The role of plasma glucose level as a confounding factor with a significant effect on the strong relationship between serum DPP4 activity and mortality in COVID-19 might, in theory, be due to very different pathophysiologic explanations – for example, an altered metabolic dysregulation in the DPP4–incretin axis or alternatively a viral inflammation-related increase in insulin resistance with subsequent hyperglycemia [38] that simply occurs in parallel with a reduction in circulating DPP4 activity. This evidence concerns the gene DPP4 and COVID-19.