Continuous Glucose Monitors: Who Benefits and What the Numbers Mean

Your clinician suggested a continuous glucose monitor, or you saw ads for a sensor you can buy without a prescription. Now you have a report full of graphs, percentages, and coloured bars. This guide explains who benefits from a CGM, what the main numbers mean, and the targets clinicians use.

What a CGM is

A continuous glucose monitor (CGM) is a small sensor worn on the skin, often on the back of the arm or the belly [4]. A tiny filament under the skin measures glucose in the fluid between cells. The sensor sends readings to a phone, reader, or insulin pump every few minutes, day and night [4].

Most sensors last about 10 to 15 days before you replace them.

CGM compared with finger-sticks

• A finger-stick meter shows one moment in time.
• A CGM shows trends: whether glucose is rising, falling, or steady.
• A CGM can sound alarms for low or high glucose.
• CGM readings lag behind blood glucose by several minutes, because they measure fluid under the skin [4].

Who benefits most

The ADA Standards of Care 2026 recommend CGM for people with diabetes who use insulin, including people with type 1 diabetes and people with type 2 diabetes on insulin [2]. It also says CGM can be useful for adults with type 2 diabetes who take other glucose-lowering medicines [2].

CGM is especially helpful if you:

• Take several insulin doses a day or use an insulin pump
• Have frequent or severe low blood sugar
• Do not feel the warning signs of lows
• Are pregnant and have diabetes

In Canada, Diabetes Canada guidance also supports CGM for many people who use insulin. Public and private coverage differs by province and plan, so check what applies to you.

What your doctor looks at

The standard CGM report

An international consensus in 2019 set standard measures and targets for CGM data [1]. Clinicians usually review at least 14 days of data, with the sensor worn at least 70% of the time [1].

Time in range

Time in range is the share of readings between 3.9 and 10.0 mmol/L (70 and 180 mg/dL) [1]. It is the main number to watch.

For most adults with type 1 or type 2 diabetes, the targets are [1]:

• Time in range: more than 70%, which is about 17 hours a day
• Time below 3.9 mmol/L (70 mg/dL): less than 4%, or about 1 hour a day
• Time below 3.0 mmol/L (54 mg/dL): less than 1%, or about 15 minutes a day
• Time above 10.0 mmol/L (180 mg/dL): less than 25%
• Time above 13.9 mmol/L (250 mg/dL): less than 5%

For older adults or people at high risk of lows, the targets are looser [1]:

• Time in range: more than 50%
• Time below 3.9 mmol/L: less than 1%

Pregnancy has its own, tighter targets, using a range of 3.5 to 7.8 mmol/L (63 to 140 mg/dL) [1].

Other numbers on the report

• Average glucose
• GMI (glucose management indicator): an estimate of A1C from your average CGM glucose [1]. It may not match your lab A1C exactly.
• Glucose variability (coefficient of variation): how much glucose swings up and down. A common goal is 36% or less [1].
• The ambulatory glucose profile: a graph that stacks all your days on top of each other to show patterns by time of day.

Why time in range matters

Each 5% rise in time in range, about 1 hour a day, is linked to meaningful benefit for people with diabetes [1]. Time in range also shows lows and highs that an A1C can hide.

CGMs for people without diabetes

In 2024, the US FDA cleared the first over-the-counter CGM [3]. It is for adults 18 and older who do not use insulin, including people without diabetes who want to see how food and activity affect glucose [3]. It is not designed for people with problematic low blood sugar, and the FDA says users should not make medical decisions from it without talking to a clinician [3].

Keep in mind:

• Glucose normally rises after meals, even in healthy people.
• The targets above were set for people with diabetes, not for people without it.
• Evidence that CGM use improves long-term health in people without diabetes is limited.

Tips for using a CGM

• Follow the insertion and warm-up instructions.
• Check a finger-stick if your symptoms do not match the reading, or during the first day of a new sensor.
• Some medicines and supplements, such as high-dose vitamin C or acetaminophen, can affect some sensors. Check your device instructions.
• Share your data with your care team before visits.

Questions to ask your clinician

• Would a CGM help me, given my medicines?
• What time in range goal is right for me?
• What low and high alarm settings should I use?
• How should I respond to a falling trend arrow?
• How often should I share my data, and how?
• Is a CGM covered by my plan?
• When should I still use a finger-stick meter?

When to get care quickly

Treat a low right away if your CGM or meter shows low glucose and you have symptoms. Call 911 if someone with low glucose is confused and cannot swallow, has a seizure, or passes out. Get urgent care for very high glucose with vomiting, deep fast breathing, or severe drowsiness.

Bottom line

A CGM shows glucose all day and night and helps people who use insulin most. Time in range between 3.9 and 10.0 mmol/L is the key number, with a common goal above 70% and time below range under 4%. CGMs are now sold over the counter in the US, but their value for people without diabetes is not clear.

Primary AI can show you the guideline and study sources behind this topic in plain language, so you can bring better questions to your appointment.