HbA1c measures how much sugar has attached to haemoglobin in your red blood cells, which reflects average glucose over roughly two to three months rather than a single moment. It is reported as a percentage (DCCT units) or in mmol/mol (IFCC units), depending on your country.
It is widely used both to diagnose diabetes and prediabetes and to track how well treatment is working over time. Because it gives an average, it complements but does not replace day-to-day finger-prick or CGM checks, which show short-term swings and hypos.
Certain conditions, such as anaemia, recent blood loss, pregnancy, or some haemoglobin variants, can make HbA1c less reliable, so results are interpreted in context. Discuss your specific target and how often to test with your clinician, since goals vary by age, health, and risk of lows.