Time in Range: The Number That Matters More Than A1C on a GLP-1

Joslin said that a century ago to make a point: the person watching your glucose day and night matters more than the specialist you see twice a year. Today that always-on nurse is a sensor on your arm, but only if you can read what it's telling you. The most useful number it reports is Time in Range.

What Time in Range actually means

Time in Range (TIR) is the share of the day your glucose sits in the healthy target zone of 70 to 180 mg/dL. A TIR of 70% means you spent about 17 of 24 hours in range and the rest either too high or too low. Two companion numbers round it out: Time Above Range (TAR), the part of the day above 180, and Time Below Range (TBR), the part below 70. Together they describe your whole day rather than its average (Battelino 2019).

Why it beats A1C

A1C gives you one number for three months, and averages hide the shape of the day. You can post a decent A1C while spiking high after meals and dipping low in between, because the highs and lows cancel. TIR counts the hours instead, so nothing cancels. That's why two people with the same A1C can have very different TIR, and why the international CGM consensus was built around it (Danne 2017). On a GLP-1, which works by flattening spikes, TIR is often where you notice the benefit first, since the spikes are exactly what pull you out of range.

The targets to aim for

The general consensus target is TIR above 70%, with Time Below Range under 4% and Time Above Range kept as low as practical (Battelino 2019). These are population targets rather than a personal prescription. Pregnancy, older age, and certain conditions carry different goals, so your clinician sets yours. For most people, though, "more than 70% in range" is the number to build toward.

The other number: coefficient of variation (CV)

Your CGM report also shows a coefficient of variation, a measure of how steady your glucose is that's separate from how high it runs. The consensus threshold is CV at or below 36%, the line between stable and unstable glucose (Battelino 2019). Read that number correctly: 36% is a threshold to stay under, not a "36% improvement." A CV of 34% puts you on the stable side; a CV of 42% means your glucose is swinging more than the target allows. Think of it as a bar to clear, never a discount.

How to read yours in one minute

Open your CGM app's summary and check three things. Your TIR (aim above 70%). Your TBR (aim under 4%, and treat this as your safety number). Your CV (aim at or under 36% for steadiness). If TIR is climbing and CV is dropping since you started a GLP-1, the drug is doing its job. If TBR is creeping up, that's the one to raise with your clinician, especially if you also take insulin or a sulfonylurea.

A1C tells you the average. Time in Range tells you the day. On a GLP-1, TIR and CV are the two numbers that show what the drug is really doing to your glucose, and both are sitting on your phone once you know what you're looking at.