CGM vs A1c: how they work together

Continuous glucose monitors (CGMs) have become widely used well beyond people on insulin, and they measure something meaningfully different from A1c — which is why the two are complementary rather than interchangeable.

What a CGM shows that A1c doesn't

A CGM tracks glucose readings every few minutes, all day and night, giving a detailed picture of:

• How your glucose responds to specific meals
• Overnight patterns, including nocturnal lows or highs
• Time spent in a target range versus too high or too low
• Glucose variability — how much your levels swing, not just their average

A1c, by contrast, compresses months of data into a single average number. Two people can have an identical A1c while one has stable, steady glucose and the other swings widely between highs and lows — a distinction A1c alone can't capture, but a CGM makes visible immediately.

Where they line up (and where they don't)

CGM data can be used to calculate a metric called "Glucose Management Indicator" (GMI), which estimates what your A1c would be based on recent CGM readings. It often tracks closely with lab-measured A1c, but not always exactly — the same factors that affect A1c accuracy (see our article on what can cause a false A1c reading) can create a gap between the two.

Using both together

Clinicians increasingly use CGM data and A1c side by side: A1c for the long-term trend and diagnostic threshold, CGM for the day-to-day detail that explains that trend and guides real-time adjustments. If you use a CGM, understanding your average glucose from it (which our calculator can also convert) alongside your lab A1c gives a fuller picture than either alone.