Implantable Fluorescence Sensing (Eversense)

The Eversense 365 is a glucose sensor implanted under the skin that reads glucose with light instead of an enzyme, and its FDA labeling still asks for fingerstick calibrations for the whole year it is worn [1]. So a blood glucose meter and test strips stay part of the routine. What changed across three generations of Eversense is how often the system asks for a fingerstick, and Senseonics' safety statement lists the times a meter reading is still required [2].

Why a light-based sensor still needs a meter

The sensor carries a fluorescent indicator that binds glucose reversibly and glows brighter as more glucose binds. It has no battery; a transmitter worn on the skin above it powers it by radio frequency and turns the optical signal into a glucose value [1]. Our CGM hub's article on Eversense fluorescence sensors covers that chemistry in depth.

Light-based doesn't mean calibration-free. The indicator loses sensitivity as it oxidizes in the body, and the investigators behind the 180-day PROMISE study wrote that factory calibration might suit a sensor worn for 2 weeks but not one worn 12 times longer [3]. Their calibration algorithm tracks that drift using fingerstick readings [3]. The factory still contributes something: for the E3, FDA describes a one-time download of calibration parameters specific to each sensor, with the user's meter readings doing the rest [4].

How the calibration schedule changed

The original Eversense (2018, up to 90 days)

FDA approved the first Eversense in June 2018 as an adjunctive device, meaning it was meant "to complement, not replace" home blood glucose monitoring. Its labeling told users to take a fingerstick reading before making treatment decisions [5]. In PRECISE II, the 90-day study, participants calibrated twice a day with a study-issued Contour Next USB meter and Contour Next test strips [6]. A June 2019 approval changed the indication so the system could replace fingerstick readings for treatment decisions, though calibration stayed [4]. That 90-day system has since been succeeded by the E3 and then the 365 [4] [1].

Eversense E3 (2022, up to 180 days)

The E3 asks for 2 calibrations a day for the first 21 days. After that, the system picks 1 or 2 a day depending on how stable the sensor's signal looks, and it tells the user through the app when the next one is due [4]. Here's what surprised us: the drop to once a day wasn't guaranteed. In PROMISE, the standard sensors were on the once-a-day schedule a median of 56% of the time after day 21, and sensors with the modified chemistry later used in the E3 reached close to two-thirds [3] [4].

Eversense 365 (2024, up to 1 year)

The 365 starts with a 24-hour warm-up, then asks for 4 fingerstick calibrations spaced 2 to 12 hours apart, all within 36 hours. Next comes one calibration every 24 hours for 13 days. From day 14, it asks once a week [1]. By our arithmetic from that schedule, that's 4 plus 13 plus about 50 weekly entries, or roughly 67 calibrations in a year, before any extra checks. On the E3, by contrast, every day after day 21 still brought one or two [4].

FDA's summary also shows why weekly can work. It compared sensor readings with a laboratory reference, grouped by time since the last calibration. In every window, from the first 12 hours out to 168 to 174 hours (a little over a week), between 90.5% and 96.1% of readings fell within 20 mg/dL or 20% of the reference [1]. In ENHANCE, a year-long study of the 365 in 110 adults, overall MARD (mean absolute relative difference, the average gap from lab values) was 8.8% "with primarily one calibration per week" [7].

What happens when a calibration is missed

The labeling sets hard edges during start-up. In the 365's initialization phase, glucose data stops displaying after 8 hours without a calibration entry. Later, the system prompts 24 hours after the last successful calibration in the daily phase, and 1 week after it in the weekly phase [1].

The PROMISE protocol shows how strict an earlier version was. If a calibration was 4 hours overdue, the system stopped showing glucose values. At 12 hours overdue, it had to be reinitialized, which took four fingersticks within 24 hours [3]. That describes the study system, so the current user guide is the reference for how a 365 handles a late entry.

Which meter readings count

FDA's summary specifies fingerstick blood glucose calibrations but doesn't name a meter brand [1]. The clinical studies did use one brand. PRECISE II issued Contour Next USB meters [6], and PROMISE used the Contour Next One meter with its test strips [3]. Both come from Ascensia Diabetes Care, which also distributed Eversense in the US until Senseonics took over sales on January 1, 2026 [8]. For the rules on entering a calibration reading, the system's user guide is the reference.

When labeling still calls for a fingerstick

Calibration isn't the only time the meter comes out. Senseonics' safety statement says fingerstick readings are required for weekly calibration after day 13, when symptoms don't match the CGM information, and when taking tetracycline-class antibiotics [2]. FDA's summary explains the antibiotic part: tetracycline, doxycycline, minocycline and tigecycline can falsely lower sensor readings, and users shouldn't rely on sensor glucose while taking them. Mannitol or sorbitol given intravenously, or in irrigation or peritoneal dialysis solutions, can falsely raise readings; the sorbitol in ordinary food doesn't [1].

The 365 is the newer generation, cleared in 2024 [1]. As of January 2026, Senseonics still listed the older E3 alongside it, describing the E3 as needing a calibration once a day after day 21 and the 365 primarily once a week after day 14 [8]. For anyone keeping strips on hand, that gap is the practical difference between the two systems. How to act on any sensor or meter reading is a decision for your care team.