March Jaume, Trujillano Javier, Tor Mercé, Sorribas Albert
Department de Ciències Médiques Bàsiques, Universitat de Lleida, Av. Rovira Roure, 44, 25198-Lleida, Spain.
Growth Dev Aging. 2003 Fall;67(2):59-72.
Reference intervals are a fundamental tool for characterizing the health status in a given population and play a central role in defining diagnostic values in clinical applications. Estimation of the conditional distribution of a variable, as the body mass index (BMI), is necessary for providing reference values when there is a trend as a function of the covariate.
We studied 1453 boys and young between 5 and 16 years old measured in a study carried out in the schools of Lleida (Spain). BMI conditional distributions with age have been derived using a new parametric method based on the one proposed by Sorribas et al. [Stat. Med. (2000) 19:697-713]. This method is based on S-distributions as a parametric model for the distribution and uses maximum likelihood estimation of the conditional distribution.
The methods commonly used for estimating reference curves are based on a smoothing of sample quantiles using different techniques. However, these methods do not provide information on the conditional distribution of the target variable. Our method provides an estimation of such distribution and the corresponding reference curves for the quantiles as a function of a covariate, in our case age.
The suggested methodology provides appropriate reference quantiles for the BMI. Our results allow characterizing the change in distribution within the age range considered. Besides describing a raise in BMI with age, we observe an increase in dispersion around puberty. This must be considered when using BMI as a diagnostic variable.
参考区间是描述特定人群健康状况的基本工具，在临床应用中确定诊断值方面发挥着核心作用。当变量（如体重指数（BMI））随协变量呈现趋势时，估计其条件分布对于提供参考值是必要的。
我们研究了在西班牙莱里达学校开展的一项研究中测量的1453名5至16岁的男孩和青少年。基于索里巴斯等人提出的方法[《统计医学》（2000年）19:697 - 713]，使用一种新的参数方法得出了BMI随年龄的条件分布。该方法基于S分布作为分布的参数模型，并使用条件分布的最大似然估计。
常用的估计参考曲线的方法是基于使用不同技术对样本分位数进行平滑处理。然而，这些方法并未提供目标变量条件分布的信息。我们的方法提供了这种分布的估计以及作为协变量（在我们的案例中为年龄）函数的分位数的相应参考曲线。
所建议的方法为BMI提供了合适的参考分位数。我们的结果能够描述在所考虑年龄范围内分布的变化。除了描述BMI随年龄升高外，我们还观察到青春期前后离散度增加。在将BMI用作诊断变量时必须考虑这一点。
