Investigation of Fasting and Postprandial C-Peptide Levels and Their Correlation with HbA1c in Type 2 Diabetes Mellitus

Background: Type 2 Diabetes Mellitus (T2DM) is a progressive metabolic disorder characterized by insulin resistance and β-cell dysfunction. C-peptide, a marker of endogenous insulin secretion, plays a crucial role in assessing β-cell function. This study evaluates the correlation between fasting and postprandial C-peptide levels with glycemic markers in T2DM patients to better understand β-cell function and its relationship with glycemic control. Methods: A cross-sectional study was conducted in the Department of Biochemistry, Patna Medical College, Bihar, India, with 86 T2DM patients. Fasting and postprandial plasma glucose, C-peptide levels, and HbA1c were measured. Correlation analysis was performed between C-peptide levels and glycemic parameters using Pearson’s correlation coefficient. Results: The mean age of participants was 52.3 ± 10.4 years, with a male-to-female ratio of 1.2:1. The mean fasting and postprandial plasma glucose levels were 152.6 ± 28.4 mg/dL and 225.7 ± 35.2 mg/dL, respectively, while the mean HbA1c was 8.3 ± 1.5%. A significant negative correlation was observed between fasting C-peptide and HbA1c (r = -0.42, p < 0.001) and postprandial C-peptide and HbA1c (r = -0.48, p < 0.001), indicating declining β-cell function with worsening glycemia. Conclusion: C-peptide levels negatively correlate with HbA1c and plasma glucose levels, highlighting the progressive decline in β-cell function in T2DM. These findings suggest that C-peptide assessment can serve as a valuable tool in evaluating β-cell reserve and optimizing treatment strategies. Further, longitudinal studies are warranted to explore its prognostic significance.