A 20-year-old male with a confirmed diagnosis of type 1 diabetes mellitus (T1DM) for 3 years has been on long-term insulin therapy. Recently, he has experienced large fluctuations in blood glucose, with frequent hypoglycemic episodes (lowest: 1.8 mmol/L) and hyperglycemia (highest: 31.0 mmol/L), requiring repeated emergency visits. To precisely and rationally adjust insulin dosage, which laboratory test is essential?