Measurement of glucose in multiple Field Laboratories requires rigorous standardization when patients and caregivers are masked, unless predefined thresholds are met. Local misclassification of participants at the thresholds can introduce recruitment bias and adversely affect the integrity of study findings. To describe the challenges and the approach to meeting them in measuring glucose, HbA1c, and C-peptide in the Hyperglycemia and Adverse Pregnancy Outcome (HAPO) Study. HAPO is an observational epidemiologic study of 25 000 pregnant women from 15 centres in 10 countries, designed to clarify unanswered questions on associations of maternal glycemia, less severe than overt diabetes mellitus, with risks of adverse pregnancy outcome. Glucose tolerance (75 g two-hour OGTT) is assessed locally at 24-32 weeks' gestation, with results masked if fasting and two-hour plasma glucose are