Time in range: how much of your day sits between two lines

Picture a glucose graph with two lines drawn across it. Time in range is simply how much of the day your trace spends between them. If you use a continuous glucose monitor (CGM), or have seen someone’s reports, you have probably come across the phrase. Here is what it means, where the commonly cited targets come from, and how it works alongside HbA1c. This is general information from the sources listed at the end, not advice for your situation.

A reading every few minutes

A CGM is a device that automatically estimates your blood glucose, using a tiny sensor under the skin that measures glucose in the fluid between your cells. It estimates glucose every few minutes and keeps a record, so you can see whether glucose is rising or falling and spot trends over hours or days [3]. A fingerstick is one dot on the graph; a sensor draws the whole line. That steady stream of readings is what makes time in range possible.

What “time in range” means

The 2019 international consensus on CGM targets defines three measures: the percentage of readings, and time per day, within a target glucose range (time in range), below it (time below range) and above it (time above range) [1]. For people with type 1 or type 2 diabetes, the consensus recommends a standard range of 70 to 180 mg/dL (3.9 to 10.0 mmol/L) [2].

The calculation is a simple share. As an illustration, if 70 out of every 100 readings fall between 70 and 180 mg/dL, time in range is 70%. A sensor that records a reading every 5 minutes makes 288 readings a day, so 70% would be about 202 of them, or 16 hours 48 minutes. The consensus recommends 14 days of CGM data, with the sensor active for at least 70% of that time, so the figure reflects enough readings to mean something [1].

Where the commonly cited targets come from

For adults with type 1 or type 2 diabetes, the consensus paper lists these targets [1]:

• Time in range (70 to 180 mg/dL): more than 70%, which is more than 16 hours 48 minutes a day.
• Time below 70 mg/dL: less than 4%, and below 54 mg/dL: less than 1%.
• Time above 180 mg/dL: less than 25%, and above 250 mg/dL: less than 5%.

The same paper gives different numbers for other groups. For older or high-risk adults it suggests more than 50% in range and less than 1% below 70 mg/dL, and says to individualize and be conservative, with a strong focus on reducing time below 70 mg/dL. For pregnancy with type 1 diabetes it uses a narrower range of 63 to 140 mg/dL [1]. These are expert targets for groups of people. Your own targets are set with your care team.

The paper notes that each incremental 5% increase in time in range is associated with clinically significant benefits, and also says that long-term trials showing how CGM metrics relate to or predict clinical outcomes have not been conducted [1].

Time in range and HbA1c: two lenses

The consensus describes HbA1c as more useful when combined with CGM data, because A1C does not show how large or how frequent the swings in glucose are within a day or between days [1]. In one analysis it cites, 70% time in range corresponded to an A1C of approximately 7%, and 50% to approximately 8%; the paper also shows that different analyses give somewhat different estimates, so the match is approximate [1].

In practice the two answer different questions. HbA1c is a long-term average (see the HbA1c post). Time in range shows how much of the time glucose sat inside the band, and how much was spent above or below it.

We made Steadion, an iPhone tracker for people with diabetes or prediabetes. It reads your blood glucose from Apple Health (from a continuous glucose monitor, if you use one) and works out your time in range. Learn more at steadion.fit.