Is HbA_1c Influenced More Strongly by Preprandial or Postprandial Glycemia in Type 1 Diabetes?

The relative contribution of preprandial and postprandial glycemia to HbA_1c has been the subject of increased interest in recent years; however, according to the American Diabetes Association, there are insufficient data to determine this accurately (1).

The objective of our study was to evaluate the accuracy of preprandial and postprandial glycemia in predicting overall glycemic control as estimated by HbA_1c in type 1 diabetes.

We analyzed 112 consecutive home blood glucose records from type 1 diabetic patients who were on an intensive diabetes therapy program and who had had diabetes for >5 years. Each home blood glucose record included the blood glucose values performed before and 2 h after breakfast, lunch, and dinner during a period of 6 weeks (mean number of glucose measures performed: 237). HbA_1c was measured at the end of each record period by high performance liquid chromatography. Home blood glucose monitoring was performed with the same glucose meters in all patients (One Touch Profile; Lifescan, Milipitas, CA).

Mean daily glycemia, mean preprandial glycemia, and mean postprandial glycemia values were obtained from each record. Records were classified in two groups, according to HbA_1c results: good glycemic control (HbA_1c <7%) and poor glycemic control (HbA_1c ≥7%). Regression lines for HbA_1c and each glycemic value were obtained. We calculated the expected values of glycemia (mean daily glucose, mean preprandial glucose, and mean postprandial glucose) for HbA_1c levels of 7% according to the corresponding regression line. We then used these values to make additional classifications of the records within the good and poor glycemic control groups. Sensitivity, specificity, and positive predictive value of mean daily glycemia, mean preprandial glycemia, and mean postprandial glycemia were calculated.

The study results showed that good glycemic control records (HbA_1c <7%) had significantly lower glycemic values than poor glycemic control records (HbA_1c ≥7%): mean daily glycemia 143.1 ± 14.8 mg/dl vs. 157.5 ± 15.4 mg/dl (P = 0.000), mean preprandial glycemia 136.2 ± 17.4 mg/dl vs. 152.8 ± 16.6 mg/dl (P = 0.000), and mean postprandial glycemia 151.9 ± 18.7 mg/dl vs. 162.8 ± 23.8 mg/dl (P = 0.008), respectively. Significant linear correlations were found between HbA_1c and mean daily glycemia (r = 0.635, P = 0.000), mean preprandial glycemia (r = 0.631, P = 0.000), and mean postprandial glycemia (r = 0.415, P = 0.000). Mean daily glycemia showed the best sensitivity (77%), specificity (71%), and positive predictive value (76%) in predicting good glycemic control. Mean preprandial glycemia had better sensitivity (74%), specificity (67%), and positive predictive value (73%) than mean postprandial glycemia (67, 61, and 67%, respectively).

The results of this study suggest that preprandial glycemia is a better predictor of overall glycemic control, as estimated by HbA_1c, than postprandial glycemia in type 1 diabetes.

Our data are in agreement with those recently reported in this journal by Bonora et al. (2) in type 2 diabetes and with a recent review of the available data on postprandial glucose by the American Diabetes Association (1). A higher contribution of preprandrial than postprandial glycemia to mean daily glycemia, due to the fact that more hours are spent in the interprandial period than in the postprandial one, could probably explain these findings. In fact, it was the mean daily glycemia that correlated the most strongly with HbA_1c in both the study by Bonora et al. and our study as well as in most of the available studies analyzed by the American Diabetes Association (1,2).

If postprandial glycemia is confirmed as an independent contributing factor in cardiovascular disease (3), the fact that preprandial glycemia is a better predictor of HbA_1c than postprandial glycemia could suggest that HbA_1c may not exactly reflect the harmful effects of hyperglycemia in diabetes macrovascular complications. This could explain why the Diabetes Control and Complications Trial (DCCT) and the U.K. Prospective Diabetes Study (UKPDS) showed a significant improvement in glycemic control (as estimated by HbA_1c) in the intensive therapy groups but did not definitely demonstrate a risk reduction in macrovascular complications.