gain velocity, and reference growth rate (50th percentile Fenton) from birth to discharge were calculated. To account for gestational age dependent growth rates, assessment of growth was standardized calculating the weight gain ratio (WGR) = weight gain velocity/reference growth rate. The primary outcome is the variation of the Zdiff-to-WGR relationship.

Zdiff and WGR showed a weak agreement with a Zdiff of -0.74 (-1.03, -0.37) at the reference growth rate of the 50th percentile (WGR = 1). A significant proportion (n = 1,585; 23%) of infants with negative Zdiff had weight gain velocity above the 50th percentile’s growth rate. Zdiff to WGR relation was significantly affected by the interaction of GA x BW%ile x LOP.

This study supports the hypothesis that Zdiff, which are calculated using birth weights, are confounded by skewed reference data and can lead to misinterpretation of growth rates. New concepts like individualized growth trajectories may have the potential to overcome this limitation.

Partial Text

Changes in percentiles or z-scores during defined observation periods, such as from birth to discharge, are commonly used to assess growth of preterm infants. Tools presently applied in clinical routine for monitoring and guiding growth include growth charts and z-score plots (Fig 1).[1–3]

This observational study was performed using a data set of very low birth weight (VLBW, birth weight < 1,500 g) infants from the German Neonatal Network (2009 to 2015). All VLBW infants that were discharged between 37 to 41 weeks were included in this study. The study was approved by the Hamilton Integrated Research Ethics Board (HiREB) (2016-1696-C). The data were anonymized, and ethics committee waived the requirement for informed consent for this retrospective study. This study was comprised of 6,926 (male n = 3,461; 50%) VLBW infants. The patient characteristics are outlined in Table 1. In this study, we found that z-score differences (Zdiff) and weight gain velocity from birth to discharge do not correlate well in a significant portion of in VLBW infants. This finding supports the hypothesis that the approach of using Zdiff to assess growth in preterm infants is confounded by skewed reference data and thus, does not provide an accurate reflection of growth. Gestational age, birth weight percentile, and the length of the observation period have significant effects on the Zdiff. The influence of these factors is supported by statistically significant mathematical findings. Thus, an unadjusted Zdiff cannot be accurately translated into growth rates or growth trajectories.   Source:


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