Change Is Good — Evidence Appears to Support Lowering Diagnostic Threshold for GDM

May 2011 Issue

Change Is Good — Evidence Appears to Support Lowering Diagnostic Threshold for GDM
By Rita Carey Rubin, MS, RD, CDE
Today’s Dietitian
Vol. 13 No. 5 P. 10

Maternal and fetal complications of clinically diagnosed diabetes in pregnancy are well documented. Excessive fetal weight gain, fetal hyperinsulinemia, preeclampsia, and neonatal intensive care are just a few of the negative and costly outcomes associated with the disease.

While there is consensus among healthcare professionals regarding the importance of maintaining ideal glucose control in women with either gestational diabetes mellitus (GDM) or preexisting diabetes, controversy exists concerning the value of treating maternal hyperglycemia that falls below blood glucose levels now diagnostic of overt disease.

This debate received renewed attention last year when the International Association of Diabetes and Pregnancy Study Groups (IADPSG) recommended revisions to existing diagnostic criteria and screening protocols used to identify women with GDM. The revised guidelines would allow for earlier detection of maternal hyperglycemia and undiagnosed type 2 diabetes and alter the blood glucose thresholds and testing used to diagnose GDM. This year, the American Diabetes Association (ADA) revised its Standards of Medical Care in Diabetes to reflect these new recommendations.

Tune in to the IADPSG’s recommendations, the ADA’s standards of care revisions, and future implications of the new screening and diagnostic protocols.

Evidence
The IADPSG formed in 1998 to facilitate research and education about diabetes during pregnancy and enhance the quality and standardization of care in the field.1 The group met in 2008 to review the results of several published and unpublished studies examining links between maternal hyperglycemia and adverse pregnancy outcomes. Conference attendees then held additional regional meetings to consider the clinical importance of these findings.

Of the studies considered, the largest and most important was the Hyperglycemia and Adverse Pregnancy Outcome Study (HAPO). This observational study utilized data from 23,316 participants in nine countries to determine whether maternal hyperglycemia that is less severe than overt diabetes (ie, maternal glucose intolerance) negatively affects pregnancy outcomes. Researchers in the HAPO study monitored numerous negative pregnancy outcomes, including the number of large-for-gestational age births and Cesarean deliveries and the frequency of neonatal hypoglycemia, fetal hyperinsulinemia, birth injury, and preeclampsia. Resulting data indicated a positive association between levels of maternal hyperglycemia and all study outcomes, with the strongest associations occurring between levels of hyperglycemia, birth weight, and serum cord blood C-peptide levels. (Elevated C-peptide levels indicate fetal hyperinsulinemia.) In fact, with climbing maternal glucose levels, the frequency of each outcome increased, although less so for clinical neonatal hypoglycemia than for the other measures.

Recommendations
The IADPSG recommendations are based on the results of the HAPO study and other studies that conclusively demonstrated a positive, linear relationship between levels of maternal hyperglycemia and negative pregnancy outcomes. Due to the high quality of trial procedures and analysis of results in the HAPO study, the data from that study were used as the basis for the new GDM diagnostic thresholds recommended in the IADPSG report.1

The IADPSG suggests a new strategy for the detection and diagnosis of diabetes and hyperglycemic disorders in pregnancy and describes the strategy in two phases.1 The first phase deals with identifying women with overt diabetes who were not diagnosed before pregnancy. The group recommends compulsory screening for diabetes in all pregnant women or, if more economically or logistically feasible, all women in high-risk populations (those with a high prevalence of type 2 diabetes). The second phase is a 75-g oral glucose tolerance test (OGTT) performed at 24 to 28 weeks in all women not previously found to have either prepregnancy diabetes or GDM. The suggested diagnostic criteria for overt diabetes and GDM are listed in Table 1.

The recommendations described by the IADPSG differ from current screening and diagnostic protocols in several ways. A diagnosis of GDM under the IADPSG guidelines can be made with one abnormal glucose measurement—fasting plasma glucose or elevated plasma glucose following a glucose load—instead of two (the current protocol). The OGTT is standardized to a 75-g carbohydrate load (down from a 100-g dose and fasting plasma glucose concentration thresholds for a GDM diagnosis are lower. Compulsory screening earlier than the third trimester is also a deviation from the current protocol.

2011 ADA Medical Standards of Care in Diabetes
The revised standards of care are as follows:

• Screen for type 2 diabetes at the first prenatal visit in those with risk factors.

• In pregnant women without diagnosed diabetes, screen for GDM at 24 to 28 weeks of gestation, using a 75-g two-hour OGTT and (the recommended) diagnostic cut points.

• Screen women with GDM for diabetes at six to 12 weeks postpartum.

• Conduct lifelong diabetes screening at least every three years for women with a history of GDM.

These revised standards, published in the February issue of Diabetes Care, reflect concern regarding negative maternal and fetal outcomes from all levels of hyperglycemia during pregnancy and also recognize the larger implications of the ongoing diabetes epidemic in women of childbearing age. Per the ADA, “As the ongoing epidemic of obesity and diabetes has led to more type 2 diabetes in women of childbearing age, the number of pregnant women with undiagnosed type 2 diabetes has increased. Because of this, it is reasonable to screen women with risk factors for type 2 diabetes … at their initial prenatal visit, using standard diagnostic criteria. Women with diabetes found at this visit should receive a diagnosis of overt, not gestational, diabetes.”2

In other words, after considering the IADPSG findings, the ADA concluded that it is reasonable and good practice to adopt protocols that identify a greater number of pregnant women with preexisting diabetes as well as impaired carbohydrate metabolism (with glucose levels previously not indicative of GDM).

Clinical Implications
According to Boyd E. Metzger, MD, corresponding author of the IADPSG and Tom D. Spies Professor of Metabolism and Nutrition in the endocrinology, metabolism, and molecular medicine division at the Northwestern University Feinberg School of Medicine, the major clinical implication of the new screening and diagnostic protocols for GDM will be the ability to identify the risk of adverse pregnancy outcomes with one abnormal glucose value instead of two. The authors of the IADPSG paper note that these new diagnostic criteria, if universally accepted, will likely increase the frequency of hyperglycemic disorders in pregnancy. They also note that an increased incidence of GDM or glucose intolerance in pregnancy is consistent with the increased prevalence of obesity and diabetes in the general population of child-bearing adults.

Negative arguments do exist regarding the cost-effectiveness of expanded screening for and treatment of GDM, and the authors of the IADPSG paper recommend clinical studies to determine the most cost-effective way of utilizing the new criteria. In the standards of care paper, the ADA concedes that no data currently exist concerning cost-effectiveness or pregnancy outcomes in women diagnosed with GDM using the recommended revised criteria. They do note, however, that most cases of mild GDM or hyperglycemia are successfully treated with diet and lifestyle therapy alone and not more expensive interventions.

— Rita Carey Rubin, MS, RD, CDE, is a dietitian practicing in northern Arizona.

References
1. International Association of Diabetes and Pregnancy Study Groups Consensus Panel, Metzger BE, Gabbe SG, et al. International association of diabetes and pregnancy study groups recommendations on the diagnosis and classification of hyperglycemia in pregnancy. Diabetes Care. 2010;33(3):676-682.

2. American Diabetes Association. Standards of medical care in diabetes—2011. Diabetes Care. 2011;34 Suppl 1:S11-S61.

Table 1: Threshold Values for Diagnosing Gestational Diabetes Mellitus (GDM) or Overt Diabetes in Pregnancy

To Diagnose GDM
Fasting plasma glucose = 92 mg/dL or less
One-hour plasma glucose = 180 mg/dL or less
Two-hour plasma glucose = 153 mg/dL or less

To Diagnose Overt Diabetes
Fasting plasma glucose = 126 mg/dL or more
Hemoglobin A1c = 6.5% or more
Random plasma glucose = 200 mg/dL or more, plus confirmation

— Adapted from Reference 1