Objective: To test whether there is a circadian rhythm of blood glucose control in critically ill patients and whether morning blood glucose is an accurate surrogate of overall blood glucose control.
Design: Retrospective multiple-center observational study.
Setting: Intensive care units of three tertiary hospitals and one affiliated private hospital.
Patients: Cohort of 8,307 consecutive critically ill patients.
Interventions: Extraction of blood glucose values from electronically stored measurements. Extraction of demographic and outcome data from unit and hospital databases. Statistical assessment of variations in blood glucose control over each 24-hr cycle.
Measurements and main results: We studied 208,362 blood glucose measurements in 8,307 patients (5.5 measurements/day/person). In each hospital, there was a circadian rhythm of blood glucose control (p<.0001). The differences between highest and lowest blood glucose concentration in different time periods in each hospital were 0.27, 0.28, 0.95, and 0.22 mmol/L. There was also significant variation in the incidence and notional duration of hyperglycemia. The differences between the lowest and highest incidence of hyperglycemia in different time periods were 3.3, 2.7, 9.9, and 2.6% in each hospital. In all four hospitals, the average blood glucose value from 5:30 am to 6:30 am was significantly lower than the 24-hr average.
Conclusions: Blood glucose values and the incidence of hyperglycemia have a circadian rhythm in critically ill patients. Morning blood glucose may not be an accurate surrogate of blood glucose control over the daily cycle.