Gestational Diabetes Trio, Featuring A Special Interview with Dr. Donald Coustan

Happy Holidays to all, and to celebrate the season we have a very sweet triple episode release today! The first two episodes are focused on the pathophysiology, diagnosis, and treatment of GDM, while the third is a special interview with Dr. Donald Coustan, Professor and Chair Emeritus of the Department of Obstetrics and Gynecology at Brown University. Dr. Coustan was recently profiled by AJOG as a “Giant in Obstetrics and Gynecology.” We hope you enjoy the interview and his perspective on GDM and OB-GYN more generally.

The ACOG PB (PB 190) on GDM was recently updated in February 2018. There is also a new bulletin on Pregestational Diabetes (PB 201), though we don’t spend much time on pregestational diabetes today.

We discuss multiple ways to diagnose GDM, based on different organization’s recommendations. The classic Carpenter-Coustan criteria endorsed by ACOG and the National Diabetes Data Group (NDDG) are based on two-step testing. An initial 50 gram glucose tolerance test is performed, and patients move on to the second screen if their 1hr glucose is measured at 130-140 mg/dL, pending on the institution. It is generally accepted that a value >200 mg/dL is diagnostic without moving on to the second step.

The three hour test is based on a 100g glucose load. The cutoffs vary by time point. Two elevated values are needed to diagnose GDM; however, there is increased risk for the patient even with just one elevated value on three hour testing. The classic Carpenter-Coustan criteria as well as the NDDG criteria are shown here from PB 190:

ACOG PB 190: The Carpenter-Coustan criteria are the most commonly used in the USA.

There is also single-step testing proposed by the International Association for the Study of Diabetes in Pregnancy, that uses a 75g, two-hour glucose tolerance test. Any one elevated value (fasting > 92, 1 hour > 180, or 2 hour > 153) is diagnostic of GDM, and no second screen is needed. The ADA has endorsed these criteria recently but also admits that there is not clear-cut evidence to support one screening strategy over another. ACOG endorses the two-step screening at this time.

Much of the research regarding treatment of GDM that we review in the podcast is well-reviewed in PB 190, so we won’t rehash it here. If non-pharmacologic treatments fail (monitored fasting and postprandial blood glucose levels are consistently elevated), an oral agent or insulin is required, with insulin being the gold-standard. How do you initiate insulin? See our guide below!

And remember — postpartum patients with GDM need a 2 hour, 75 gram glucose tolerance test between 4 and 12 weeks postpartum to rule out type 2 diabetes. A fasting > 125 or a 2 hour > 200 is diagnostic. A fasting between 100-125 or a 2 hour between 140-199 demonstrates impaired glucose tolerance. And even with normal values, anyone with GDM has a 15-70% chance of developing T2DM later in life, so it’s an important part of the pregnancy history to correspond back to the patient’s PCP.