An HbA1c blood test checks for glycated haemoglobin (haemoglobin + sugar molecule) attached to the RBC. When sugar levels in the blood are high, more sugar molecules get attached to the RBC. The glycation process (addition of sugar molecule to haemoglobin) is an irreversible process, meaning once attached, the sugar remains there until the RBC is destroyed or removed from your body. Now the average lifespan of each RBC is around three months. Thus by measuring the glycated haemoglobin levels in the blood, a doctor would be able to tell you what your sugar levels have been in the past three months. The higher the percentage, the higher is your blood sugar level and vice versa.