Do positive screen rates differ with fasting versus fed 1-hour glucose tolerance tests in second-trimester pregnancy?
The positive screen rate for the 1-hour 50-g oral glucose tolerance test was more than double among participants at greater than 24 weeks' gestation who had fasted for at least 6 hours compared with those who had eaten within 2 hours prior to testing. This study suggests, but doesn't prove, that prolonged fasting prior to 1-hour glucose tolerance testing leads to false positive results and unnecessary second-step testing. A larger study, powered to evaluate clinically meaningful differences in maternal and infant outcomes, is needed.
Randomized controlled trial (nonblinded)
Self-funded or unfunded
Outpatient (primary care)
The two-step method of oral glucose tolerance testing to screen for gestational diabetes in the second trimester of pregnancy is used nearly universally in the United States. These investigators conducted a randomized controlled trial to compare the positive screening rate for the first step in fasting versus fed conditions. They compared the results for participants who were instructed to fast for at least 6 hours before testing with the results for those who were instructed to eat normally within 2 hours prior to testing. The study included patients 18 years or older with a singleton pregnancy who were undergoing screening at 24 or more weeks' gestation. The authors excluded patients with pregestational diabetes, those given a diagnosis of diabetes in first trimester, those with an elevated 1-hour glucose tolerance test result before 24 weeks', those who used diabetes medication prior to pregnancy, those with daily oral steroid use for at least 4 weeks in the prior year, and those with history of bariatric surgery. A positive result was defined as glucose measurement of 140 mg/dL to 179 mg/dL 1 hour after a 50-g oral glucose load. Results of 180 mg/dL were considered diagnostic for gestational diabetes. Baseline characteristics were similar between groups, including age, body mass index, multiparity, and insurance type. Analysis was by intention to treat. The positive screen rate was 31/97 (32%) in the fasting group and 13/98 (13%) in the fed group (P = .002). There were no differences in maternal or neonatal outcomes between groups, though the study was not powered to detect them. The majority of participants expressed a preference of eating without restriction prior to testing (138/195 [71%]).
Linda Speer, MD
Professor and Chair, Department of Family Medicine
University of Toledo