CGM vs Fingerstick Accuracy: How Close Should the Values Be?

The Importance of Trend Arrows

Knowing the direction your blood sugars are heading is just as important as knowing what your blood sugar numbers are. We can’t emphasize this enough – it’s SO IMPORTANT to look at your CGM trend arrows!! 

When a trend arrow is diagonally up, your blood sugar is typically rising at 1-2 mg/dL per minute. One arrow straight up is rising at 2-3 mg/dL per minute, and two arrows up is rising at over 3 mg/dL per minute. So if you do not take the trend arrow into account, you cannot compare the values!

The above scenario where a CGM value reads 65 mg/dL and the fingerstick value says 85 mg/dL is a perfect example of when it looks like the values are way off. However, if the 65 mg/dL value on the CGM had a trend arrow diagonally up or straight up, it might be very accurate. 

The chart below shows how you can use trend arrows to determine how quickly your blood glucose is changing.

How Does a CGM Measure Glucose Compared to a Fingerstick?

CGMs and fingersticks measure sugar concentrations from different sources in the body.

Fingersticks measure glucose in the capillary blood, which reflects close to real-time glucose levels (a real-time value is when your blood is drawn at a lab). CGMs use a tiny sensor inserted under the skin to measure glucose levels in the interstitial fluid, which is a thin layer of fluid (not blood) that surrounds the cells of the tissues below your skin.

How Accurate Are CGMs Required to Be Compared to Blood Glucose Meters?

The national standards for CGM accuracy in the United States are largely based on guidelines and performance criteria from the FDA, the International Organization for Standardization (ISO), and professional organizations like the American Diabetes Association. 

While there’s no single unified U.S.-specific CGM accuracy standard, CGMs must meet certain international and regulatory benchmarks to be approved and trusted for clinical use.

Most people use the following national standards:

• For glucose values less than 100 mg/dL: the CGM value should be within 15 mg/dL of the blood glucose meter number. For example, if the blood glucose meter number is 90 mg/dL, then the CGM value should be between 75 and 105 mg/dL.
• For glucose values greater than 100 mg/dL: the CGM value should be within 15% of the meter value. For example, if the blood glucose meter value is 180 mg/dL, then the CGM value should be between 153 and 207 mg/dL. 

So you really need to control your expectations. Before you complain to your doctor that your values are different, use the above formula and know that based on the ISO national standards, there’s an acceptable degree of error that would not alter clinical decisions. 

Is It Normal for CGM and Fingerstick Values to Be Different?

Yes. Once you’re beyond ISO standards, there are additional reasons why CGM and fingerstick values can be different. Besides the difference in how values are measured (capillary blood vs interstitial fluid), other factors can come into play.

CGMs can have variances due to:

• Sensor lag time (see below)
• Sensor placement
• Compression (e.g., lying on the sensor while sleeping)
• Sensor age (the difference in values can be greater during the first 24 hours of a new sensor)
• A defective sensor
• Environmental factors like temperature (check out our article on diabetes device hacks that discusses false highs from heat)

Nothing irks me more than when CGM and fingerstick numbers are way off, especially when I’m getting automatic insulin delivery. If I’m wearing an AID system and the sensor number is falsely high (due to a hot bath, for example), I’m going to get inappropriate insulin delivery that could cause a low blood sugar.

Fingerstick accuracy can be affected by:

• Hand cleanliness (were your hands clean and dry before you tested?)
• Insufficient blood sample
• Test strip quality (were the strips exposed to air for an extended period, or were they expired?)
• Environmental factors like temperature 

CGM Accuracy and Sensor Lag Time

Interstitial fluid lags behind blood glucose by about 5 to 20 minutes (depending on the CGM), and it can be even more pronounced if your blood sugar is rising or falling quickly (like after you eat or during exercise). A CGM may show a lower value during a rise or a higher value during a drop compared to a fingerstick.

For example, after eating a high-carb meal, fingerstick glucose might read 180 mg/dL, while the CGM could still show 150 mg/dL. This isn’t a malfunction — it’s simply the interstitial fluid “catching up” to the blood glucose.

It’s more dangerous when your blood sugars are dropping rapidly. Your CGM may say 70 mg/dL, but you could really be 55 mg/dL depending on your trend arrows – whether you have one arrow down, two arrows down, or a diagonal arrow down. 

So Which One is “Right”? 

As I often say…it’s like adult diapers. It Depends.

Fingerstick readings are typically considered the gold standard for real-time blood glucose and are generally used for dosing decisions in situations where your symptoms don’t match your CGM value.

CGMs, while slightly lagged, provide a broader picture of glucose patterns and trends. As we mentioned earlier, one of the key advantages of CGMs are the trend arrows, which provides context beyond a single number at a single moment in time.

How to Dose Insulin When Blood Sugar Values Don’t Match

If your CGM and fingerstick values are significantly different and you’re worried about the correct insulin dose to give, err on the side of caution and use the blood glucose value that calls for the lower dose of insulin.

The Benefits of CGM for People with Type 2 Diabetes

CGMs can help you learn how lifestyle factors like food, meal timing, exercise, sleep, stress, and alcohol can impact blood sugar. They’re an invaluable tool for anyone living with any type of diabetes. 

In a recent study, CGM use in people with type 2 diabetes resulted in a larger reduction in A1c and improved time in range.

Currently you need to be on at least one shot of insulin per day for CGMs to be covered by insurance or Medicare, but we hope that requirement will change soon.

Dexcom and Abbott recently developed CGMs specifically for people with type 2 diabetes who are not on insulin. Dexcom’s Stelo is available to purchase online without a prescription, and Abbott’s Rio should be coming out soon. For information on Stelo CGM costs, visit Stelo.com.