Gestational diabetes mellitus: why screen and how to diagnose

Dear Editor,

We appreciate the interest of Sampanis and collaborators in our editorial¹. Gestational diabetes mellitus (GDM) is indeed a significant health issue, associated with increased perinatal morbidity. Diagnosis of GDM has always been an issue of controversy due to the use of different diagnostic criteria and methodology. Hence, the proposal by the International Association of Diabetes and Pregnancy Study Groups (IADPSG) of a uniform set of simple diagnostic criteria, if adopted, could simplify diagnosis and provide a common ground for GDM for healthcare professionals around the world.

The main feature of the newly proposed IADPSG criteria, which clearly differentiates them from previous criteria, like those proposed by the American Diabetes Association (ADA), is the diagnostic strategy rationale. Current ADA diagnostic criteria² are based on the later risk of developing type 2 diabetes in the mother after gestation, as proposed in 1964 by O' Sullivan and Mahan and later modified by Carpenter and Coustan, during a 100 g oral glucose tolerance test (OGTT). Moreover, the ADA criteria could be used with a 75 g OGTT, however "... this test is not as well validated as the 100 g OGTT"². The IADPSG criteria introduce a shift in the diagnostic strategy by suggesting cut-off values that correspond to an odds ratio for adverse pregnancy outcomes of at least 1.75 compared with women with the mean glucose levels during a 75 g OGTT³, as demonstrated in the HAPO study⁴.

Moreover, however minimal the difference between the IADPSG and ADA cut-off values may seem, using the value of 95 mg/dl for fasting plasma glucose (FPG) as an alternative to the 92 mg/dl, could indeed result in a substantial decrease in the percentage of women diagnosed with GDM³. Thus, the IADPSG diagnostic strategy can indeed increase the prevalence of GDM. Today, there is mounting evidence that treating even mild GDM reduces morbidity for both mother and baby⁵. The American Diabetes Association is therefore "...working with U.S. obstetrical organizations to consider adoption of the IADPSG diagnostic criteria and to discuss the implications of this change"².

When applying the IADPSG criteria on the HAPO cohort, the majority of women diagnosed with GDM were identified by the FPG and the 1-hour measurement of the 75 g OGTT (8.3% and 5.7% of the entire study population, respectively)³. Adding the 2-hour measurement identified another 2.1%. Thus, alternative diagnostic strategies such as measuring FPG alone or omitting the 2-hour blood draw can indeed be appealing. It would thus be interesting to verify their cost-effectiveness in comparison with the unmodified IADPSG diagnostic recommendations in a large prospective multicentre trial.