What does an A1c test measure?

An A1c test measures the proportion of haemoglobin in your blood that has glucose attached to it. Because red blood cells live for roughly three months, the result reflects average blood glucose across that period rather than what it was on the morning of the test.

That is what makes it different from a fasting glucose reading, and why it is not affected by whether you ate breakfast. It is also why a single A1c cannot tell you whether your glucose has been swinging, only where the average sat.

Some conditions affecting red blood cells can make an A1c misleading, which is one reason results are interpreted by a clinician alongside your history rather than read straight off a range. If yours has changed, that is a conversation to have rather than a number to act on alone.