Reply to the Letter to the Editor "Accurate Direct Measures Are Required to Validate Derived Measures"

Dear Sir,

We appreciate and agree with the concerns expressed by Dr. Pereira-da-Silva and Dr. Virella [1]. The precise method by which length was measured for each infant in the study cohort used for our growth curves [2, 3] is not reported in the Pediatrix Clinical Data Warehouse. When we surveyed our sites, almost all reported using tape measurements. We are actively involved in promoting a more accurate and uniform approach to measurements of premature infants using length boards, especially in light of newer data on the impact of linear growth on important health outcomes, e.g., neurodevelopment [4, 5].

We recognize that measurements of weight, length, and head circumference are difficult and that there were likely some errors made in making these measurements. For this reason, we excluded extreme outliers for any of the growth measures (weight, length, or head circumference), defining these as infants with values greater than 2 times the interquartile range above the 75th percentile or below the 25th percentile for each gestational age. Interestingly, including these outliers does not meaningfully change our charts or our conclusions (paper in progress). It is our hypothesis that most measurements are accurate and that the errors are uniform in direction: both over- and underestimates of the true measurement occur with the same frequency. Others share this opinion and have discussed this issue [6]. We believe the median values we report are accurate reflections of prematurely born infants.

In 2 previous publications [7, 8], we discussed the limitations of using administrative data to define what...