Diurnal variation in fasting plasma glucose: implications for diagnosis of diabetes in patients examined in the afternoon.

CONTEXT Current diagnostic criteria for diabetes are based on plasma glucose levels in blood samples obtained in the morning after an overnight fast, with a value of 7.0 mmol/L (126 mg/dL) or more indicating diabetes. However, many patients are seen by their physicians in the afternoon. Because plasma glucose levels are higher in the morning, it is unclear whether these diagnostic criteria can be applied to patients who are tested for diabetes in the afternoon. OBJECTIVES To document diurnal variation in fasting plasma glucose levels in adults not known to have diabetes, and to examine the applicability to afternoon-examined patients of the current diagnostic criteria for diabetes. DESIGN, SETTING, AND PARTICIPANTS Analysis of data from the US population-based Third National Health and Nutrition Examination Survey (1988-1994) on participants aged 20 years or older who had no previously diagnosed diabetes, who were randomly assigned to morning (n = 6483) or afternoon (n = 6399) examinations, and who fasted prior to blood sampling. MAIN OUTCOME MEASURES Fasting plasma glucose levels in morning vs afternoon-examined participants; diabetes diagnostic value for afternoon-examined participants. RESULTS The morning and afternoon groups did not differ in age, body mass index, waist-to-hip ratio, physical activity index, glycosylated hemoglobin level, and other factors. Mean (SD) fasting plasma glucose levels were higher in the morning group (5.41 [0.01] mmol/L [97.4 ¿0.3¿ mg/dL]) than in the afternoon group (5.12 [0.02] mmol/L [92.4 ¿0.4¿ mg/dL]; P<.001). Consequently, prevalence of afternoon-examined participants with fasting plasma glucose levels of 7.0 mmol/L (126 mg/dL) or greater was half that of participants examined in the morning. The diagnostic fasting plasma glucose value for afternoon-examined participants that resulted in the same prevalence of diabetes found in morning-examined participants was 6.33 mmol/L (114 mg/dL) or greater. CONCLUSIONS Our results indicate that if current diabetes diagnostic criteria are applied to patients seen in the afternoon, approximately half of all cases of undiagnosed diabetes in these patients will be missed.