Continuous Glucose Monitoring FAQs

Full update September 2020

Continuous glucose monitoring (CGM) is the newest way to monitor glucose levels. Most CGMs check glucose about every one to five minutes, display readings, and can alert patients when glucose levels are higher or lower than customizable, preset thresholds. Currently available CGMs can show trends as glucose levels rise or fall. CGMs can display graphs showing glucose levels over specified periods of time. Some CGMs still require fingerstick glucose readings in order to make treatment decisions. The Dexcom G5 Mobile, Dexcom G6, and FreeStyle Libre systemsa are approved to be used alone (without fingerstick glucose readings) to adjust meds and make treatment decisions.4,20,25,36,43 For CGMs to be most beneficial, the collected data should be used to guide lifestyle or med changes. The chart below answers common questions about continuous glucose monitoring.

Topic/Question

Answer/Pertinent Information

What different types and options for continuous glucose monitoring are available?

Professional CGM may be available at some prescriber’s offices, usually an endocrinologist. It can be provided to patients on a short-term basis in order to analyze glucose trends (e.g., three days).2 Examples include the FreeStyle Libre Pro, Dexcom G6 Pro, and iPro2 Professional.2,33

Personal CGM is used long-term by a patient as a part of their diabetes management.2 Two different types of personal CGMs are available.

  • Stand-alone CGM: provides continuous glucose levels, with accessible tracking and trending. Examples are:
    • Dexcom G4 Platinum:c compatible with Tandem T:slim X2 systems WITHOUT basal- or IQ-technologyb (U.S. only) insulin pumps.
    • Dexcom G5 Mobilec and Dexcom G6: compatible with both Apple and Android technology.
    • Medtronic Guardian Connect (U.S. only): compatible with most Windows and Apple operating systems.1
    • Senseonics Eversense (U.S. only): compatible with both Apple and Android technology.1
    • FreeStyle Libre systemsa: use flash glucose monitoring (glucose readings captured when scanned for).10
      • Glucose values are reported when the user scans the sensor by passing a reader close to the sensor.10
      • Has a built-in glucose meter compatible with FreeStyle Precision Neo strips.4,44
  • Combination insulin pump/CGM: integrates continuous glucose readings with insulin pump technology. Examples are:
    • Medtronic MiniMed 530G (U.S. only; discontinued, but patients may still use), 630G, and 670G. Uses Medtronic Enlite (530G and 630G) and Guardian Sensor 3 (630G and 670G).
      • Compatible with Bayer’s Contour Next Link 2.4 wireless meter (630G and 670G).17,18
    • Tandem T:slim X2 systemsb (U.S. only [uses Dexcom G6 CGM sensors]).
      • Compatible with Dexcom G6 CGM.38,45

How does continuous glucose monitoring work?

Most CGM systems consist of three basic parts to measure, record, and display glucose readings. These parts usually include a sensor (with a fine needle for insertion into the skin), transmitter, and receiver.

  • The FreeStyle Libre systemsa are slightly different. Instead of the transmitter and receiver, they use a portable reader. The reader is manually waved over the sensor to capture glucose readings.4,25
  • The FreeStyle Libre 2 (U.S. only) system has optional alarms that can be turned on for highs and lows, as well as for loss of signal between the reader and the sensor. Highs and lows are only captured when the sensor is within 20 feet of the reader.49,50

Sensor: acts similar to a test strip to measure glucose values.2

  • Measures glucose level within interstitial fluid (fluid just below the surface of the skin).2
    • Test strips measure glucose levels within blood.
    • Interstitial fluid glucose levels may lag behind blood glucose readings by a few minutes, especially when glucose levels are changing.2,5
  • Sends an electric signal through a wire to the transmitter (except for FreeStyle Libre systemsa).
  • Most sensors need to be changed every seven or ten days (14 days for FreeStyle Libre 14-day and Libre 2 systems, up to 90 days [sensor implanted by a healthcare provider] for Senseonics Eversense).1,4,25,41

Transmitter: attaches to sensor base and sends glucose levels to the receiver or reader to display results.1,2

  • Transmitters must be within range of the receiver for proper functioning. Acceptable distances are:
    • within six feet (2 meters [Canada]):16-18 MiniMed 530G (U.S. only; discontinued, but patients may still use), 630G, and 670G.
    • within 20 feet (6 meters [Canada]):19,20,35,37,38,43,45 Dexcom G4 Platinum,c Dexcom G5 Mobile,c Dexcom G6, Medtronic Guardian Connect (U.S. only), and Tandem T:slim X2 systemsb (U.S. only).
  • The Senseonics Eversense (U.S. only) transmitter is applied daily over the implanted sensor and must be within 24.9 feet (7.6 meters [Canada]) of the app for effective wireless communication.41

Receiver or Reader: displays current and stored glucose readings.1,4,25

  • Updates in real time (receivers), or when scanned within 1.5 inches (4 cm) of sensor (FreeStyle Libre systemsa).
    • Patients should scan FreeStyle Libre sensors at least once every eight hours.4,24,25,50
    • Patients can use the FreeStyle LibreLink app on their smartphone (compatible with most Apple and Android phones) instead of the reader to scan for results with the FreeStyle Libre sensors.32
  • Displays trends to aid in decisions about treating highs and lows.2
  • Receiver not required for Senseonics Eversense. Instead, data are sent to mobile device.1,41

Which patients are most appropriate for continuous glucose monitoring?

Most of the data supporting the use of CGM are in patients with type 1 diabetes.12

U.S. endocrinology experts believe CGM should be available to all patients using insulin regardless of diabetes type.12

CGMs can be used to guide lifestyle and med changes, but may be most appropriate for the following patients:

  • patients at risk from hypoglycemia (e.g., athletes, elderly, patients with renal impairment)12
  • patients with hypoglycemia unawareness10,24
  • patients with frequent hypoglycemia10,24
  • patients unable to reach A1C goals because of hypoglycemia31

What are the approved ages with available continuous glucose monitors?

Dexcom G4 Platinum Pediatric is approved for patients between the ages of two and 17 years old.37

Dexcom G4 Platinum,c FreeStyle Libre, Freestyle Libre 14-day, and Senseonics Eversense are approved for patients ≥18 years old.4,19,25,41

Dexcom G5 Mobile and Dexcom G6 are approved for patients at least two years old.20,36,43

FreeStyle Libre 2 (U.S. only) is approved for patients at least four years old.49,50

Medtronic MiniMed 530G (U.S. only; discontinued, but patients may still use) is approved for patients ≥16 years old.16

Medtronic MiniMed 630G (with Guardian Sensor 3) is approved for patients ≥14 years old.1,17

Medtronic MiniMed 670G is approved for patients ≥7 years old.18

Medtronic Guardian Connect (U.S. only) is approved for patients between the ages of 14 and 75 years old.35

Tandem T:slim X2 systems (U.S. only) is approved for patients ≥6 years old.40

What instructions should be given to patients about glucose sensors?

To avoid irritation, instruct patients to avoid placing sensors on areas that experience rigorous movement during exercise, under a belt, or on the waistline.14,15

Scarred or tattooed areas should not be used for sensor insertion/placement.13

The abdomen is the most common part of the body used for sensor placement.14,15,19,20

  • The abdomen is not used for FreeStyle Libre sensors. Apply these on the back of the upper arm.4
  • Dexcom G5 Mobile or Dexcom G6 sensors can also be placed on the upper buttock for children ages two to 17 years old.20,43
  • Guardian Connect sensors can also be placed on the back of the upper arm.35

For most sensors, patients need to wait two hours after sensor placement for glucose readings to be available.1

  • It takes 1 hour for the FreeStyle Libre systemsa to provide glucose readings, but readings in the first 12 hours of initial placement need to be confirmed with a finger-stick glucose before they’re used to make treatment decisions.4,25,50
  • It takes 24 hours after sensor implantation (by healthcare provider) for the Senseonics Eversense to provide glucose readings.1,41

Teach patients to clean skin thoroughly (e.g., first with soap and water, then with an alcohol swab) and dry completely prior to placing sensors.

Patients may have difficulty maintaining sensor adhesion. For example:

  • Skin Tac can be used to ensure adhesion with Guardian sensors.15
  • Medical tape can be used to ensure adhesion with Dexcom sensors (e.g., Tegaderm, Skin Tac, 3M tape).39
    • Be sure not to tape over the transmitter to ensure proper communication to the receiver.39
  • FreeStyle Libre sensors should not be reused if they fall off or need to be removed for any reason.4
    • If patients struggle with adhesion, consider products to help (e.g., Over-Bandage, Skin Tac, Mastisol, Tegaderm).48
    • New sensors should be applied on rotating sites to avoid irritation.4

To remove sensors (other than Senseonics Eversense sensors, as these are removed by healthcare providers), instruct patients to gently pull off like an adhesive bandage, starting at the edge.4,14,15,19,20

  • Most transmitters are reusable. Remind patients to remove transmitters from sensors.14,15,19,20
  • Sensors should be disposed of in a container appropriate for sharps and blood exposure.4,14,15,19,20

What calibration is needed with continuous glucose monitors?

Most CGMs need to be calibrated at least every 12 hours using fingerstick blood glucose levels.1,19,20,27

  • Blood glucose levels must be between 40 and 400 mg/dL (2.2 to 22.2 mmol/L) for proper calibration.1,19,20,27
  • MiniMed 530G (U.S. only; discontinued, but patients may still use) should be calibrated three or four times per day.16

FreeStyle Libre systemsa and Dexcom G6 do NOT require fingerstick blood glucose level calibration.4,25,36,43

What travel considerations are important with continuous glucose monitors?

Sensors should not be exposed to x-ray technology.

  • It is safe for patients to go through metal detectors. However, patients should request a manual pat down or remove sensors prior to going through airport security.4,19,20,43
  • It is safe to wear the Senseonics Eversense sensor and transmitter when going through metal detectors at the airport.41

Remind patients to adjust the time if changing time zones.4

How does water exposure affect continuous glucose monitoring?

Flexibility with water exposure may be an important factor for patients to consider when selecting a CGM.

Sensors are water resistant to different depths and durations of time depending on the model. For example:

  • FreeStyle Libre sensors are water resistant in up to three feet of water (1 meter) for up to 30 minutes.1,4,25
  • Dexcom G5 Mobile and Dexcom G6 sensors are water resistant in up to eight feet of water (2.4 meters) for up to 24 hours.1,43
  • Senseonics Eversense transmitter is water resistant in up to three feet of water (1 meter [3.2 feet]) for up to 30 minutes.41

When should continuous glucose monitors NOT be used?

CGM is not generally recommended for use in pregnant, dialysis, or critically ill patients due to lack of safety information.4,13,16-19,25

Sensors and the Eversense transmitter (not the Eversense sensor, as this in implanted) should be removed prior to Computed Tomography (CT) scans, Magnetic Resonance Imaging (MRI), or x-rays.4,13,16-19,41

When are fingerstick blood glucose values needed with continuous glucose monitors?

Even with CGM, check blood glucose using a fingerstick during the following conditions:4,14-20,25

  • during times of rapidly changing glucose (e.g., more than 2 mg/dL [0.11 mmol/L] per minute), as interstitial fluid glucose levels may not accurately reflect blood glucose levels.
  • if patients suspect that a CGM reading may be inaccurate for any reason.
  • to calibrate CGMs (not required for the FreeStyle Libre systemsa or Dexcom G5 Mobile or Dexcom G6).
  • to confirm hypoglycemia or impending hypoglycemia as reported by the CGM readings.
  • when patients experience symptoms that may be due to low or high blood glucose or do not match CGM readings.
  • when the CGM readings do not indicate a trend.
  • before making treatment decisions based on CGM readings (exceptions FreeStyle Libre systemsa and Dexcom G5 Mobile and Dexcom G6).4,6-9,24,43

What information about glucose values can be obtained from continuous glucose monitors?

Specific terms and available information may vary slightly among the CGMs. Examples of some information that can be found on standardized CGM reports include:24,31,42

  • Average glucose level: an average of glucose values over a specified period of time (e.g., days, overnight, fasting, postprandial). Average glucose level is used to calculate glucose management indicator.
  • Glucose or glycemic variability (GV): fluctuation in blood glucose throughout the day.34 Aim for glucose variability of 36% or less to indicate “stable” blood glucose values.42,47
    • Some data suggest GV may be an independent risk factor for some diabetes-related complications, possibly due to vascular damage from fluctuations. However, more data are needed to confirm this.28
  • Glucose management indicator (GMI): replaces the estimated A1C (eA1C) and provides an approximate A1C over a shorter period of time. GMI is based on the average glucose level from CGM readings over about 14 days (A1C estimates glucose control over about two to three months).30 For example, a GMI of 7.5% corresponds to a mean glucose of about 175 mg/dL (9.7 mmol/L).30
    • Use an online calculator (https://www.jaeb.org/gmi/) or calculate GMI with a formula:
      • GMI (%) = 3.31 + 0.02392 x mean glucose (mg/dL).30
      • GMI (mmol/mol) = 12.71 + 4.70587 x mean glucose (mmol/L).30
    • GMI and A1C may differ by at least 0.3% about 50% of the time. For example:30
      • Conditions that affect red blood cell lifespan (e.g., hemolytic anemias) may impact A1C, but not GMI.30
      • GMI may be higher than the A1C shortly after periods of hyperglycemia (e.g., illness, ketoacidosis, stress), as A1C reflects glucose control over a two to three-month period not just the last 14 days or so.
      • GMI may be lower than the A1C shortly after periods when glucose readings are lower than normal (e.g., after starting a low-carb diet, periods of intense exercise, starting a new glucose-lowering med).
    • Avoid using GMI by itself to guide diabetes management decisions.30 Use GMI as one of the available tools (along with A1C, time in range, etc) to assess treatment and as part of the decision-making process to determine if therapy changes are needed. For example:30
      • GMI will trend downward before an A1C. Use GMI to confirm that dietary or med changes are improving glycemic control. Don’t use GMI alone, it is still important to look for hypoglycemia.
      • GMI may look good, but time in range may indicate adjustments in therapy are needed to prevent significant hypoglycemia.
  • Time in range (TIR): percent of readings and time measured glucose values fall within the specified target range (e.g., 70 to 180 mg/dL [3.9 to 10 mmol/L]).47 TIR may be expressed as a percentage of time or in hours per day.31
    • Aim for a TIR of 70% (correlates with an A1C of 7%) for most patients.42
    • Aim for a TIR of 50% (correlates with an A1C of about 8%) for older or high-risk patients.31,42
  • Time above range (TAR):47 percent of readings and time measured glucose values fall above 180 mg/dL (10 mmol/L). Aim for less than 25% (six hours) for most patients or <50% for older or high-risk patients.42,47
    • level one: (high)47 TAR with glucose values between 181 and 250 mg/dL (10.1 to 13.9 mmol/L)
    • level two (very high):47 TAR with glucose values above 250 mg/dL (13.9 mmol/L). Aim for less than 5% (72 minutes) for most patients or consider a goal of less than 10% for older or high-risk patients.42,47
  • Time below range (TBR):47 percent of readings and time measured glucose values fall below 70 mg/dL (3.9 mmol/L). Aim for less than 4% (~1 hour) for most patients or less than 1% for older or high-risk patients.42,47
    • level one (low):47 TBR with glucose values between 54 and 69 mg/dL (3 to 3.8 mmol/L)
    • level two (very low):47 TBR with glucose values below 54 mg/dL (3 mmol/L). Aim for less than 1% (14 minutes) for most patients and to avoid completely for older or high-risk patients.42,47
  • TBR may be especially useful for patients who are at their goal A1C, but experience frequent low readings.31 For example, changes in therapy may be needed for a patient who has an A1C of 6.8%, but who has hypoglycemia 10% of the time. However, changes in therapy may NOT be needed for a patient who has an A1C of 6.8%, but who only has hypoglycemia 1% of the time.30

You can view an example of a standardized CGM report (e.g., Ambulatory Glucose Profile) at https://care.diabetesjournals.org/content/diacare/42/8/1593/F2.large.jpg)

Do any medications interfere with continuous glucose monitoring?

Acetaminophen may falsely raise CGM readings from Dexcom G4 Platinumc and G5 Mobilec and Medtronic Guardian Connect and their sensors.13,20,35 Acetaminophen doses ≤1,000 mg every six hours do not affect Dexcom G6 readings.13,43

  • To be on the safe side, patients should not rely on readings from the Dexcom G4 Platinumc or G5 Mobilec or Medtronic Guardian Connect if they have recently taken any product containing acetaminophen.13,35

Taking ascorbic acid (vitamin C) may falsely raise all FreeStyle Libre systema readings. Salicylic acid (aspirin) may falsely lower FreeStyle Libre and FreeStyle Libre 14 day system readings.4

One small pilot study found some evidence that the following medications may interfere with accuracy of CGM readings with the Dexcom 4G Platinum sensors: acetaminophen, albuterol, atenolol, and lisinopril.3

Some evidence suggests that some medications used on an inpatient basis may also interfere with accurate CGM readings (e.g., dopamine, mannitol, maltose [breakdown product from peritoneal dialysis]).23

Considerations for continuous glucose monitoring in hospitalized patients.

Use of CGM in hospitalized patients is still a work in progress.10,24

  • CGM is not recommended in hospitalized patients until more safety and efficacy data are available.24
    • There is concern that some devices may not provide accurate readings with changes in pH, hematocrit levels, blood oxygen tension, and peripheral perfusion.23
    • Studies comparing the efficacy and clinical benefits of CGM devices vs using fingerstick blood glucose monitoring have yielded mixed results.24
      • Several studies have been unable to show improved glycemic control with inpatient use of CGM.24
      • Some studies indicate CGM seems to improve identification of hypoglycemia in hospitalized patients.11,24 This may be a time saver compared to checking fingerstick blood glucose values.
  • Check to see if your hospital has a policy for handling patients who would like to continue to use their personal CGM while in the hospital.29
    • If use is allowed, consider requiring a waiver to ensure hospital glucose monitoring devices are used to validate glucose values before making treatment decisions.
    • Work with patients to properly remove and replace sensors as needed. Most CGMs have reusable transmitters. Remove transmitters from sensors to avoid accidental disposal.14,15,19,20
      • Sensors should be disposed of in an approved sharps container.4,14,15,19,20

What are the costs associated with continuous glucose monitoring?

Costs for CGM equipment and supplies will vary among manufacturers.

Patients should check with their specific insurance carrier to ask about coverage.

Medicare and insurance coverage are becoming more common as newer CGMs are approved to be used to adjust diabetes medications without patients needing to get additional glucose values.

  • For example, Medicare Part B covers CGMs approved to be used in place of blood sugar monitors for treatment decisions.7,21,22 Medicaid coverage will vary by state.
  • In Canada, most private insurers cover the FreeStyle Libre.25

What billing codes should be used with continuous glucose monitors?

In the U.S., use the following billing codes when working with CGMs:26

  • Professional CGM: 95250 (e.g., sensor placement, hook-up, calibration, patient training, sensor removal, download)
    • Can be completed by a healthcare professional under the direct supervision of a physician, physician assistant, or nurse practitioner. Modifier 52 should be used if the monitoring was for a duration of less than 72 hours.
  • Personal CGM: 95249 (e.g., sensor placement, hook-up, calibration, patient training, sensor removal)
    • Can be completed by a healthcare professional under the direct supervision of a physician, physician assistant, or nurse practitioner.
  • Interpretation of CGM data: 95251 (personal or professional use)
    • Physician, physician assistant, or nurse practitioner must be involved (e.g., direct involvement, co-signature) in order comply with Medicare requirements.
    • Can only be used one time/month per patient.
  1. FreeStyle Libre systems include the FreeStyle Libre (Canada; discontinued in the U.S., but may still be in use.), FreeStyle Libre 14 Day (U.S. only), and FreeStyle Libre 2 (U.S. only).49,50 The FreeStyle Libre 14 Day (U.S. only) reader is black while the FreeStyle Libre 2 (U.S. only) reader is blue.
  2. Tandem T:slim X2 systems (U.S. only) include the Tandem T:slim X2 Basal-IQ Technology (designed to assist patients with low blood glucose levels) and Tandem T:slim X2 Control-IQ Technology (designed to assist patients with high and low blood glucose levels).40
  3. Dexcom stopped selling G4 Platinum and G5 mobile CGM transmitters in June 2020. Dexcom will stop selling G4 Platinum and G5 Mobile sensors by the end of 2020.46

Project Leader in preparation of this clinical resource (360904): Beth Bryant, Pharm.D., BCPS, Assistant Editor

References

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Cite this document as follows: Clinical Resource, Continuous Glucose Monitoring FAQs. Pharmacist’s Letter/Prescriber’s Letter. September 2020.

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