Gestational diabetes: what is the relevance of the glucose challenge test?

To the Editor: The recent letter by McElduff and Hitchman1 has some very practical implications. They were able to show that pregnant women having a glucose challenge test (GCT) in the afternoon were nearly twice as likely to have a positive result as women tested in the morning, so that more women tested in the afternoon were diagnosed with gestational diabetes mellitus (GDM). If the function of the GCT is to aid in the diagnosis of GDM, then either all women should be tested in the afternoon or the glucose "cut-point" for the morning test should be reduced.

But does the GCT now have any relevance? In the United States, where testing for GDM often still involves a three-hour glucose tolerance test (GTT) using a 100 g glucose load and four blood samples, the GCT was introduced to reduce the number of women who had to have this long and, because of the higher dose of glucose, relatively unpleasant procedure. In Australia, where a two-hour, 75 g GTT is used (requiring two blood samples), it is not as important to offer a simpler initial test.

With the use of an initial GCT, about a quarter of women will need to have a GTT for confirmation, and the definitive diagnosis of GDM will be delayed. Further, the GCT is not specific and some women who may have GDM will not have a GTT. In addition, there will inevitably be some women who are GCT-positive, some of whom will have GDM, who do not return for the definitive GTT.

Thus, while a GCT may be convenient for a busy hospital clinic with space limitations, it may not necessarily be in the best interests of the patient. Whether a GCT is ultimately helpful or possibly a hindrance requires further evaluation.