What is the Standard for Determining Blood Glucose Meter Accuracy?

The International Organization for Standardization (ISO) determines specifications for various products, including blood glucose meters (BGMs).¹ BGMs need to fulfill certain criteria to meet its latest standard of ISO 15197:2013, listed below:

• For blood glucose levels >100 mg/dL, 95% of data pairs (ie, reference measurement and BGM measurement) must be within 15%
• For blood glucose levels <100 mg/dL, 95% of data pairs must be within 15 mg/dL
• 99% of blood glucose results must fall within the Consensus Error Grid’s zones A or B

 The ISO 15197:2013 standard is not used by the U.S. Food and Drug Administration (FDA) to clear BGMs for over-the-counter use.^2,3 However, the FDA has developed a standard similar to that of ISO. For all blood glucose levels,

• 95% of data pairs must be within 15%
• 99% of data pairs must be within 20%

Why Might Two Blood Glucose Meters Show Different Readings?

Two BGMs that meet the ISO or FDA standard can seem to have vastly different readings and still be considered accurate. This is because of the allowance of either 15% or 15 mg/dL deviation in measurement when compared to the reference value.

As a simplified example, if a lab BGM measures the true value of blood glucose level to be 150 mg/dL, and BGM “A” shows a reading of 130 mg/dL (<15% below the true value) while BGM “B” shows a reading of 170 mg/dL (<15% above the true value), both meters A and B are still considered accurate. This is because both meter A and B’s measurements are within 15% of lab value and meet the ISO and FDA standard of accuracy.

This discrepancy may be raised as a concern for consumers starting to use a new device, alarmed at the difference between the new product and the product they have been using in the past. They may be led to think that the new BGM is either less accurate or even defective. Since different BGMs may show different but accurate readings and cause unnecessary confusion, individuals may benefit from using one accurate BGM consistently.⁴ One meter should not be compared to another meter in a home setting.

What Other Factors Affect the Accuracy of Blood Glucose Meters?

As BGMs are complex devices with many components, individuals should be informed of various other factors that can affect the accuracy of BGMs.⁴

1. Test Strip Problems – Test strips contain the enzyme glucose oxidase that reacts with the glucose in the blood sample, which creates an electric signal when the strip is inserted into the meter, generating an electrical current that is read by the BGM.⁴ The complex process is affected by sample well size, enzyme coverage, mediator reduction, among others, which can be influenced by factors such as duration and technique of storage. To ensure the most accurate reading, always make sure to follow proper storage instructions, and do not use expired strips.

2. Extreme Temperatures and Altitudes – Because complex biochemical reactions take place in the test strips, high temperature or humidity can affect the accuracy.⁴ BGM and test strips should be stored in dry places and at room temperature. The strips are also sensitive to oxygen concentration, and the results may vary based on altitude, with the value usually being overestimated at lower oxygen concentration and underestimated at higher oxygen concentration.

3. Hematocrit Variation – Because red blood cells carry significantly more intracellular glucose than other cells, hematocrit (percentage of red blood cells in the blood) variation – caused by dehydration or anemia, among other factors – can drastically affect blood glucose measurements, even though most BGMs make calibrations to adjust for this factor.⁴

4. Testing Site Location – The most common testing site is the fingertip as its high capillary flow and density provides rich blood supply.⁵ Individuals may choose to obtain blood from other parts of the body such as the palm, upper forearm, thigh, calf, and abdomen. However, especially in cases of rapid changes in blood glucose levels, alternate sites may be more delayed in reflecting those changes than the fingertip and can affect the test results.

5. Other Substances on Testing Site – Even the smallest amount of foreign substance (eg, dirt, alcohol, food residues, etc.) on the testing site can drastically increase the blood glucose levels detected by a BGM.⁴ It is thus important to practice good hand hygiene especially when taking blood glucose measurements.

6. Insufficient Amount of Blood Applied to Test Strip – Sufficient amount of blood must be used for BGMs to provide an accurate reading.⁶ When low blood volume (eg, < 3 ml) is applied, BGMs may give a misleading result that underestimates the actual value, even with the built-in detection of insufficient blood sample. A generous drop of blood should be applied to the test strip in measuring blood glucose levels.