The one hour oral glucose challenge test [GCT] is a screening test for gestational diabetes that measures plasma or serum glucose concentration 1 hour after a 50-g oral glucose load.  A blood sugar value >140 mg/dl (7.8 mmol/l) identifies approximately 80% of women with gestational diabetes (GDM),  a cutoff of >130 mg/dl (7.2 mmol/l) identifies 90% of women with GDM.

The California Diabetes and Pregnancy Program recommends that women with values 140 mg/dl to 179 mg/dl should have  a 3-hour 100 gram oral glucose tolerance test (OGTT) within one week.  If the patient has a screening value > 180 mg/dl, a fasting blood sugar should be checked as soon as possible. If the fasting is > 95 and the one hour screening test > 180 mg/dl this provides two abnormal values and the 3 hour OGTT is not required.  If the fasting value is less than 95 then proceed with the 3 hour OGTT.

A glucose challenge test is not necessary in patients with a fasting plasma glucose level >126 mg/dl (7.0 mmol/l) or a casual plasma glucose >200 mg/dl (11.1 mmol/l) since these values meet the threshold for the diagnosis of diabetes.

References:

1.  Guidelines for Care, California Diabetes and Pregnancy Program,2002
2. American Diabetes Association.Diagnosis and classification of diabetes mellitus. Diabetes Care. 2009 Jan;32 Suppl 1:S62-7. PMID:19118289