The group deliberated how much risk to address, or cover, with new cut points. Is a 150% increase in risk, for example, too much? Or a doubling of newborn fatness? In looking at a possible lifetime of obesity, type 2 diabetes, and heart disease, how much testing and treatment is just right? In the end, the group chose cutoff points for the fasting, 1-hour, and 2-hour plasma glucose measurements that conveyed an odds ratio for adverse outcomes of at least 1.75.
This means that a fasting plasma glucose of 92 mg/dL or more almost doubles the adverse fetal outcome risk; so does a 1-hour value after the 75-g OGTT of at least 180 mg/dL, and a 2-hour value of at least 153 mg/dL. If any one of these values is elevated, according to the IADPSG, a fetus is at risk and the mother should be treated for hyperglycemia (Diabetes Care 2010;33:676–82).
The Near Future
With the new criteria proposed by the IADPSG, the number of women who will be defined as having GDM using the 75-g OGTT will double to approximately 16%, compared with about 8% today. This doubling of incidence obviously will require additional resources and intervention.
The question now is, Are we going to adopt these new criteria? The practice approach for GDM in the United States normally follows guidelines for diabetes put forth by the American Diabetes Association and/or guidelines for pregnancy developed by the American College of Obstetricians and Gynecologists. Although the ADA has revised its recommendations for diagnosis of GDM to embrace the criteria of IADPSG, neither body has issued a directive or a formal set of guidelines for clinicians.
The National Institute of Child Health and Human Development is planning a workshop on GDM for next year, and it is quite possible that the proceedings from this NICHD workshop will inform future statements or guidelines from these organizations. In all likelihood, new screening criteria will be widely adopted within several years.
In the meantime, providers must decide what to do. There is nothing wrong with continuing two-stage testing. However, those who do should realistically consider its disadvantages: For one, this process identifies only 80%–90% of the women who actually have abnormal glucose levels, so many at-risk newborns will be missed even though their mothers were tested.
Secondly, the timing of the two-step process is problematic. Most women are given lab orders for the OGTT at about 28 weeks' gestation. By 29 or 30 weeks, they'll have results. If abnormal, the office staff must call and tell the patient to schedule the second OGTT test. Our own studies have shown that each step takes about 7–12 days to complete. In our system, it can then take up to 10 days for a woman diagnosed with GDM to receive care. She will be instructed in glucose monitoring and her care team will check with her every week.
In the end, it may be 6–8 weeks after initial testing before the woman's glucose intolerance is effectively addressed. The maximal time of fetal fat accretion is at about 34 weeks. If we do not have a diagnosis made and treatment plan underway by 32 weeks, we will have significantly decreased our chance of preventing obesity in her newborn.
Aggressive efforts to get screening done at about 26 weeks would be worthwhile, especially if you are working within a system that can accommodate a greater number of women with identified glucose intolerance. To ensure the outcomes that we're seeking, we must ensure that our patients receive adequate dietary and other interventions.
There also are questions about whether the identification of more women at risk of an adverse pregnancy outcome could itself create risk, particularly since it is well documented that women with GDM are more likely to be delivered earlier or through cesarean section, regardless of the level of achieved glucose control. (In the Landon study, interestingly, the rate of cesarean delivery was reduced in the intervention group.)
On the other hand, wider identification offers such hope for reducing fetal adiposity, and its many adverse consequences, that it should be immediately considered.
'Old' (Current) vs. 'New' (Upcoming)
Two-Step Approach to GDM Dx:
▸ Initial screening with a 50-g glucose challenge test at 24–28 weeks' gestation in women at greater than low risk of GDM. Women at very high risk should be screened as soon as possible after confirmation of pregnancy.
▸ Diagnostic 100-g oral glucose tolerance test (on separate day, after overnight fast) in women who meet or exceed chosen threshold on 50-g screening (140 mg/dL or more, or 130 mg/dL or more for higher sensitivity).