The Contemporary Role of Masked Continuous Glucose Monitoring in a Real-Time World

Real-time continuous glucose monitoring (RT-CGM) has, in the span of just a few years, established an essential role in the contemporary management of type 1 diabetes. Benefits of RT-CGM include improvement in overall glycemic control as measured by reduction in HbA1c, reduction in both daytime and nocturnal hypoglycemia, avoidance of impending hypo- and hyperglycemia, and avoidance of severe hypoglycemia (both of its own accord and also when coupled with use of suspended insulin delivery via continuous subcutaneous insulin infusion).^1-9

Unlike real-time continuous glucose monitoring, masked (also referred to as “blinded” or “professional”) continuous glucose monitoring (CGM) provides data only for retrospective analysis. Use of masked CGM has many proven benefits. It detects postprandial hyperglycemia, ¹⁰ unrecognized hypoglycemia in both children ¹¹ and adults ¹² with type 1 diabetes, lowers HbA1c in adults living with type 2 diabetes,^13,14 reduces macrosomia rates in pregnant women with either type 1 or type 2 diabetes, ¹⁵ and provides important, actionable information leading to health-care-provider-recommended changes in therapy.^16,17

Given that real-time continuous glucose monitoring is either proven to—or would logically be expected to—provide the same benefits as masked CGM while, at the same time, also providing additional safety regarding hypo- and hyperglycemia avoidance as well as allowing for moment-to-moment decision making, it can reasonably be asked what, if any, role remains for masked CGM in a “real-time world.” In this commentary I discuss why masked CGM does, indeed, retain—at least for now and likely for a number of years to come—an important role in the contemporary management of diabetes.

Perhaps most important, masked CGM is simply more affordable—and thus more readily accessible to potential CGM users—than real-time CGM. Indeed, financial barriers are a major obstacle for many patients who would otherwise benefit from and wish to use real-time CGM. To use RT-CGM a patient must buy a transmitter, a receiver (or, as of very recently, for some RT-CGM systems, have a smartphone capable of serving this role), and purchase (disposable) sensors. The annual costs involved with using real-time CGM measure in the thousands of dollars. In addition, third-party reimbursement for RT-CGM is not uniform and, often, not available at all.

Conversely, use of masked CGM places a comparatively minor financial burden on a patient. The transmitter is owned by the health care professional and there is no receiver required. The only costs incurred by the patient are those for the purchase of a single-use sensor and the fee, where applicable, for the professional’s interpretation of the data. Even performing multiple masked CGM studies per year would be orders of magnitude less expensive to the patient than using real-time CGM.

The contemporary role of masked CGM is not, however, confined to matters of cost, and masked CGM should not be thought of as a technology to be used only if a person cannot afford real-time CGM. Cost issues aside, masked CGM, in fact, has important roles to play in lieu of real-time CGM.

Masked CGM is an invaluable tool in helping determine the reason for a person with diabetes having an HbA1c value that is disproportionately above or below what is expected from their self-monitored blood glucose (SMBG) readings. There are a number of medical disorders including hemoglobinopathies that renders the HbA1c less useful as a guide to glycemic control. ¹⁸ In these situations, a 1-time masked CGM study will reveal what the person’s true glycemic control is, and will therefore indicate if the person’s HbA1c is or is not of value for ongoing assessment of their glycemic control. In the event a person’s HbA1c is found to not reliably indicate his or her true glycemic control, potentially hazardous over- or undertreatment with antihyperglycemic medication can be avoided.

Also not to be overlooked is the utility of masked CGM in helping determine the glycemic control of the commonly seen individual whose HbA1c is chronically and markedly elevated, but whose blood glucose log documents consistently excellent readings and whose blood glucose meter is always “forgotten” at home at the time of clinic appointments. In this situation masked CGM typically reveals glucose values far higher than the person’s recalled SMBG readings.

Masked CGM is also a helpful tool to determine the cause of suboptimal HbA1c readings in people living with diabetes who decline to perform regular SMBG testing or real-time CGM; and may reveal unsuspected hypoglycemia ¹⁹ and hyperglycemia. ¹⁰

Despite real-time continuous glucose monitoring’s proven value, the great majority of people living with type 1 diabetes (and virtually all people living with other forms of diabetes) do not, in fact, use real-time continuous glucose monitoring and as many as 40% of people who have used real-time CGM abandon its use. ²⁰ Reasons for discontinuing RT-CGM include cost, frustrations over inaccuracy, the burden of using the technology, frequent alarms (“alarm fatigue”), and other factors.^20,21 Masked CGM shares few of these downsides of real-time CGM and in particular is far less cumbersome or onerous for the user. As such, it is reasonable to expect that many patients who decline using RT CGM may well find it more acceptable to (intermittently) use masked CGM thereby still accruing some of the benefits of CGM.

When masked CGM data is downloaded and reviewed in concert by both provider and patient, it can be an effective teaching tool providing information that allows health care providers to inform and empower patients regarding the impact of food choices, exercise, and medications on blood glucose levels, and what measures can be undertaken to improve glycemic control. ²²

Masked CGM is also as an effective tool to motivate patients who would benefit from—but have declined—real-time continuous glucose monitoring to overcome their reticence and to adopt use of real-time CGM. Quite striking is the response of the individual who has consistently declined real-time CGM when, having performed a masked CGM study, he or she bears witness, for example, to masked CGM tracings revealing hitherto undocumented episodes of prolonged and asymptomatic nocturnal hypoglycemia. An instant change of heart and quick pursuit of real-time CGM often follows (personal observation).

Masked CGM can provide computerized data analysis and automatically generated therapeutic options. This feature serves both as an effective teaching tool to help health care providers gain better insight into CGM data analysis and also helps the provider to better offer guidance to their patient.

Both the American Association of Clinical Endocrinologists and the Endocrine Society have concluded that masked CGM has a role to play in the contemporary management of people living with diabetes.^23,24 Specifically, masked CGM is noted to be helpful in the management of youth with type 1 diabetes with hypoglycemia unawareness, nocturnal hypoglycemia, the dawn phenomenon, or postprandial hyperglycemia, and in patients “experiencing important changes in their diabetes regimen.” The Endocrine Society also points out that masked CGM is an alternative to real-time CGM for “patients who cannot safely and effectively take advantage of the information provided to them by real-time CGM.” And, of course, masked CGM is a requisite over real-time CGM for those research studies where patient access to real-time data would unduly influence certain metrics that are being examined.

Ultimately, the relative clinical benefit and value of real-time and masked CGM are dependent on the skills and abilities of the individual(s) interpreting and using the information provided. Those patients who are sufficiently empowered, knowledgeable, and skilled to make use of the copious data provided by real-time CGM are most likely to maximally accrue the benefits of this technology. And those health care providers who are most adept at interpreting masked CGM data and translating this into actionable, clinically meaningful, and understandable information for patients will be able to most effectively make use this technology. Whether it be real-time or masked CGM that is being used, health care providers will best serve their patients by recognizing which patient in which situation will benefit most from one or the other technology; and then using this technology in a mentoring and educational manner.

I believe that in a few years, when real-time continuous glucose monitoring has become easier to use and more affordable, it will usurp most roles that presently remain for masked CGM. For now, however, masked continuous glucose monitoring retains an important place in the contemporary management of diabetes.