With great interest, we read the study by Wilcox et. al. “The limits of small‐for‐gestational‐age as a high‐risk category” [1], which showed that small‐for‐gestational age (SGA) was a poor marker for predicting neonatal mortality and cerebral palsy, whilst gestational age performed well. Following the same selection criteria, we used birth certificate data of Aus‐ trian newborns between 1984 and 2019 (n = ~ 2,920,000) (provided by Statistics Austria) and found an AUC value of 85.3 (95% CI: 84.7–85.9) for gestational age as a predictor of neonatal mortality. This estimate perfectly mirrors the ROC values reported in Wilcox et al. analyzing data of the US and Norwegian births. In addition to confirming the findings reported in Wilcox et al. [1] we would like to draw attention to two additional points as follows: In the discussion outlined in Wilcox et al. [1], authors used the term “birthweight” several times; a few examples are “The discriminating power of birthweight is even fur‐ ther reduced in preterm weeks”, “This suggests that ROC analyses of birthweight for virtually any other birth out‐ comes are likely to show even worse prediction,” and “It may seem incongruous that birthweight so poorly discrimi‐ nates neonatal mortality and cerebral palsy”. We believe the “birthweight” mentioned in the discussion stands for SGA because their method sections explicitly noted that birthweight percentiles (i.e., SGA) were analyzed. Although birthweight was not analyzed in Wilcox et al., we were able to conduct this analysis in our data based on Austrian births. We examined the effect of birthweight (in grams) on neo‐ natal mortality and calculated the corresponding AUC and maximum of the Youden index. We found an AUC value of 87.2 (95% CI: 86.7,87.7) for birthweight as a predictor of neonatal mortality (see Online Resource 1), which was even numerically higher than that for gestational age. In the “History of SGA” section, Wilcox et al. described the difference between birthweight (specifically low birth‐ weight that defines babies < 2500 g) and SGA, and that SGA was preferred to using low birthweight as a binary variable. Interestingly, in our analysis using birthweight as a continu‐ ous variable in lieu of gestational age the maximum of the Youden index turned out to be at a cutoff of 2240 g, which is close to the cutoff of 2500 g to define low birthweight. Dif‐ ferent criteria can be used to identify optimal cut‐offs to cre‐ ate subgroups using the Youden index. Wilcox et al. use the maximum of the Youden index as a criterion to group births into pre‐and full‐term deliveries [2]. Using this criterion in our analysis, we observed a cutoff of < 36 weeks which is similar as in Wilcox et al. [1]. Another well‐known criterion is to use the Youden index with minimal distance to the top‐ left corner of the ROC curve. Following this criterion, our data showed a shift of the optimal cutoff from < 36 weeks to < 37 weeks and from 2240 to 2435 g for defining pre‐term delivery. The cutoff value of < 37 weeks for preterm delivery with the highest Youden index found in the study by Wil‐ cox et al. as well our cutoff surprisingly coincides well with the often‐used “accumulated clinical judgment”. Without a doubt, grouping newborns into pre‐and full‐term deliveries by gestational age, SGA, or exact birthweight makes sense in a clinical setting in practical terms. Nevertheless, using a cutoff to create a binary variable for gestational age as the outcome (dependent variable) or exposure (independent variable) in an analysis may lead to arbitrary effect estimates * Thomas Waldhoer thomas.waldhoer@meduniwien.ac.at

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回复：小于胎龄儿作为高风险类别的限制

我们怀着极大的兴趣阅读了 Wilcox 等人的研究。人。“小于胎龄儿作为高风险类别的界限”[1]，这表明小于胎龄儿 (SGA) 是预测新生儿死亡率和脑瘫的不良标志物，而胎龄则用于预测新生儿死亡率和脑性瘫痪。出色地。按照相同的选择标准，我们使用了 1984 年至 2019 年间奥地利新生儿的出生证明数据（n = ~ 2,920,000）（由奥地利统计局提供），发现妊娠期的 AUC 值为 85.3（95% CI：84.7-85.9）。年龄作为新生儿死亡率的预测指标。这一估计完美地反映了 Wilcox 等人报告的 ROC 值。分析美国和挪威的出生数据。除了确认 Wilcox 等人报道的发现之外。[1] 我们想提请注意以下两点：在 Wilcox 等人概述的讨论中。[1]，作者多次使用“出生体重”一词；一些例子是“出生体重的判别力在早产周内进一步降低”、“这表明 ROC 分析对几乎任何其他出生结果的出生体重可能会显示出更差的预测”和“它可能出生体重很难区分新生儿死亡率和脑瘫，这似乎是不协调的”。我们认为讨论中提到的“出生体重”代表 SGA，因为他们的方法部分明确指出分析了出生体重百分位数（即 SGA）。虽然 Wilcox 等人没有分析出生体重，但我们能够在基于奥地利出生的数据中进行这项分析。我们检查了出生体重（以克为单位）对新生儿死亡率的影响，并计算了相应的 AUC 和约登指数的最大值。我们发现出生体重的 AUC 值为 87.2（95% CI：86.7,87.7）作为新生儿死亡率的预测因子（参见在线资源 1），在数值上甚至高于胎龄。在“SGA 的历史”部分，Wilcox 等人。描述了出生体重（特别是定义婴儿 < 2500 克的低出生体重）和 SGA 之间的差异，并且 SGA 优于使用低出生体重作为二元变量。有趣的是，在我们使用出生体重作为连续变量而不是胎龄的分析中，Youden 指数的最大值原来是在 2240 g 的临界值，这接近定义低出生体重的临界值 2500 g。可以使用不同的标准来确定最佳截止值，以使用约登指数创建子组。威尔科克斯等人。使用 Youden 指数的最大值作为标准，将分娩分为足月分娩和足月分娩 [2]。在我们的分析中使用这个标准，我们观察到小于 36 周的截止时间，这与 Wilcox 等人的相似。[1]。另一个众所周知的标准是使用距离 ROC 曲线左上角最小的约登指数。按照这一标准，我们的数据显示，用于定义早产的最佳截断值从 < 36 周变为 < 37 周，从 2240 g 变为 2435 g。Wilcox 等人的研究中发现，约登指数最高的早产截止值 < 37 周。同样，我们的截止值与经常使用的“累积临床判断”非常吻合。毫无疑问，在临床环境中，按胎龄、SGA 或确切出生体重将新生儿分为足月分娩和足月分娩是有意义的。然而，在分析中使用截止值创建一个二元变量作为结果（因变量）或暴露（自变量）可能会导致任意效应估计 * Thomas Waldhoer thomas.waldhoer@meduniwien.ac.at