Diabetes Laboratory Testing Guidelines: From Blood Glucose Tests to Glycated Hemoglobin

Blood glucose testing is an essential step in diagnosing diabetes. Clinically, blood glucose testing can be divided into three methods: random blood glucose testing, fasting blood glucose testing, and oral glucose tolerance testing (OGTT). ① Random blood glucose testing: This is the most convenient method, unaffected by time of day, and can be performed at any time. If the blood glucose level is >11.1 mmol/L, coupled with symptoms of diabetes, a diagnosis of diabetes can be suspected. ② Fasting blood glucose testing: Fasting refers to at least 8 hours without calorie intake. A fasting blood glucose level ≥7.0 mmol/L can be diagnosed as diabetes. ③ Oral glucose tolerance test (OGTT): This requires the patient to drink 75g of glucose powder after fasting blood glucose testing, and then check the changes in blood glucose levels at 30 minutes, 1 hour, and 2 hours. Based on the fasting and 2-hour blood glucose levels, it can be determined whether the subject has diabetes. In OGTT, a 2-hour postprandial blood glucose level ≥11.1 mmol/L is the diagnostic criterion. A 1-hour postprandial blood glucose level cannot be used as a diagnostic criterion, but if the blood glucose level at this time is >10 mmol/L, it also indicates a potential for developing diabetes. Glycated hemoglobin (HbA1c) reflects the average blood glucose level over a certain period. HbA1c is not a precise blood glucose value, but rather represents the percentage of total hemoglobin composed of glycated hemoglobin. HbA1c is the product of the combination of hemoglobin and blood glucose. The binding process of blood glucose and hemoglobin is slow and irreversible, existing until red blood cells undergo apoptosis. Red blood cells have a lifespan of 120 days, with an average of 60 days. Therefore, HbA1c reflects the average blood glucose level over the previous 1-2 months and is unaffected by random factors. Other tests for understanding average blood glucose levels include glycated serum albumin (GA) and 1,5-acid-glucanol (1,5-AG). Glycated serum albumin reflects the patient's average blood glucose level over the past 1-2 weeks. Reference range: 1.5–2.4 mmol/L. 1,5-Acid-glucanol levels help understand the average blood glucose level over the past few days. Reference range: 14.0 μg/ml (lower limit of normal). Pancreatic function and autoantibody tests include the insulin release test (IRT) and the C-peptide release test (CPRT). Measuring the levels of insulin and C-peptide secretion at various time points, both fasting and postprandial, can reveal the degree of pancreatic islet dysfunction. Pancreatic beta cell autoantibody tests include glutamate decarboxylase antibody (GADA), insulin autoantibody (IAA), and islet cell antibody (ICA). GADA appears early in the blood and lasts the longest, making it clinically significant. Urinalysis includes multiple indicators such as urine glucose, urine ketones, urine protein, and white blood cells. A positive urine glucose test does not necessarily indicate diabetes. Glucose can only be excreted in urine when blood glucose exceeds 8.9 mmol/L. Normal urine ketones are negative. In urine protein, a microalbumin excretion rate >20–200 μg/min in diabetic patients is a diagnostic indicator of early diabetic nephropathy. Blood ketones, blood lactate, and blood osmolality are mainly used to diagnose acute complications of diabetes. In fundus examination, diabetic retinopathy fundus changes include hemorrhage, microaneurysms, and exudative macules.