# Does an overnight CGM reading of 3.8 mmol/L prove hypoglycemia? No. One CGM reading of **3.8 mmol/L (about 68 mg/dL)**, especially when you felt well, does not by itself prove that you have a hypoglycemic disorder. For people treated for diabetes, a glucose value at or below **3.9 mmol/L (70 mg/dL)** is an alert level. It deserves attention because glucose may fall further. A value below **3.0 mmol/L (54 mg/dL)** is considered clinically important hypoglycemia. [ISPAD hypoglycemia guidance][source-1] But a CGM measures glucose in the fluid under the skin, not directly in blood. Its value can lag behind blood glucose and can sometimes be inaccurate. An overnight low may be a **compression low**: pressure from lying on the sensor temporarily produces a falsely low reading. This often looks like a sudden drop during sleep and improves after you change position. [UAMS CGM guide][source-2] [Guide to continuous glucose monitors]( "A BMJ Learning guide explains the practical use and limitations of continuous glucose monitors.") For someone **without diabetes or glucose-lowering medicines**, clinicians generally diagnose a hypoglycemic disorder only when *Whipple’s triad* is documented: - symptoms or signs consistent with low glucose; - a low **plasma glucose** result; and - symptoms improving when glucose is raised. A low sensor number without symptoms does not meet those criteria. A clinician may need a laboratory plasma-glucose measurement during an episode to investigate unexplained hypoglycemia. [Endocrine Society guideline][source-7] [RACGP review][source-4] ## What to do if it happens again 1. **Check your CGM trend and the sensor site.** If you were lying on the sensor, change position. 2. **Confirm with a finger-stick meter when you can**, especially if the result is unexpected, you have no symptoms, or it would change what you do. CGM education guidance specifically recommends a meter check for a suspected compression low or when symptoms and sensor results do not match. [UAMS CGM guide][source-2] 3. **If the finger-stick confirms a value below 3.9 mmol/L**, follow your own diabetes plan. A common approach is 15 g of fast-acting carbohydrate, then recheck in 15 minutes. Examples include glucose tablets, glucose gel, or about 120 mL (½ cup) of juice or regular soda. Repeat if still low. [UAMS CGM guide][source-2] 4. **Do not change insulin, pump settings, or other glucose-lowering medicine based on one uncertain reading** unless your prescribed plan tells you to do so. ![Illustration of a person wearing a CGM sensor and viewing glucose readings on devices]( "This diagram shows how a CGM sensor sends glucose information to a phone, watch, or handheld reader.") [Image source: Continuous Glucose Monitoring (CGM): What It Is]() Treat a low reading as potentially real if you have shaking, sweating, fast heartbeat, confusion, dizziness, or trouble waking. Get emergency help if there is a seizure, unconsciousness, severe confusion, or inability to swallow safely; another person may need to give glucagon if it has been prescribed. [ISPAD hypoglycemia guidance][source-1] Contact your diabetes clinician promptly if confirmed overnight lows recur, if readings are below 3.0 mmol/L, or if you use insulin or a medicine that can lower glucose. Bring the CGM overnight graph and any matching meter results. That record can help distinguish a recurring true low from sensor compression. [source-1]: [source-2]: [source-7]: [source-4]: