It’s normal for your CGM and fingerstick readings to differ, and sometimes the CGM may appear lower or higher than your BGM reading. This doesn’t mean anything is wrong with the CGM or the BGM. CGMs and BGMs measure glucose in different places, and both have normal, expected variability that still meets accuracy expectations. Some of the most common reasons a CGM reading may look different are:
Where glucose is being measured:
Blood glucose changes before interstitial fluid glucose does, so during times when glucose is falling like after exercise or a large carbohydrate-based meal, the CGM may temporarily show a lower number than a fingerstick. Similarly, during times when glucose is rising rapidly – like after meals or during higher-intensity exercise the CGM reading may temporarily show a higher reading than a fingerstick. This is a normal effect of where the measurement happens.
Pressure on the biosensor:
Lying on the biosensor or having pressure on it can reduce local blood flow, which may make the CGM reading appear lower for a short time.
Normal, expected CGM and BGM variability:
CGMs and BGMs each have accuracy expectations that allow for some differences between readings.
CGMs like Lingo must have about 87% of readings within 20% or 20 mg/dL of a lab reference value. (1) BGMs have similar accuracy ranges, where 95% of readings must fall within ±15 mg/dL (below 100 mg/dL) or ±15% (above 100 mg/dL) of a lab value. (2)
Because both devices allow for this normal variability, it is expected that readings won’t match perfectly, and either device may appear slightly higher or lower at times.
Remember, CGMs are meant to show patterns, trends, and changes over time. The power of a CGM comes from seeing how your glucose responds throughout the day and over time, not from matching every fingerstick value.
References:
1. Alva S, et al. Accuracy of a 14-Day Factory-Calibrated Continuous Glucose Monitoring System With Advanced Algorithm in Pediatric and Adult Population With Diabetes. J Diabetes Sci Technol. 2022 Jan;16(1):70-77. https://pubmed.ncbi.nlm.nih.gov/32954812/
2. Jendrike N, et al. ISO 15197: 2013 Evaluation of a Blood Glucose Monitoring System's Measurement Accuracy. J Diabetes Sci Technol. 2017 Nov;11(6):1275-1276. https://pubmed.ncbi.nlm.nih.gov/28849677/