Time in range, GMI, and A1C: three clocks on the same glucose

Time in range is the most immediate of the three: the share of your readings that land inside your target band, commonly 70–180 mg/dL (3.9–10 mmol/L). The consensus goal is spending more than 70% of the day in range with very little time low, though your own target is set with your clinician. Because it's built from individual readings, it shows the texture an average hides — whether an out-of-range day was a single spike after lunch or a long overnight low.

A lab A1C sits at the other end of the clock. It measures the share of hemoglobin with glucose attached, and since red blood cells live about three months, it compresses roughly 90 days into one number. It has been the standard for over 30 years and it's what large studies tie to long-term outcomes — but it is an average, so two very different months can produce the same A1C, and it says nothing about lows, spikes, or when they happen.

GMI — the glucose management indicator — is the bridge between them. It converts a recent average glucose into an A1C-style percentage using a published formula, so you get a familiar number without a blood draw. On CGM reports it leans toward the most recent stretch of data, which makes it a snapshot of what you're working with right now if current patterns continue, where a lab A1C is a photograph of the past quarter.

That difference in clocks is why the two numbers rarely match exactly — and why a gap between them is information, not an error. If you changed something recently (a new medication, a food pattern, more walking), GMI reflects it months before the lab number catches up. A lab A1C that runs higher than your recent estimate is mostly telling you about the months before the change; many clinicians suggest not clinging to an old A1C when the recent data says something new.

Reading them together: use time in range week to week to steer daily decisions, watch GMI or an estimated A1C to see whether the recent trend is heading where you want, and treat the lab A1C as the long-exposure confirmation your clinician uses to set targets. When all three agree, you have a stable picture; when they disagree, the disagreement usually points at what changed and when.

In SteadyRange, the glucose page shows all three side by side: the time-in-range bar with the 70% goal marked, a GMI computed from your recent average, and an estimated A1C in the lab's ~90-day style. None of these estimates replaces a blood test — this is general information for reading your own numbers, and targets belong in a conversation with your clinician.