Am J Epidemiol. 2014 Jan 1;179(1):4-11. doi: 10.1093/aje/kwt228. Epub 2013 Oct 31.
To estimate the possible direct effect of birth weight on blood pressure, it is conventional to condition on the mediator, current weight. Such conditioning can induce bias. Our aim was to assess the potential biasing effect of U, an unmeasured common cause of current weight and blood pressure, on the estimate of the controlled direct effect of birth weight on blood pressure, with the help of sensitivity analyses. We used data from a school-based study conducted in Switzerland in 2005-2006 (n = 3,762; mean age = 12.3 years). A small negative association was observed between birth weight and systolic blood pressure (linear regression coefficient βbw = -0.3 mmHg/kg, 95% confidence interval: -0.9, 0.3). The association was strengthened upon adjustment for current weight (βbw|C = -1.5 mmHg/kg, 95% confidence interval: -2.1, -0.9). Sensitivity analyses revealed that the negative conditional association was explained by U only if U was relatively strongly associated with blood pressure and if there was a large difference in the prevalence of U between low-birth weight and normal-birth weight children. This weakens the hypothesis that the negative relationship between birth weight and blood pressure arises only from collider-stratification bias induced by conditioning on current weight.
为了估计出生体重对血压的可能直接影响，通常需要对当前体重这个中介因素进行条件分析。这种条件分析可能会导致偏差。我们的目的是通过敏感性分析来评估未测量的共同原因 U（当前体重和血压的共同原因）对出生体重对血压的控制直接效应估计值的潜在偏差影响。我们使用了 2005-2006 年在瑞士进行的一项基于学校的研究的数据（n=3762；平均年龄为 12.3 岁）。观察到出生体重与收缩压之间存在微弱的负相关关系（线性回归系数βbw=-0.3mmHg/kg，95%置信区间：-0.9，0.3）。在调整当前体重后，这种关联得到了加强（βbw|C=-1.5mmHg/kg，95%置信区间：-2.1，-0.9）。敏感性分析表明，只有在 U 与血压有较强的关联且在低出生体重和正常出生体重儿童之间 U 的患病率存在较大差异的情况下，U 才能解释负条件关联。这削弱了出生体重与血压之间的负相关关系仅源于对当前体重进行条件分析而产生的混杂分层偏差的假设。
