Continuous glucose monitoring: revision 1 to 2

Revision 1

Revision 2

categories: ["longevity", "cybernetics"]

tags: ["diabetes", "biosensors", "metabolic health", "wearables", "self-tracking", "measurement"]

summary: "A wearable sensor that reports glucose in the fluid between cells every few minutes, well established in diabetes and of unproven value to people without it."

summary: "A wearable sensor that reports glucose in the fluid between cells every few minutes, well established in diabetes and of unproven value to people without it."

updated: "2026-07-27"

updated: "2026-10-29"

humanEvidence: "Randomized trials in type 1 and insulin-treated type 2 diabetes show lower HbA1c and less hypoglycemia; no trial in people without diabetes has shown a benefit on any health outcome."

humanEvidence: "Randomized trials in type 1 and insulin-treated type 2 diabetes show lower HbA1c and less hypoglycemia; no trial in people without diabetes has shown a benefit on any health outcome."

access: "Prescribed and widely reimbursed for insulin-treated diabetes in wealthy countries; over-the-counter sensors have been sold in the United States since 2024, paid out of pocket."

access: "Prescribed and widely reimbursed for insulin-treated diabetes in wealthy countries; over-the-counter sensors have been sold in the United States since 2024, paid out of pocket."

reversibility: "reversible"

"caption": "Wearable biosensor",

"rows": [

{ "label": "Measures", "value": "Interstitial fluid glucose" },

{ "label": "Sampling interval", "value": "1–5 minutes" },

{ "label": "Sampling interval", "value": "1-5 minutes" },

{ "label": "Sensor life", "value": "Roughly 10–15 days" },

{ "label": "Sensor life", "value": "Roughly 10-15 days" },

{ "label": "First FDA approval", "value": "1999 (retrospective)" },

{ "label": "Real-time systems", "value": "Mid-2000s" },

{ "label": "Over-the-counter clearance", "value": "2024 (US)" },

```keyfacts

[

  { "value": "5–15 min", "label": "Typical lag behind blood glucose", "note": "diffusion into interstitial fluid plus algorithmic smoothing" },

  { "value": "5-15 min", "label": "Typical lag behind blood glucose", "note": "diffusion into interstitial fluid plus algorithmic smoothing" },

  { "value": "2024", "label": "First over-the-counter monitors cleared in the US", "note": "Dexcom's Stelo, then Abbott's Lingo" },

  { "value": "2024", "label": "First over-the-counter monitors cleared in the US", "note": "Dexcom's Stelo, then Abbott's Lingo" },

  { "value": ">70%", "label": "Consensus time-in-range target", "note": "for most adults with diabetes, 70–180 mg/dL" }

{ "value": ">70%", "label": "Consensus time-in-range target", "note": "for most adults with diabetes, 70-180 mg/dL" }

]

```

period after insertion before they display anything, and are discarded after roughly ten to fifteen

days.

The crucial detail is what the sensor is standing in. Glucose reaches interstitial fluid by

The sensor reads interstitial fluid, not blood. Glucose reaches interstitial fluid by diffusion out

diffusion out of capillaries, so the interstitial value trails the blood value by something on the

of capillaries, so the interstitial value trails the blood value by something on the order of five

order of five to fifteen minutes, and manufacturers' smoothing algorithms add their own delay. When

to fifteen minutes, and manufacturers' smoothing algorithms add their own delay. When glucose is

glucose is stable the two track closely. When it is moving fast, during exercise, in the half hour

stable the two track closely. When it is moving fast (during exercise, in the half hour after a

after a meal, or while treating a low, the displayed number describes the recent past. This is why

meal, or while treating a low), the displayed number describes the recent past. For that reason the

the trend arrow on the screen carries more decision-relevant information than the digit beside it,

trend arrow on the screen carries more decision-relevant information than the digit beside it, and

and why insulin users are taught to treat a falling reading differently from a flat one at the same

insulin users are taught to treat a falling reading differently from a flat one at the same value.

value.

The clinically useful outputs are aggregate rather than instantaneous: how much of the day is spent

The clinically useful outputs are aggregates, not instantaneous values: how much of the day is spent

in a target band, how variable the readings are, how much time is spent low overnight. The same

in a target band, how variable the readings are, and how much time is spent low overnight. The same

signal also closes a control loop. Paired with an insulin pump and a dosing algorithm, a monitor

signal can also close a control loop. Paired with an insulin pump and a dosing algorithm, a monitor

becomes the sensor half of an automated insulin delivery system, the sense-and-respond architecture

also used by closed-loop [[deep-brain-stimulation]] and proposed for

[[medical-nanorobots|nanoscale therapeutic devices]].

> [!note] Terminology

> Time in range is the share of the day spent between 70 and 180 mg/dL (3.9–10.0 mmol/L). The

> Time in range is the share of the day spent between 70 and 180 mg/dL (3.9-10.0 mmol/L). The

> glucose management indicator converts mean sensor glucose into an HbA1c-like percentage; it is an

> estimate, not a laboratory value, and the two can disagree. The ambulatory glucose profile is the

> estimate rather than a laboratory value, and the two can disagree. The ambulatory glucose profile

> standard one-page report that stacks two weeks of traces into a single median-and-percentile day.

> is the standard one-page report that stacks two weeks of traces into a single

> median-and-percentile day.

## Development history

[

  { "year": "1999", "title": "First CGM approved", "text": "The FDA approves MiniMed's system, which records glucose blind and is downloaded at a clinic days later. It is a diagnostic tool for physicians, not a display for patients." },

  { "year": "1999", "title": "First CGM approved", "text": "The FDA approves MiniMed's system, which records glucose blind and is downloaded at a clinic days later. It is a diagnostic tool for physicians, not a display for patients." },

  { "year": "2001", "title": "GlucoWatch reaches the market", "text": "Cygnus wins approval for a wrist device that draws fluid through the skin by reverse iontophoresis. Skin irritation and unreliable readings sink it, and it is off the market within a few years." },

  { "year": "2001", "title": "GlucoWatch reaches the market", "text": "Cygnus wins approval for a wrist device that draws fluid through the skin by reverse iontophoresis. Skin irritation and unreliable readings sink it, and it is off the market within a few years." },

  { "year": "2005–2006", "title": "Real-time systems arrive", "text": "Medtronic's Guardian RT and DexCom's STS put a live number, a trend arrow, and alarms in the user's hand for the first time." },

  { "year": "2005-2006", "title": "Real-time systems arrive", "text": "Medtronic's Guardian RT and DexCom's STS put a live number, a trend arrow, and alarms in the user's hand for the first time." },

  { "year": "2008", "title": "Randomized benefit demonstrated", "text": "A JDRF-funded trial in type 1 diabetes reports improved HbA1c with CGM, with the effect concentrated in adults aged 25 and over." },

  { "year": "2008", "title": "Randomized benefit demonstrated", "text": "A JDRF-funded trial in type 1 diabetes reports improved HbA1c with CGM, with the effect concentrated in adults aged 25 and over." },

  { "year": "2014", "title": "FreeStyle Libre launches in Europe", "text": "Abbott's factory-calibrated, scan-to-read sensor cuts cost and eliminates fingerstick calibration, and drives the first mass adoption outside intensive insulin users." },

  { "year": "2014", "title": "FreeStyle Libre launches in Europe", "text": "Abbott's factory-calibrated, scan-to-read sensor cuts cost and eliminates fingerstick calibration, and drives the first mass adoption outside intensive insulin users." },

  { "year": "2016", "title": "Dosing from the sensor allowed", "text": "The FDA permits the Dexcom G5 to be used for insulin dosing without a confirmatory fingerstick, making the sensor the primary measurement rather than an adjunct. Calibration fingersticks are still required." },

  { "year": "2016", "title": "Dosing from the sensor allowed", "text": "The FDA permits the Dexcom G5 to be used for insulin dosing without a confirmatory fingerstick, making the sensor the primary measurement rather than an adjunct. Calibration fingersticks are still required." },

]

```

Each step turned less on the electrochemistry than on trust in it. The 1999 system was blinded

Each step turned less on improvements in the electrochemistry than on how far its readings could be

because nobody was prepared to let a patient act on the number; permitting insulin dosing from a

trusted. The 1999 system was blinded because nobody was prepared to let a patient act on

sensor reading in 2016 was the decision that made the automated pumps, and eventually the retail

the number. Permitting insulin dosing from a sensor reading in 2016 was the decision that made the

shelf, possible.

automated pumps, and eventually the retail shelf, possible.

## Evidence in diabetes

For people who take insulin, a reading changes an action. The 2008 JDRF trial found improved

For people who take insulin, a reading leads to an action. The 2008 JDRF trial found improved

glycemic control in adults but not in the children and young adults enrolled alongside them, a

glycemic control in adults but not in the children and young adults enrolled alongside them, a split

split usually attributed to how much the younger groups actually wore the sensor.[^jdrf2008] The

usually attributed to how much the younger groups actually wore the sensor.[^jdrf2008] The DIAMOND

DIAMOND trial extended the result to adults on injections rather than pumps, the larger population,

trial extended the result to adults on injections rather than pumps, the larger population, with a

with a modest but consistent HbA1c reduction.[^diamond2017] In type 2 diabetes treated with basal

modest but consistent HbA1c reduction.[^diamond2017] In type 2 diabetes treated with basal insulin

insulin in ordinary primary care, the MOBILE trial found a similar direction of

in ordinary primary care, the MOBILE trial found a similar direction of effect.[^mobile2021] Less

effect.[^mobile2021] Less time spent hypoglycemic is a second and arguably more important benefit,

time spent hypoglycemic is a second benefit, arguably the more important one, because severe lows

since severe lows are the acute danger of insulin therapy and often go unnoticed overnight.

are the acute danger of insulin therapy and often go unnoticed overnight.

An international consensus group in 2019 converted the raw traces into targets that trials and

clinics could share, chiefly the recommendation that most adults with diabetes spend more than 70%

of the day between 70 and 180 mg/dL.[^battelino2019] Time in range is not a validated surrogate for

of the day between 70 and 180 mg/dL.[^battelino2019] Time in range has not been validated as a

complications in the way HbA1c is, but it correlates with it and captures variability that a

surrogate for complications in the way HbA1c has. It correlates with HbA1c, and it captures

three-month average hides.

variability that a three-month average hides.

> [!debate] Where the indication ends

> The strongest evidence is in people whose reading changes an insulin dose. In type 2 diabetes

> managed with diet, [[metformin]], or a [[glp-1-receptor-agonists|GLP-1 receptor agonist]], trials

> are fewer, effects on HbA1c smaller, and reviewers disagree about whether continuous wear is worth

> the cost against periodic testing. That boundary, rather than the wellness market, is where most

> the cost compared with periodic testing. Most of the live clinical argument concerns that

> of the live clinical argument sits.

> boundary, not the wellness market.

## Use without diabetes

In 2024 the FDA cleared the first monitors sold without a prescription, Dexcom's Stelo followed by

Abbott's Lingo, both indicated for adults who do not use insulin.[^fda2024] The consumer proposition

Abbott's Lingo, both indicated for adults who do not use insulin.[^fda2024] The consumer pitch is

is that watching one's own curve teaches which foods, workouts, and bad nights produce which

that watching one's own curve shows which foods, workouts, and bad nights produce which responses,

responses, and that flattening it is a route to [[healthspan|longer healthy life]]. Companies

and that flattening the curve is a route to [[healthspan|longer healthy life]]. Companies including

including Levels and Signos sell subscriptions built on Dexcom and Abbott hardware, adding food

Levels and Signos sell subscriptions built on Dexcom and Abbott hardware; they add food logging,

logging, meal scores, and coaching rather than sensors of their own. The sensor joins the other

meal scores, and coaching, not sensors of their own. The sensor joins the other

[[wearable-health-sensors|consumer physiological sensors]] as a fixture of [[biohacking]] and of the

[[quantified-self|self-tracking]] culture that surrounds it, alongside

[[consumer-blood-testing|direct-to-consumer blood panels]] and

Institute study which showed that postprandial glucose responses to identical meals differ markedly

between people, and that an algorithm trained on personal data could design meals that blunted those

responses.[^zeevi2015] The endpoint there was the glucose curve itself. The larger PREDICT study

later reported the same between-person spread and, importantly, substantial variation within the

later reported the same between-person spread, and also substantial variation within the same person

same person eating the same food on different days.[^berry2020] Reference data from people without

eating the same food on different days.[^berry2020] Reference data from people without diabetes

diabetes wearing blinded sensors show that normal glucose tolerance means spending nearly all of the

wearing blinded sensors show that normal glucose tolerance means spending nearly all of the day in a

day in a narrow band, with brief post-meal excursions that a consumer app may flag as

narrow band, with brief post-meal excursions that a consumer app may flag as alarming.[^shah2019] A

alarming.[^shah2019] A monitor that pathologizes normal physiology has a specificity problem, not a

monitor that flags those excursions treats normal physiology as abnormal, which is a failure of

sensitivity one.

specificity rather than sensitivity.

> [!caution] What a spike does not mean

> No threshold for a post-meal glucose excursion has been validated as a risk factor in a person

> with normal glucose tolerance. The same meal, eaten by the same person on two days, can produce

> visibly different curves through sleep, prior activity, stress, sensor site, and simple noise. A

> number that moves is not the same as a number that matters, and the inference from a curve to a

> movement in the number does not by itself indicate a change in risk, and inferring a health

> health outcome runs through evidence that does not exist yet.

> outcome from a curve would require evidence that does not exist yet.

## Accuracy and its limits

## Risks and second-order effects

The physical risks are minor: adhesive dermatitis, occasional infection, rarely a retained filament

The physical risks are minor: adhesive dermatitis, occasional infection, and rarely a retained

fragment. The interesting risks are behavioural. Clinicians have raised concerns that continuous

filament fragment. The less obvious risks are behavioural. Clinicians have raised concerns that

feedback encourages food restriction and anxiety in people predisposed to disordered eating, and

continuous feedback encourages food restriction and anxiety in people predisposed to disordered

alarm fatigue is a documented problem even among those with a clinical reason to wear a sensor.

eating, and alarm fatigue is a documented problem even among those with a clinical reason to wear a

Attention spent on curves is also attention not spent on [[exercise-and-aging|structured exercise]],

sensor. Time spent attending to curves is also time not spent on

which has randomized evidence on function in older adults, or on [[sleep-and-longevity|sleep]],

[[exercise-and-aging|structured exercise]], which has randomized evidence on function in older

whose mortality epidemiology is far larger than anything assembled for glucose curves in people

adults, or on [[sleep-and-longevity|sleep]], whose mortality epidemiology is far larger than

without diabetes.

anything assembled for glucose curves in people without diabetes.

A glucose trace also reveals meal timing, sleep, illness, and activity, and it sits with

manufacturers and app companies under commercial privacy policies rather than the rules that govern

clinical records. Cost pushes in the other direction: sensors are a recurring out-of-pocket

clinical records. Cost raises a separate problem: sensors are a recurring out-of-pocket expense, so

expense, so a technology with proven benefit in diabetes is distributed partly by ability to pay, an

a technology with proven benefit in diabetes is distributed partly by ability to pay, an instance of

instance of the pattern examined under [[access-and-inequality]].

the pattern examined under [[access-and-inequality]].

## Outlook

Hardware is moving toward longer wear and more analytes. Senseonics' implanted fluorescence-based

sensor runs for months to a year rather than days, and manufacturers have announced sensors

reporting ketones alongside glucose, which matters for users of automated insulin delivery. Non-invasive optical

reporting ketones alongside glucose, which matters for users of automated insulin delivery.

measurement remains the field's perennial promise; the FDA warned in 2024 against smartwatches and

Non-invasive optical measurement is a promise the field has repeatedly made and not yet kept; the

rings claiming to measure blood glucose without piercing the skin, and no such device has been

FDA warned in 2024 against smartwatches and rings claiming to measure blood glucose without piercing

authorized.

the skin, and no such device has been authorized.

The more consequential question is interpretive. Sensors generate dense longitudinal metabolic data

The larger open question is what the data mean. Sensors generate dense longitudinal metabolic data

on large numbers of people outside clinical research, the kind of input that

[[human-digital-twins|individualized physiological models]] require. Whether that yields anything

beyond diabetes depends on studies that have not been run: trials in people without diabetes,

powered on health outcomes rather than glucose curves, long enough to show whether early detection

of impaired glucose tolerance changes anything. Until those exist, wellness monitoring occupies

of impaired glucose tolerance changes anything. Until those exist, wellness monitoring occupies the

the same position as consumer [[epigenetic-clock|methylation age tests]] and other

same position as consumer [[epigenetic-clock|methylation age tests]] and other

[[aging-biomarkers|biomarker panels]] sold on the promise of measuring

[[biological-age|biological aging]]: a real measurement of a real quantity, with an unestablished

[[biological-age|biological aging]]: a real measurement of a real quantity, with no established link

link between moving the number and living better. That gap is the standing challenge to the

between moving the number and living better. The [[geroscience-hypothesis|geroscience]] programme as

[[geroscience-hypothesis|geroscience]] programme as a whole, and glucose is simply its most widely

a whole faces the same gap, and glucose is its most widely worn example.

worn instance.

## See also