Wednesday, September 12, 2007

MacDorman's feeble response to continued criticism

I have written before about the MacDorman paper, Infant and Neonatal Mortality for Primary Cesarean and Vaginal Births to Women with "No Indicated Risk",United States, 1998–2001 Birth Cohorts, MacDorman, et al., Birth,Volume 33 Page 175 - September 2006. MacDorman's conclusion that C-section leads to an increased risk of neonatal death is unsupported by the data and actually disingenuous. (Disingenuous and Why try to trick people?)

Simply put, MacDorman et al. compared outcomes of C-sections with "no indicated risk" (a blank space on the risk section of the birth certificate) with outcomes from vaginal deliveries with "no indicated risk" and found that the neonatal death rate was higher in the C-section group. However, MacDorman neglected to mention that it is well known that the risk section of the birth certificate is often left blank even when there are serious risk factors and complications. Indeed, in 50% or more of serious risk factors (heart disease, kidney disease, etc.) the space is left blank. So their assumption that "no indicated risk" means no risk is completely unjustified.

In the current issue of Birth, fully a year after the original paper was published, MacDorman and al. are still attempting to respond to serious criticism of the paper. The most recent critical letter involves the presentation of EVEN MORE data that shows that MacDorman's decision to claim that "no indicated risk" means no risk is completely insupportable. MacDorman persists in replying with yet another disingenuous claim:
Although underreporting of medical risk factors in
the United States data has been documented, we do not find evidence of a selectively greater level of underreporting for cesarean deliveries in the U.S. data. Table 1 presents data for all singleton, term U.S. births from 1999-2001 with no prior cesarean (over 8 million births)... Reported rates of pregnancy-associated hypertension, chronic hypertension, eclampsia, and all medical risk factors and complications of labor and/or delivery combined were at least twice as high for primary cesarean as for vaginal deliveries. A comparison to medical records data would provide stronger evidence, but it is unavailable; however, the high rates of reported medical risk factors and complications in the primary cesarean data do not support a hypothesis of selective underreporting for primary cesarean deliveries.
This reply is disingenuous for two reasons:

First, no one is claiming that there has been selective underreporting of complications in primary cesarean data. Equivalent underreporting both of C-section and vaginal delivery groups has a disparate impact. Imagine that both groups are underreported by 50%. However each group does not have the same number of complicated cases. If the C-section group had 500 complicated cases, and the vaginal delivery group had 20 complicated cases, underreporting by 50% would mean that 250 complicated cases would erroneously make it into the "no indicated risk" C-section group, while only 10 complicated cases would erroneously be included in the vaginal delivery group.

Second, the number of women in the C-section group was only 311,927. If only 358 records were erroneously categorized as no risk, the mortality rates would have been exactly the same in both groups. In other words, if only 0.1% of the C-section records were erroneously categorized, it could entirely account for the difference in mortality rates. The entire number of deaths in the C-section group constitutes only 0.17% of the group. It is possible that every single one of those deaths is mischaracterized and that the neonatal mortality rate in the C-section group was actually far lower than in the vaginal delivery group.

The MacDorman study is junk. It shows nothing and it is very clear why it shows nothing. MacDorman et al. deliberately relied on data that they knew to be seriously flawed.

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