A continuous glucose monitor shows a stream of readings. A fasting plasma glucose test captures one point in time. A1C reflects average glucose over roughly the previous three months. They offer different views of glucose, with different uses.[1][2]

Having more readings can reveal a pattern. Diagnosis still depends on the appropriate clinical tests and interpretation.[1]

Fasting glucose: a defined testing condition

Fasting plasma glucose is measured from a blood sample after a fasting period specified for the test. It is one of the laboratory tests used to diagnose diabetes or prediabetes. A provider commonly uses another test to confirm a diagnosis.[1]

Its strength is that the sampling conditions and interpretation are defined. It does not describe every rise and fall over the rest of the day.

A1C: the longer view

A1C measures glucose attached to hemoglobin in red blood cells. It summarizes a longer period rather than a single meal or morning.[3]

Some conditions affecting red blood cells or hemoglobin can make A1C less reliable. A mismatch between A1C and other glucose results is a reason to investigate the difference, rather than simply choosing whichever number looks more reassuring.[1][3]

CGM: the pattern over time

A CGM uses a wearable sensor and an app or reader to display repeated glucose measurements and trends. Device indications matter: different systems are designed for different users and monitoring needs.[2]

In its announcement of the first over-the-counter CGM, FDA explained that the system was intended for adults who did not use insulin and did not have problematic low blood sugar. It also said users should discuss the output with a provider before making medical decisions. Those details apply to that system, not automatically to every CGM.[2]

What makes the combined review useful

A provider can compare the laboratory results with the sensor record, the device used and what was happening during the recorded period. Meals, activity and symptoms provide context for the graph.

A screenshot of one high reading leaves out duration, repetition and the rest of the record. A laboratory report adds its own units, reference information and testing conditions. Keep the original lab report and sensor record together so the review can compare the same dates and circumstances.