Is a glucose spike after a meal a sign of prediabetes?

Usually not. In the 2019 reference study of 153 healthy people, the median time above 140 mg/dL was 2.1% of the day, about 30 minutes. Short rises after carbohydrate-heavy meals are normal. Prediabetes is defined by fasting glucose, A1C, or an oral glucose tolerance test, not by a sensor peak.

Can I use an over-the-counter CGM if I have type 1 diabetes?

No. Stelo and Lingo are cleared only for adults 18 and older who do not use insulin, and the FDA says Stelo is not for people with problematic hypoglycemia. Stelo shows no alerts for low glucose. People who use insulin need a prescription CGM with alarms.

Why does my CGM show a low reading in the middle of the night?

Lying on the sensor site can press fluid away from the wire and produce a false low, called a compression low or pressure-induced sensor attenuation. The trace drops fast and recovers fast once you move. If you have no symptoms, it is most likely an artifact rather than true hypoglycemia.

How long should a healthy person wear a CGM to learn something useful?

One sensor, 14 or 15 days, is enough to see how your usual meals, sleep, and exercise affect glucose. A second sensor lets you test changes. There is no evidence that healthy people benefit from wearing one all year, and the cost adds up at $89 to $99 a month.

Does a CGM reading match a finger-stick meter?

Not exactly. The sensor reads interstitial fluid, which lags blood by roughly 5 to 10 minutes, and the Stelo app updates only every 15 minutes. Both tools also have their own error margins. Dexcom reports 93% of Stelo readings fall within 20% of a lab reference, so a small gap between devices is expected.

Do Continuous Glucose Monitors Help People Without Diabetes?

The short answer: Not in any proven way, yet. The FDA cleared two over-the-counter glucose sensors in 2024 for adults 18 and older who do not use insulin. They show reliable trends, but evidence that they improve health in people without diabetes is thin. Healthy adults already spend about 96% of the day between 70 and 140 mg/dL. The people most likely to benefit have prediabetes and a specific question about meals or exercise.

What does a continuous glucose monitor actually measure?

A CGM does not measure blood. A thin wire under the skin reads glucose in the interstitial fluid, the fluid between cells. The NIDDK notes this level is very similar to blood glucose, but not identical, so a finger-stick check is sometimes needed.

Interstitial glucose also runs behind blood glucose. A tracer study in healthy adults put the delay at 5 to 6 minutes. A 2014 study found a median lag of 6.8 minutes in type 1 diabetes. The FDA 510(k) summary for Stelo adds that the app updates every 15 minutes, so "the most recent displayed glucose value might be up to 15 minutes old." The number on your phone describes the recent past.

Which CGMs can you buy without a prescription, and who are they for?

On March 5, 2024, the FDA cleared the Dexcom Stelo Glucose Biosensor System, the first over-the-counter CGM in the United States. The label covers "anyone 18 years and older who does not use insulin," and the system "is not for individuals with problematic hypoglycemia." Each sensor lasts up to 15 days. On June 10, 2024, Abbott received clearance for Lingo and Libre Rio. Lingo is for wellness users 18 and older not on insulin, worn on the upper arm for 14 days. Libre Rio is for adults with type 2 diabetes not on insulin.

Read the indication closely. The Stelo 510(k) summary says the device "may also help the user better understand how lifestyle and behavior modification, including diet and exercise, impact glucose excursion." The user "is not intended to take medical action based on the device output" without a clinician. Stelo shows no alerts, and its clearance study enrolled adults with diabetes, not healthy volunteers. Abbott says Lingo "is not intended for diagnosis of diseases, including diabetes."

What does normal glucose look like in people without diabetes?

The best reference data come from Shah and colleagues, 2019, Journal of Clinical Endocrinology and Metabolism. Twelve centers put a blinded Dexcom G6 on 153 healthy people aged 7 to 80 for up to 10 days:

Mean glucose was 98 to 99 mg/dL for every age group except those over 60, who averaged 104 mg/dL.
Median time between 70 and 140 mg/dL was 96% (interquartile range 93 to 98).
Median time above 140 mg/dL was 2.1%, about 30 minutes a day.
Median time below 70 mg/dL was 1.1%, about 15 minutes a day.
Time above 180 mg/dL and below 54 mg/dL had a median of 0.0%.
Mean within-person coefficient of variation was 17%, plus or minus 3%.

So a healthy person does go above 140 mg/dL, briefly, most days. A spike after a bowl of rice is not a diagnosis. A 2025 review in Life (Basel) states: "There is no CGM-derived definition of normoglycaemia in people without diabetes." An app's colored bands are a design choice, not a clinical threshold. Our guide to metabolic health markers worth tracking covers the tests that do have thresholds.

How accurate are over-the-counter CGMs?

Accurate enough for trends, not exact. Dexcom reports that 93% of Stelo readings fall within 20/20% of a laboratory reference: within 20% of the lab value above 70 mg/dL, or within 20 mg/dL below 70. A lab value of 120 mg/dL could read anywhere from 96 to 144 and still count as accurate. The FDA summary lists local infection, skin irritation, and pain as the device-related adverse events in the trial.

Sensors also produce false lows. Lying on the sensor site squeezes fluid away from the wire. Researchers call this pressure-induced sensor attenuation; users call it a compression low. The trace drops fast, then recovers once you move. Treat a single reading as an estimate and the daily pattern as the signal.

Do CGMs improve health in people without diabetes?

The trial evidence is small and mostly in people who already have diabetes. A 2024 meta-analysis in the International Journal of Behavioral Nutrition and Physical Activity pooled 25 randomized trials with 2,996 participants. Only 3 of the 25 trials (12%) enrolled people without diabetes, all with overweight or obesity. Across all trials, HbA1c fell 0.28 percentage points and time in range rose 7.4%. Weight fell 0.7 kg, not significant. Heterogeneity was high, 44% of studies reported CGM-related conflicts of interest, and every intervention bundled CGM with coaching. The authors concluded that "further research on the use of CGM-based biological feedback in populations without diabetes is needed."

A 2025 systematic review in Cureus looked only at people without diabetes and found 7 studies with about 1,127 participants. CGM helped people time a walk before their post-meal peak. But interventions ran "from a single night to several weeks," and evidence on cardiovascular endpoints "remains limited." The Life review agrees: "there is no strong evidence to support the global implementation of CGM in individuals who are at risk of developing diabetes."

The American Diabetes Association agrees on the diagnostic question. Its Standards of Care in Diabetes 2025 states: "There is presently insufficient evidence to support the use of continuous glucose monitoring (CGM) for screening or diagnosis of prediabetes or diabetes." Fasting glucose, A1C, and the oral glucose tolerance test still define prediabetes.

What can go wrong when a healthy person wears a CGM?

The main risk is misreading normal. A 2025 study in Obesity Research and Clinical Practice surveyed 56 adults not on insulin who used CGM for lifestyle change. More than two thirds, 68%, reported fear of type 2 diabetes when they saw a high reading, whether or not they had diabetes. Younger age, obesity, and eating disorder symptoms predicted more distress. Most, 89%, also reported positive diet or activity changes. Both were true at once.

Other predictable problems: cutting out fruit, oats, or beans because they cause a normal 30-minute rise above 140 mg/dL, treating a compression low as an emergency, and judging food by its glucose peak alone. Glucose is one input to energy balance and diet quality, not a verdict. The Life review adds that faulty sensors can cause "inappropriate actions and related anxiety."

Who is most likely to benefit, and what does it cost?

The strongest case is people with prediabetes. The CDC estimates 115.2 million American adults have prediabetes, and 8 in 10 do not know it. Weight loss through diet and activity can cut the risk of type 2 diabetes in half. Dexcom's launch materials say Stelo is "designed for adults with Type 2 diabetes not using insulin or prediabetes who seek behavior change." For that group, watching a post-meal peak flatten after a walk is a concrete lesson. The habits that fix it are in our metabolic health basics guide, and aerobic exercise helps whether or not you watch it happen.

Cost is real. Dexcom priced Stelo at $99 for a two-sensor pack (about 30 days) or $89 per month on subscription. Abbott priced Lingo at $49 for one 14-day biosensor, $89 for two, and $249 for six. Both are FSA and HSA eligible. One two-week or one-month run, aimed at a specific question about your meals, is enough for most healthy people. Year-round wear has no evidence behind it.

The bottom line

Over-the-counter CGMs are reasonably accurate and labeled for adults 18 and older who do not use insulin. They do not diagnose anything, and the ADA calls the evidence for screening prediabetes insufficient. Healthy adults spend about 96% of the day between 70 and 140 mg/dL, and short rises above 140 are normal. Trials in people without diabetes are few, short, and bundled with coaching. If you have prediabetes or a clear question about your meals, a one-month trial for $49 to $99 can teach you something. If not, the sensor is more likely to create worry than to change your health.

Last updated: September 2026. This article is for informational purposes only and does not constitute medical advice. Over-the-counter glucose sensors are not diagnostic tools. If a CGM shows repeated readings above 180 mg/dL, or you have symptoms of high or low blood sugar, see a clinician for standard testing.