Manufacturing of Continuous Glucose Monitoring (CGM) Sensors

Manufacturing of Continuous Glucose Monitoring (CGM) Sensors

A CGM sensor runs on chemistry that test strip users already rely on. An enzyme, glucose oxidase, reacts with glucose and passes electrons to a metal electrode, and the strength of the resulting current tracks how much glucose is present[1]. The hard part of manufacturing is making thousands of sensors respond to glucose almost identically. Once a company can do that, it can calibrate sensors at the factory instead of asking you to calibrate them with fingerstick readings from your meter[2]. That change explains most of why fingersticks went from a routine CGM requirement to a backup check. (For who makes which sensors, see our guide to major US CGM manufacturers and product lines.)

What a sensor is made of

A test strip gives a measurement of fingerstick blood at a single point in time[1]. A sensor has to keep measuring for days in interstitial fluid, the fluid that surrounds cells just under the skin. The first FreeStyle Libre sensor, for example, combined a thin sensor tail inserted under the skin of the upper arm, an outer casing holding the electronics, and a medical-grade adhesive layer[1].

Two parts decide how an electrochemical sensor responds to glucose, Abbott engineers wrote in 2017: the sensing layer, which holds the enzyme on the working electrode, and a membrane over it that limits how fast glucose reaches the enzyme[2]. Libre sensors use Abbott's Wired Enzyme chemistry, in which an osmium compound (a mediator) shuttles electrons from the enzyme to the electrode[2]. Here's what surprised us: the sensing area is smaller than 1 square millimeter, and the membrane is typically thinner than 100 micrometers. Keeping both nearly identical from sensor to sensor takes high-precision coating equipment[2].

Not every sensor is electrochemical. The implanted Eversense 365 uses a fluorescent indicator molecule grafted onto the sensor's surface. Glucose binds to it reversibly, the glow brightens, and the sensor's optical system measures the change[3].

Made sterile, released lot by lot

The parts that go under the skin ship sterile. FDA's summary for the Dexcom STS, approved in 2006, lists the sensor as a sterile device but not the transmitter, which doesn't touch the skin[4]. Dexcom validated electron-beam sterilization at a minimum dose of 19.3 kGy, with a sterility assurance level of 10⁻⁶, meaning a one-in-a-million chance of a non-sterile unit[4]. For the first FreeStyle Libre, Abbott validated electron-beam sterilization of the sensor pack at a target dose of 25 kGy, reaching the same assurance level[1].

Factory-calibrated sensors are made in lots, and each lot is tested before release[2]. A recall Dexcom started on May 27, 2026 shows what can happen to a lot that never should have left. Certain Dexcom G7 lots marked as scrap were stolen during destruction and later sold to pharmacies and distributors[5]. According to FDA's recall record, one lot had been scrapped because its sensors weren't properly sterilized, and the other because of a higher risk of missing or inaccurate readings[5].

How calibration moved from your meter to the factory

Early sensors drifted, meaning their response to glucose changed over the days of wear, so users had to recalibrate them. After a 2-hour start-up, the Dexcom STS asked for two fingerstick readings from a OneTouch Ultra meter, uploaded by cable; it worked with no other meter[4]. Calibration had to be updated at least every 12 hours, and the labeling said readings had to be confirmed with a blood glucose meter before any therapy change[4]. As late as 2017, Abbott's engineers wrote that drift was the reason every CGM on the market except FreeStyle Libre still required fingerstick calibrations, typically twice a day[2].

Factory calibration, as those engineers describe it, has four manufacturing steps[2]:

• Make sensor lots with low sensor-to-sensor variability.
• Pull a sample of sensors from each lot and measure their response to known glucose solutions in the lab.
• Convert the lot's glucose sensitivity into a sensor code.
• Show that the sensitivity doesn't change over the sensor's shelf life.

The code can be programmed into the sensor's electronics so nobody has to type it in[2]. If you've read about calibration codes on [test strips](/diabetic-test-strips/calibration-codes-vs-no-code-technology/), the idea will sound familiar: one code stands for a whole manufacturing lot.

User calibration had its own weak spot. If a sensor's signal happened to be briefly off when a meter reading was entered, for example because of an interfering substance, the calculated sensitivity came out wrong and the readings after it stayed falsely high or low[2].

The FDA approved the first FreeStyle Libre in 2017 as a factory-calibrated system that neither required nor accepted calibration with blood glucose values[1]. The Dexcom G6, authorized in 2018, came with a calibration code entered at sensor start. Skipping the code meant two fingerstick calibrations at startup and every 24 hours after that[6]. Dexcom stopped manufacturing the G6 after July 1, 2026, though pharmacies may have stock for a while[7]. FDA's decision summary for the newer Dexcom G7 15 Day sensor describes it as factory calibrated, with an optional fingerstick calibration if the user wants one[8].

Where meters and strips still fit

Factory calibration removed the routine fingerstick, not the meter. The first FreeStyle Libre reader had a built-in blood glucose meter that used FreeStyle Precision Neo test strips[1]. In Abbott's studies, readings from that strip port were independent of the sensor and didn't change its readings[2]. As of October 2026, Abbott directs prescribers to its FreeStyle Libre 3 Plus and 2 Plus sensors, and the earlier Libre 2 and Libre 3 sensors have been discontinued[9].

Labeling still names times for a meter check. FDA's summary of the first Libre says users were told to test with a meter when their symptoms didn't match the sensor, and to use the sensor alongside a meter until they were comfortable with it[1]. The G6 labeling listed times not to rely on the sensor for treatment decisions, including the first two hours of warm-up, when a meter was to be used instead, and any time no reading or trend arrow appeared[6]. Which checks apply to you, and what to do with the result, is a decision for your care team.

The implanted Eversense 365 still depends on strips by design. It needs 4 fingerstick calibrations at the start, then one a day through day 13, then one a week from day 14 on[3].

FDA reviewers flagged a trade-off when they approved the first factory-calibrated sensor. With no required daily meter checks, some users get fewer chances to learn how a particular sensor performs against their meter[1]. Better manufacturing made sensors more consistent, and it also took away the daily reason to pick up the meter.