Types of Continuous Glucose Monitoring Systems
There are four main types of CGM systems used in clinical practice: real-time CGM (rtCGM), intermittently scanned CGM (isCGM), professional CGM, and implantable CGM. 1, 2, 3
Real-Time CGM (rtCGM)
Real-time CGM systems measure interstitial glucose every 1-5 minutes and automatically transmit data continuously to a receiver, smartphone, or insulin pump without requiring user action 1. These devices provide:
- Immediate glucose readings displayed continuously on a monitor
- Predictive alerts and alarms for impending hypo- or hyperglycemia
- Real-time feedback enabling immediate therapeutic adjustments
- Data storage for retrospective analysis 3, 4
The key advantage is timeliness—users can intervene immediately based on glucose trends and directional arrows showing rate of change. The latest versions come equipped with customizable alerts that help users make real-time insulin dosing decisions 1.
Intermittently Scanned CGM (isCGM)
Intermittently scanned CGM measures glucose continuously every 1-5 minutes but requires the user to actively scan the sensor with a reader or smartphone to visualize and store glucose values 1, 2, 3. Critical distinctions include:
- Glucose data are NOT transmitted automatically—scanning is mandatory
- Newer versions provide optional predictive alerts and alarms; older technology may lack this feature 1
- Must be scanned at minimum every 8 hours to capture full 24-hour data 3
- Generally lower daily costs and may not require calibration 5
The flash glucose monitoring system (FreeStyle Libre) is the prototypical isCGM device, combining features of retrospective and real-time systems but requiring active user engagement 4.
Professional CGM
Professional CGM serves as a diagnostic tool placed by healthcare providers in the clinic setting (or with remote instruction) and worn for discrete periods, typically 7-14 days 1, 2, 3. Key characteristics:
- Clinic-based devices NOT owned by the patient—they belong to the healthcare facility
- Data may be blinded (patient cannot see results during wear) or unblinded
- Used for short-term investigation of glycemic patterns to guide therapeutic management
- Data collected retrospectively after the monitoring period for interpretation by the diabetes care team 1
This "Holter-style" glucose monitoring objectively records glucose patterns without patient behavior modification from real-time feedback 4.
Implantable CGM
The newest category involves sensors implanted subcutaneously that can remain in place for extended periods (versus externally worn disposable sensors) 3. While less extensively discussed in current guidelines, these represent an emerging technology designed to reduce the burden of frequent sensor changes.
Clinical Decision Framework
For patients on intensive insulin therapy (multiple daily injections or insulin pumps): rtCGM is preferred and should be used as close to daily as possible for maximal benefit 2. The evidence is strongest here for reducing hypoglycemia and improving time-in-range.
For patients requiring diagnostic assessment: Professional CGM provides objective glycemic profiling without the behavioral modification that occurs when patients see real-time data 4.
For cost-sensitive situations or when alarms are not desired: isCGM offers continuous measurement at lower cost, though it requires active scanning discipline and may miss critical glucose excursions if scanning frequency is inadequate 5.
Important Caveats
- All CGM systems measure interstitial glucose, which can lag behind blood glucose by 5-15 minutes during rapid changes 2
- Skin reactions (irritation or allergy) occur and must be assessed to ensure successful device use 2
- Users must be educated on interfering substances that affect accuracy 3
- The improvement in glycemic control with rtCGM is positively correlated with frequency of sensor use—intermittent use yields less benefit 4
- Patients should have uninterrupted access to supplies to minimize gaps in CGM coverage 3
The choice between rtCGM and isCGM should be based on the patient's need for real-time alerts, ability to respond to alarms, cost considerations, and preference for passive versus active data retrieval 2, 5.