The Association between Hba1c- Derived Estimated Average Glucose (Eag) with Fasting Blood Sugar (FBS) and Post Prandial Blood Sugar (PPBS) in Patients with Type 2 Diabetes in A Cohort of Patients in a Tertiary Care Hospital in Sri Lanka

Introduction: Type 2 Diabetes mellitus (T2DM) is a chronic illness, caused due to resistance to insulin or poor production. FBS, PPBS and Glycated Haemoglobin (HbA1c) are the major tests that are used to monitor chronic glycemia worldwide. HbA1c level remains the gold standard test for assessment of glycemic control at follow up and it reflects the mean glucose values in the previous three - month period. HbA1c is expressed as a percentage, whereas the day-to-day monitoring and treatment of diabetes are based on blood glucose levels expressed as milligrams per deciliter (mg/dl) or millimoles per liter (mmol/L). Due to the difference in the denominators of FBS/PPBS with HbA1c, it causes confusion to the patients regards their glycaemic control. “Estimated average glucose” or eAG derived from HbA1c has been promoted by the American Diabetes Association (ADA). American Association of Clinical Chemists concludes that the correlation (r =0.92) is strong enough to justify reporting both HbA1c and eAG which indicate the 3- month control of the average sugar of the patient. This is easy for the patient as both FBS/PPBS and eAG are expressed with the same denominator for daily glucose checks and long-term control respectively, enhancing the glycaemic control.

Objectives: To determine the statistical correlation between eAG derived from HBA1C using the Nathan’s regression equation with FBS and PPBS in patients with T2DM. To analyze the significance of eAG as opposed to HbA1C as a marker of long-term glycemic control in T2DM.

Methodology: A retrospective analytical study done at the Department of Haematology of the Sri Jayewardenepura General Hospital, Sri Lanka. A simple random sampling technique was used, over a period of one year commencing June 2019 to June 2020, to obtain the laboratory records through laboratory information system of 201 adult patients both males and females who were diagnosed with T2DM. HbA1c, FBS, PPBS done on the same day were recorded. The eAG(mg/dl) was calculated using the Nathan’s regression equation (eAG = 28.7 x HbA1c – 46.7). Three groups were generated according to the patients’ levels of FBS and PPBS.

Statistical Analysis: Data were double entered and analyzed using (SPSS) version 20. Descriptive statistical methods were used to calculate the median, mean standard deviation of age, HBA1c, FBS, and PPBS. Correlations between study variables were done with Pearson’s correlation method. The p value, lower than 0.05 was considered as statistically significant. Coefficient of determination (R Sq) was used to a statistical measure of how close the data are to the fitted regression line.

Results: The total population of patients in both the FBS and PPBS groups showed a significant statistical correlation with eAG. The individual subgroups did not conform to this correlation. The results reveal no statistical correlation in both FBS and PPBS with eAG in patients with good glycaemic control. There was a significant statistical correlation in both FBS and PPBS with eAG in the groups of patients with moderately poor control. In those with markedly poor control the FBS did not show a statistical correlation with eAG, as opposed to the PPBS.

Conclusion: The clinical importance of HbA1C/eAG in diagnosis and management of T2DM can be re-emphasized by this study. HbA1C along with eAG may be added as a test in the management of T2DM, for the better understanding and maintenance of good glycemic control. As eAG values, derived by the Nathan’s regression equation in our study scattered very close to the regression line, it provided a more complete and representative measure of average glucose in past 3 months. We conclude that HbA1c values were reliably translated into eAG measurements which would be easily understood by the patients being of the same denominator as the FBS/PPBS.