胎儿停育危险因素研究中的 Immortal time bias：以妊娠期糖尿病为例。
Immortal time bias in the study of stillbirth risk factors: the example of gestational diabetes.
机构信息
From the aUniversity of British Columbia, Department of Obstetrics & Gynaecology, Vancouver, Canada; and bUniversity of British Columbia, School of Population and Public Health, Vancouver, Canada.
出版信息
Epidemiology. 2013 Nov;24(6):787-90. doi: 10.1097/EDE.0b013e3182a6d9aa.
BACKGROUND
Current understanding of the increased risk for stillbirth in gestational diabetes mellitus is often based on large cohort studies in which the risk of stillbirth in women with this disease is compared with the risk in women without. However, such studies could be susceptible to immortal time bias because, although many cohorts begin at 20 weeks' gestation, pregnancies must "survive" until 24-28 weeks in order to be screened and diagnosed with gestational diabetes.
METHODS
We describe the theoretical potential for immortal time bias in studies of stillbirth and gestational diabetes and then quantify the magnitude of the bias using 2006 United States vital statistics data.
RESULTS
Although gestational diabetes was protective against stillbirth when including all births (relative risk = 0.88 [95% confidence interval = 0.79-0.99]), restricting analyses to births at >28 weeks' gestation reversed the effect and diabetes became associated with an increased risk of stillbirth (1.25 [1.11-1.41]).
CONCLUSION
Immortal time before diagnosis of gestational diabetes may bias our understanding of the stillbirth risk associated with this condition.
背景
目前对于妊娠期糖尿病患者死产风险增加的理解通常基于大型队列研究，其中将患有这种疾病的女性的死产风险与没有这种疾病的女性的风险进行比较。然而，这些研究可能容易受到不朽时间偏倚的影响，因为尽管许多队列在 20 周妊娠时开始，但妊娠必须“存活”到 24-28 周才能进行筛查和诊断为妊娠期糖尿病。
方法
我们描述了不朽时间偏倚在死产和妊娠期糖尿病研究中的理论可能性，然后使用 2006 年美国生命统计数据量化了偏倚的幅度。
结果
尽管包括所有分娩在内，妊娠期糖尿病对死产有保护作用（相对风险=0.88[95%置信区间=0.79-0.99]），但将分析仅限于 28 周以上的分娩会逆转这种效果，并且糖尿病与死产风险增加相关（1.25[1.11-1.41]）。
结论
在诊断为妊娠期糖尿病之前的不朽时间可能会影响我们对与这种情况相关的死产风险的理解。
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