At first glance, screening pregnant women for gestational diabetes mellitus (GDM) seems like it should be straightforward. After all, the tests are designed to identify pregnant woman with high concentrations of glucose (sugar) in their blood and laboratory tests that measure glucose are accurate and precise. So what’s the problem?
For one, experts don’t agree on how best to screen pregnant women for GDM. While nearly everyone agrees that both mom and baby can have adverse outcomes if GDM goes undetected and untreated, there is lack of consensus on the best way of identifying GDM.
Consider how it has been done for several years here in the United States using either a 1 or 2 step process. In the 2-step approach, a screening test is done first followed by a diagnostic test if the screening test is abnormal. To do the screening test, blood glucose is measured 1 hour after the non-fasting patient drinks a 50-gram dose of glucose. A glucose result that is greater than 140 mg/dL is usually used as the cutoff although a lower cutoff of 130 mg/dL is also used (again, no consensus). A woman that has an abnormal screening test (i.e. glucose concentration greater than the cutoff) will go on to have the diagnostic test. In the 1-step approach the screening test is skipped completely and only the diagnostic test is performed.
The test used to diagnose GDM is the oral glucose tolerance test (OGTT). The OGTT requires women to be fasting and then drink either a 75- or a 100-gram dose of glucose. Blood samples are collected every hour for 2 or 3 hours if using the 75- or 100-gram dose, respectively. The test is considered positive, and GDM confirmed, if 2 or more of the glucose results are above designated cutoffs (which differ depending upon the glucose dose given).
Now, new criteria have recently been advocated.
The International Association of Diabetes in Pregnancy Study Groups (IADPSG) has made recommendations for glucose tolerance testing in pregnancy based on the results of the Hyperglycemia and Adverse Outcomes (HAPO) study. That study clearly demonstrated that the risks of adverse maternal and fetal outcomes continually increase as maternal glucose concentrations increased. Importantly, the relationship between glucose concentration and risks were continuous. That is, there were no obvious glucose cutoffs above which risks increased. The new recommendations from the IADPSG address this issue.
The IADPSG advocates for the use of the 75-gram OGTT in pregnant women between 24 and 28 weeks gestation. The test is performed following an overnight fast of at least 8 hours and blood is collected at 1 and 2 hours after the glucose load. A diagnosis of GDM is made when any of the following glucose results are met:
- Fasting: greater or equal to 92 mg/dL
- 1 hour: greater or equal to 180 mg/dL
- 2 hour: greater or equal to 153 mg/dL
A couple of questions are called for here:
- Why were those cutoff selected? These are the glucose concentrations above which the adverse risks of hyperglycemia were 1.75-fold higher than for women whose glucose results were lower. Other thresholds were considered but higher cutoffs missed lots of women with adverse pregnancy outcomes and lower cutoffs identified 25% of women as having GDM.
- What is the impact will this test have on the prevalence of GDM? It will definitely increase. Currently, about 7% of pregnant women are diagnosed with GDM in the US each year. Using the IADPSG approach that will more than double to about 18% of pregnant women.
Although the American Diabetes Association adopted the IAPDSG criteria and recommends that approach to identifying women with GDM, it does recognize that there is the potential for harm. For example, more interventions such as earlier delivery and increased C-section rates are likely to occur due to the increase in the prevalence of GDM. Also, an increased number of women being diagnosed with GDM will be accompanied by a rise in health care costs. Despite those considerations, the ADA supports the new criteria in light of the increased rates of obesity and diabetes throughout the US and the world.