Contributions of Overnight Glycemia to the Overall Hyperglycemia of Type 2 Diabetic Patients Variations With HbA1c Levels

HbA1c is considered the gold standard on which to base monitoring and treatment decisions in T2DM; nonetheless, it has several limitations, among which is that it represents mainly mean glycemia exposure overtime, whereas optimal diabetes management involves control of fasting, preprandial, and postprandial glucose levels. Thus, an elevated HbA1c measurement signals a need for a change in therapy, but it cannot indicate the type of change. Evidence points to the importance of postprandial hyperglycemia contribution on both macro and microvascular diabetic complications. The availability of a continuous glucose monitoring system (CGMS) offers better opportunity for diabetic patients to manage this condition. Several published studies have aimed at assessing the correlation between HbA1c and glycemia levels at several times of the day. Most of these studies have focused on fasting and preprandial and postprandial glucose levels. The contribution of specific time of the day in T2DM patients to the overall glycemic control has never been studied. Our study is the first investigating the contribution of overnight glycemia to HbA1c levels using CGMS technology. We present a retrospective study on available CGMS data from T2DM patients. CGMS was performed on 84 patients (50 males and 34 females). Patients were divided to 3 subgroups depending on levels of HbA1c. Group 1 patients had an HbA1c level ≤ 8% (mean age, 58 ± 6 years). Group 2 patients had an HbA1c level between 8% and 10% (mean age, 59 ± 8 years), and group 3 patients had an HbA1c level ≥ 10% (mean age, 56 ± 8 years). Duration of diabetes was documented as well as the number of hypoglycemic events defined as episodes of glucose < 50 mg/dl. Weight, BMI, and type of treatment were recorded. Mean glucose levels at different times of the day/night were collected from the data provided by CGMS and categorized into 6 hours periods. Plotting the data (Figure 1) showed that the mean overnight glucose tends to correlate with the extent of diurnal hyperglycemia thus showing a greater contribution of the overnight glycemia to overall daytime glycemia in T2DM patients. In group 1, overnight glucose was significantly lower than the morning, afternoon, and evening glucose. In group 2, mean glucose levels during the morning, afternoon, evening, and overnight were all similar (P < .05). In group 3, mean overnight glucose was significantly higher than mean morning, afternoon, and evening glucose. As a conclusion, we found that overnight glycemia correlated best with both mean daytime glycemia and HbA1c levels, whereas the mean nighttime glycemia was the highest, with the highest HbA1c levels. Figure 1. (A) Group 1 patients with an HBA1C 10%; mean ... This finding is in accordance with the Bonora et al study1 as well as the Borg et al study2 and the Hillman et al study,3 where mean preprandial glucose correlated best with HbA1c when compared with postprandial glucose. Although our study is limited by the small number of patients, it sheds light on the limitations of HbA1c as a sole indicator of diabetes and shows the importance of daily glucose and CGMS in treatment monitoring. Larger studies are needed to better investigate the association between daily glucose and HbA1c.