Seeing a glucose reading that doesn't seem right?
In most cases, your sensor is working normally and there's a simple explanation. Use the guide below to identify the most common causes and what to do next.
Important: If you feel unwell (for example, shakiness, sweating, dizziness or palpitations), trust how you feel rather than the number on your CGM. If you have diabetes or take glucose-lowering medication, always follow your doctor's advice and never adjust medication based solely on CGM readings.
Low reading overnight, but you feel fine
Possible cause: Compression low
Sleeping on the arm wearing your sensor can temporarily compress the tissue around the sensor filament, causing falsely low readings.
How to recognise it
A sudden drop during sleep
Readings return to normal shortly after changing position
You feel completely well
What to do
Nothing.
Your glucose hasn't actually dropped.
For your next sensor, consider applying it to the arm you are less likely to sleep on.
My sensor is new (less than 48 hours old)
Possible cause: Sensor settling period
New sensors require time to settle after insertion.
During the first 24–48 hours, readings may run slightly lower than expected or fluctuate more than usual while your body adjusts to the sensor filament.
What to do
Give your sensor up to 48 hours before placing too much emphasis on individual readings.
During this period, focus on overall trends rather than exact glucose values.
My reading doesn't match how I feel
Possible cause: Physiological lag
A CGM measures glucose in the fluid beneath your skin rather than directly from your bloodstream.
Because of this, readings typically lag behind blood glucose by approximately 5–15 minutes, particularly after:
eating
exercise
rapid glucose changes
What to do
Rather than focusing on a single number, pay attention to:
the trend arrow
whether glucose is rising or falling
the overall glucose curve
These patterns provide the most meaningful insights.
My readings seem consistently higher than expected
Possible cause: Supplements or medications
Some supplements and medications may influence CGM readings.
For example:
high-dose Vitamin C (approximately 1000 mg/day or more)
certain medications, including high doses of paracetamol
What to do
Review any supplements or medications you're taking and let your Vively Dietitian know so they can interpret your results appropriately.
My readings are erratic or jump around
Possible cause 1: Dehydration
CGMs measure glucose within the fluid surrounding your cells.
If you're dehydrated, readings can become more variable.
What to do
Maintain good hydration throughout the day.
Possible cause 2: Sensor adhesion
If the sensor has partially lifted, shifted or the adhesive is peeling, readings may become less reliable.
What to do
Check that the sensor remains:
flat against your skin
firmly attached
not lifting around the edges
To improve adhesion:
Use an over patch on the sensor.
Pat the sensor dry after showering rather than rubbing it.
Avoid friction from backpacks, clothing straps or tight sleeves.
If needed, secure the sensor with medical tape or a bandage, ensuring the centre opening remains uncovered.
During the first 24 hours after application, minimise activities that may loosen the adhesive, such as excessive sweating or repeated rubbing against clothing.
If the sensor has visibly shifted, the readings from that point onwards may no longer be reliable.
Signal dropouts or missing data
Possible cause
Poor sensor placement or lifting of the adhesive may affect performance.
For your next sensor
For the best results:
Apply the sensor to the back of your upper arm.
Avoid scars or irritated skin.
Wash the area using warm water and a fragrance-free, non-moisturising soap.
Clean the area thoroughly with two alcohol wipes.
Allow the skin to dry completely before applying the sensor.
Avoid moisturisers, oils or lotions before application.
My readings still don't look right after 48 hours
If:
your sensor has completed its 48-hour settling period,
the sensor remains firmly attached,
and the readings still appear clearly inaccurate,
please contact the Vively Support Team through the app.
We'll investigate the issue and, if your sensor is confirmed to be faulty, we'll organise a replacement.
Proactive Sensor Care & Adhesion Tips
Taking a few extra steps when applying your sensor can significantly improve both adhesion and performance.
Before applying your sensor:
Wash the application area with warm water and a fragrance-free, non-moisturising soap.
Dry the skin thoroughly.
Clean the area using two alcohol wipes.
Allow the skin to dry completely before applying the sensor.
Avoid moisturisers, oils or lotions before application.
After applying your sensor:
Consider using medical tape or a sensor patch for additional security, ensuring the centre opening remains uncovered.
Avoid excessive rubbing from clothing or bags.
Minimise excessive sweating during the first 24 hours where possible.
Pat the sensor dry after showering rather than rubbing it.
These simple steps can significantly improve sensor adhesion and reduce the likelihood of early sensor movement.
Remember: Patterns Matter More Than Individual Readings
Your CGM is designed to help you understand how your body responds to:
food
exercise
sleep
stress
lifestyle changes
An occasional unexpected reading is usually not a cause for concern.
The greatest value comes from recognising trends over time rather than focusing on a single glucose value.
If you're unsure how to interpret your glucose patterns, your Vively Dietitian can help you understand what they mean and identify practical ways to improve your metabolic health.
Important: Your CGM is designed to support health education and lifestyle optimisation. It is not intended to diagnose, treat or manage medical conditions and should not replace advice from your healthcare professional.