HbA1c

When you eat, glucose enters your blood and a little of it sticks to hemoglobin, the protein inside your red blood cells. The more sugar around over time, the more of your hemoglobin gets coated. The test reads that coated fraction.

Because red blood cells live about three to four months, the result reflects your typical glucose across that window, like a built-in logbook. The same blood result is reported two ways: the older percent scale (NGSP/DCCT) and the international mmol/mol scale (IFCC). They are the same number on different rulers, so the mmol/mol figure always looks larger and does not convert digit-for-digit.

Your HbA1c tells you how your blood sugar has been running, often years before you would feel anything, and it responds well to change. The prediabetes band (5.7-6.4%, or 39-47 mmol/mol) is an early heads-up, and it is also the window where diet, movement, and weight shifts can bring the number back down.

A result of 6.5% or higher (48 mmol/mol or higher) points to diabetes, where sugar running high over time raises the risk to the eyes, kidneys, nerves, and heart. Caught early, that risk is largely modifiable. Treat this as one clue read alongside your other markers and with your clinician, not a diagnosis on its own. A single high result is usually confirmed with a repeat test, and a genuinely low A1c is uncommon, often pointing to red-blood-cell conditions like anemia.