Factors That Mediate Racial/Ethnic Disparities in US Fetal Death Rates
Scott A. Lorch, MD, MSCE, Charlan D. Kroelinger, PhD, Corinne Ahlberg, MS, and Wanda D. Barfield, MD, MPH
What is the primary discipline of the authors?
The first author is a pediatrician, the others are in the field of reproductive health.
Draw a DAG representing the implicit or explicit causal model explored in this paper (you do not need to post your DAG, but we will try to discuss in class).
See figure one in the paper ;)
What is the exposure of interest?
Race/ethnicity
What is the outcome of interest?
Risk of fetal death
What is the hypothesized mediator of interest and how is it measured?
SES: maternal insurance status, education, trimester started prenatal care, age.
Pre-existing morbid conditions: ICD-9 codes listed on discharge records
Anetpartum/Intrapartum conditions: ICD-9 codes
Fetal factors: ICD-9 codes
Delivery hospital: Only included as a mediator when the hospital accounted for more than 15% of change in effect. Otherwise was treated as confounder.
Describe the modeling approach and briefly report the estimated total, direct, and indirect effects (if these are reported).
They used Baron and Kenny’s framework of:
- Race/ethnicity is associated with fetal death risk
- Race/ethnicity is associated with a set of potential mediating factors
- A set of potential mediating factors are associated with the risk of fetal death
- Including both race/ethnicity and the set of mediating factors changes the association between fetal death and racial ethnic group observed in #1
The measured the unadjusted association for #1 and #2 above using logistic regression and then used chi2 test to assess the association between race/ethnicity and each set of potential mediating factors. To test #3 and #4 the use sequential logistic regression models and added each group of mediating factors in the temporal order that they appear in pregnancy. For #4 they reported the percentage of the fetal death disparity in each of the mediated factors.
Unadjusted total effect: Blacks OR 2.24, Hispanics OR 1.37, Asians, 1.18 (reference, whites)
Couldn’t find the direct effect? It may be that when the added SES, AP/IP complications, and fetal factors the entire direct effect for black race disappeared (OR 1.04 95%CI 0.95-1.14). Delivery hospital did not further change the OR. So, I think they are saying that after adding these mediating factors, there is no direct effect of race/ethnicity on risk of fetal death.
Indirect effects: Blacks 49% due to fetal factors; Hispanics 35.8% due to SES factors; Asians 62% due to antepartum and intrapartum factors.
If the direct effect is reported, would you describe this as a natural direct effect, a controlled direct effect, or something else?
Not reported as a direct effect, or there appears to be no direct effect left after the mediating factors are all added.
Do you think there is potential measurement error in the mediator and how would that affect the results?
There could be the potential for measurement error whenever using birth certificate data. It may be possible that ICD-9 coding for cases of fetal death is more intensive, so more complications are listed because these cases “need explanation” compared to cases that did not have a fetal death (misclassification of the mediators). Per Vander-Weele, measurement error and misclassification of the mediator will weaken the relationship between the mediator and the outcome. If this is true, then the investigators could have underestimate the relationship between the mediators and the outcomes in this case.
Do you think there are unmeasured confounders of the mediator-outcome association and how would that affect the results of the mediation analysis?
They did not conduct a sensitivity analysis to investigate unmeasured confounding, so I do believe there could be unmeasured confounding. I am especially interested with how they treated SES as a mediator in this situation and not a confounder, given the known confounding of race with maternal insurance status, maternal education, and starting trimester of prenatal care.
Do you have any critiques of the paper?
The concern I have with this paper relates to Vander-Weele’s discussion of multiple mediators, who says that you cannot use this “summed multiple mediator” method used by these investigators when the mediators affect one another, or if there are interaction between the effects of the mediators on the outcomes. Vander-Weele says you can use regression techniques (which these authors did use) to examine an entire set of mediators. The set of mediators here seem very likely to affect each other, for instance pre-existing high blood pressure can increase the chance of intrapartum complications. I am not sure how adequately the authors dealt with mediator-to-mediator interactions.