Exact timing of the one-hour glucose sample as a factor in the screen for gestational diabetes

We examined the exactness of the timing of the 1-hour glucose sample following a 50-g oral glucose challenge as a critical variable in interpretation of the test. Heparin locks were placed in 45 pregnant patients between 25 and 28 weeks' gestation, and 5 patients between 30 and 33 weeks' gestation. Venous samples were taken at intervals of 50, 60, and 70 minutes from completion of the ingestion of a 50-g oral glucose load. We found all of the nine possible patterns of blood glucose values that can derive from three sequential values. There was no consistent relationship between these 60 +/- 10 minute values. Two of the patients had a 60-minute value greater than 140 mg/dL, but a 50- or 70-minute value that was less than 140 mg/dL. Four of the patients had a 50- or 70-minute value that was greater than 140 mg/dL, but a 60-minute value that was less than 140 mg/mL. The range of results in this study reflects a continuum of values that change rapidly over time and in patterns that are not predictable. We conclude that accurate timing is important to avoid erroneous interpretation of the 1-hour glucose screen.