New Recommendations for Screening for GDM
Every year, the American Diabetes Association publishes Clinical Practice Recommendations. Typically, some recommendations remain unchanged; sometimes older recommendations disappear (like the one for urine glucose testing); and sometimes there are significant changes to the recommendations from the previous year.
This year, the most significant change is a change in the recommendation on how pregnant women without diabetes are tested for the possibility of having gestational diabetes mellitus (GDM).
The 2011 version suggests that physicians screen pregnant women as follows: "Screen for undiagnosed type 2 diabetes at the first prenatal visit in those with risk factors, using standard diagnostic criteria. (The risk factors are spelled out: severe obesity; prior history of GDM or delivery of large-for-gestational-age infant; presence of urine sugar; diagnosis of polycystic ovary syndrome; or family history of type 2 diabetes.) In pregnant women not known to have diabetes, screen for GDM at 24-28 weeks of gestation, using a 75-g 2-h OGTT... Screen women with GDM for persistent diabetes 6-12 weeks postpartum. Women with a history of GDM should have lifelong screening for the development of diabetes or prediabetes at least every 3 years."
The 2010 version had merely advised physicians to "Screen for GDM using risk factor analysis and, if appropriate, an OGTT. Women with GDM should be screened for diabetes 6-12 weeks postpartum and should be followed up with subsequent screening for the development of diabetes or pre-diabetes."
A comparison of the recommendations from 2010 and 2011 shows the changes:
1) Women should be screened for undiagnosed diabetes at their first prenatal visit. Actually, although not mentioned in the 2011 recommendations, it would be even better to screen for diabetes before conception: if a woman has undiagnosed diabetes, it would be best to become aware of it and institute optimal treatment before conception rather than while pregnant.
2) A glucose tolerance test is now a standard recommendation for all women without diabetes, rather than leaving the decision about doing the GTT to local discretion. And explicit instructions on how much glucose, when to do it, and what values are abnormal are defined. Here's where the biggest problem in the new recommendations shows up: the criterion for diagnosing GDM have changed, and will result in more women being diagnosed with GDM. And more women being screened almost guarantees that more women will be found to have GDM, even if the criteria didn't change.
3) Long-term follow-up of women with GDM is more clearly advised, with diabetes screening at least every three years. This makes sense, as many women with GDM get "lost to follow-up" -- they wrongly assume that if they haven't continued with diabetes after delivery, they are "off-the-hook" and won't develop diabetes later -- but indeed they may. GDM is a risk factor for future diabetes, and the revised wording of this recommendation now appropriately emphasizes the need for regular follow-up indefinitely.
The ADA admits that the new recommendations "will significantly increase the prevalence of GDM, primarily because only one abnormal value, not two, is sufficient to make the diagnosis." The chair of the ADA's Professional Practice Committee claims that "the new definition will enable us to ward off preventable complications in both mother and child." But the problem is that there's not much data supporting the logic that identifying more pregnant women with mild GDM will change outcomes -- especially if the care provided to the woman with mild GDM is suboptimal. What level of care should be provided? It's unclear, as there is little data from clinical trials regarding therapeutic interventions in women who will now be diagnosed with GDM, and expected benefits to their pregnancies and offspring is inferred. Additionally, the frequency of follow-up and the frequency of blood glucose monitoring to be recommended is not yet clear, but is likely to be less intensive than for women diagnosed by the previous criteria.
But it is clear that additional studies will be necessary to determine the optimal intensity of monitoring and the optimal treatment of women with GDM who are diagnosed by the new criteria (that would not have met the prior definition of GDM). Until then, the new recommendations make partial sense: find the cases of T2DM (or perhaps T1DM) that were present at time of conception, and screen every women with a GTT. On the other hand, it's inevitable that these new recommendations will also mean that the medical community will have to cope with an increased number of women diagnosed with GDM, without having a clear picture of how to treat the women who are diagnosed solely by the new GTT criteria.