A1C

A1C, sometimes written HbA1c, measures the proportion of your hemoglobin that has glucose permanently attached to it. Because red blood cells live about three months, the figure reflects your average glucose over roughly that period, weighted toward the most recent weeks.

It is reported as a percentage. Below 5.7 percent is the usual normal range, 5.7 to 6.4 is described as prediabetes, and 6.5 and above is one of the criteria for a diabetes diagnosis. It is the measure most treatment decisions have historically been built on, because one blood test summarizes three months.

It can also be estimated rather than measured, from the average of your own glucose readings, using the relationship established by the A1c-Derived Average Glucose study. An estimate from three months of continuous monitor data gets close to a laboratory result; an estimate from a handful of finger sticks taken at convenient moments does not.

What it is not

It is not a complete picture of glucose control, and this is its main weakness. Two people with the same A1C can have very different days, one steady and one swinging between high and low, and the swings carry risks the average hides. Time in range and glucose variability are what fill that gap.

Where you see it in FitnessSyncer

An estimated A1C is worked out from your glucose readings in the built-in Diabetes report alongside your time in range, and a laboratory result can be recorded with your blood panels. Our A1C calculator estimates one from an average you already have.