In clinical practice, an unexplained discordance between percentage haemoglobin A1c (HbA1c) and the progression of diabetes and its complication is observed. HbA1c is determined by the blood glucose level and the red blood cell (RBC) lifespan. Whether the RBC lifespan changes in diabetic patients remains undefined because of the lack of a convenient and accurate measurement method. In the present study, we aim to observe the RBC lifespan in type 2 diabetic patients with poor blood glucose control by an endogenous carbon monoxide (CO) measurement using a rapid and simplified CO breath test machine. The RBC lifespan, age, RBC count, haemoglobin, haematocrit, fasting blood glucose (FBG) level, HbA1c, blood lipids and the liver and kidney function were compared between 38 diabetic patients and 40 healthy individuals. Compared with the control group, in the diabetic patients, the RBC lifespan was significantly decreased by 17.52 ± 4.58 (86.08 ± 18.13 d versus 103.6 ± 22.02 d, p = 0.00). Although a univariate linear correlation analysis showed that the RBC lifespan was negatively correlated with the FBG level (r = -0.386, p = 0.000), haemoglobin A1c (r = -0.346, p = 0.002) and age (r = -0.291, p = 0.010), a stepwise multiple linear regression analysis showed that the RBC lifespan was most affected by the FBG level (t = -3.554, p = 0.001), but not by HbA1c or age, while HbA1c was most affected by the FBG level (t = 13.989, p = 0.000), but not the RBC lifespan. The RBC lifespan in diabetic patients with poor glycaemic control was reduced. The decrease in the RBC lifespan caused by hyperglycaemia was not associated with HbA1c. Thus, a decrease in the RBC lifespan will lead to an underestimation of the actual level of hyperglycaemia and the progression of disease by HbA1c in type 2 diabetic patients if we do not adjust the RBC lifespan.