Earlier this week, something strange happened. Despite eating my usual low-carb meals, exercising daily, sleeping well, and feeling great, my Freestyle Libre CGM was consistently telling me that my blood glucose was sitting between 10–12 mmol/L. That’s a huge red flag for me – especially given my most recent HbA1c was a tight 5.2 mmol/L.

I double-checked everything. No new medications, no illness, no stress to speak of. Still, the readings remained stubbornly high for over 24 hours.

Eventually, I decided to grab my Accu-Chek Guide Me blood glucose meter and do an old-school finger prick test. The result? 5.4 mmol/L. Spot on.

After speaking with my pharmacist, I was reminded that finger prick testing remains the gold standard for accuracy. Continuous glucose monitors (CGMs) like the Freestyle Libre and Dexcom are fantastic tools – they offer real-time insights and patterns – but they’re not infallible. Here are a few reasons why a CGM might give inaccurate readings:

1. Sensor Placement Issues

Poor placement, hitting a blood vessel, or injecting into an area with too much scar tissue or fat can skew readings.

2. Compression Lows or Highs

Lying on your sensor while sleeping (common for arm placements) can reduce blood flow and give artificially low or high readings.

3. Lag Time Between Blood and Interstitial Fluid

CGMs measure glucose in the interstitial fluid, which lags behind actual blood glucose levels by about 10–15 minutes. If your blood sugar is rising or falling quickly, the CGM might not keep up.

4. Sensor Age or Expiry

As sensors near their end of life, their accuracy can degrade. Also, if stored improperly or used after expiry, performance can be unreliable.

5. Dehydration or Poor Hydration

Interstitial fluid volume can be affected by hydration levels, impacting accuracy.

6. External Pressure or Movement

Excessive movement, tight clothing, or bumps to the sensor can lead to faulty readings.

This experience reminded me of something critical: always listen to your body and don’t hesitate to double-check. Technology is a brilliant ally in managing diabetes, but it’s not perfect. I will now keep my finger prick kit on hand for moments like this and encourage my clients to do the same.

Moral of the story: If something doesn’t feel right, trust your instincts – and your trusty finger prick.