Impact of time since last meal on the gestational glucose challenge test. The Toronto Tri-Hospital Gestational Diabetes Project.

Objective: The purpose of the study was to evaluate the impact of time since the last meal on the glucose challenge test and to find cut points that are most likely to predict the outcome of the oral glucose tolerance test in patients screened for gestational diabetes.

Study Design: This prospective analytic cohort study was carried out at the University of Toronto Perinatal Complex. A 50 gm glucose load was given at 26 weeks' gestation and the time since previous meal ingestion was recorded. At 28 weeks' gestation a 100 gm oral glucose tolerance test was administered. A total of 4274 eligible patients were screened.

Results: Time since the last meal had a marked effect on mean plasma glucose. Receiver-operator characteristic curve analysis with National Diabetes Data Group criteria to interpret the oral glucose tolerance allowed the selection of the most efficient cut points for the glucose challenge test on the basis of time since the last meal. These cut points were 8.2, 7.9, and 8.3 mmol/L for elapsed postprandial times of < 2, 2 to 3, and > 3 hours, respectively. With this change from the current threshold of 7.8 mmol/L the number of patients with a positive screening test dropped from 18.5% to 13.7%. There was an increase in positive predictive value from 14.4% to 18.7%. The rate of patient misclassification fell from 18.0% to 13.1%.

Conclusion: We suggest that screening strategies for detection of gestational diabetes be reconsidered, to account for the impact of variable postprandial status on the test results.