Determining the Accuracy of Your Glucose Meter

Understanding the difference between whole blood glucose and plasma glucose

Woman testing for diabetes
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If you're like most people with diabetes, you probably assume that your glucose meter gives you accurate readings every time you check your blood. You base your insulin dose, food intake, and activity plans off that number.

Fortunately, most glucose meters are well designed and give reasonably accurate test results. But there are some things you should know about your glucose meter to help you make the most educated decisions about your diabetes management.

Test Results Are Not Exact Measures

If you’ve ever taken your blood sugar twice or three times in a row without any delay in between tests, you’ve probably noticed that you don’t get the same exact number each time. That doesn’t mean your meter isn’t operating correctly. It does, though, reflect the variance that is built into each meter.

Within the medical community, home blood glucose meters are considered clinically accurate if the result is within 20 percent of what a lab test would indicate. For example, if your glucose meter result was 100 mg/dL, it could vary on the downside to 80 mg/dL or on the upside to 120 mg/dL and still be considered clinically accurate.

Your Glucose Meter Measures Blood Differently Than the Lab

All blood glucose meters use whole blood to measure glucose. Whole blood is simply a blood sample that contains the red blood cells. In a lab glucose test, only the plasma portion of the blood is used to measure glucose levels; the red blood cells are removed.

Whole blood glucose test results are approximately 12 percent lower than the lab plasma results. But there is a way to compare the lab result with your meter. Before you do that, first you need to learn more about your meter.

Your Meter Is Calibrated to Whole Blood or Plasma Blood

Though all home glucose meters measure whole blood, newer meters are designed to automatically convert the result into plasma results.

The first thing you want to find out is whether your meter is calibrated for whole blood or plasma blood.

If your meter is calibrated for whole blood, you'll need to do a simple conversion to compare your results with a lab result. To compare a lab result with a home test you must convert the lab result into its whole blood equivalent by dividing it by 1.12. For example, if your lab glucose result was 140 mg/dL, you divide 140 by 1.12 and you get 125 mg/dL. This number represents the whole blood equivalent of the lab result, which you can compare to the number on your meter.

If your glucose meter is calibrated to give a plasma result, there's no need for you to do a manual calculation. The meter does it for you. This makes it easy to do an apples-to-apples comparison of your lab test result and your glucose meter result.

Whether your glucose meter is calibrated for whole blood or plasma, you must still factor in the 20 percent variance. For example, if your lab result is 140 mg/dL, a clinically accurate reading would range from 112 on the low side and up to 168 on the high side.

Learn How Your Monitor Is Calibrated

The instructions that came with your glucose meter should tell you whether your meter is calibrated for whole blood or plasma results.

If you don't have that information available, contact the customer service division of the company that makes your glucose meter. They will be able to tell you whether the meter you have is calibrated to whole blood or plasma. If you have an older meter that measures glucose in whole blood only, some companies will gladly send you a newer meter that automatically converts your result into a plasma result at no charge or for a nominal cost.

Comparing Your Meter Result With a Lab Test

The best way to measure the accuracy of your meter is to take it with you and check your blood with it immediately after you have a lab glucose test.

Once the blood has been drawn for your lab test, prick your finger and perform a test with your meter. For the best results, request that the lab processes your blood sample within 30 minutes of drawing your blood.

New FDA Recommendations for Blood Glucose Meter Accuracy

The U.S. Food and Drug Administration (FDA) released its new recommendations on blood glucose meters saying that new meters need to be more accurate. These new accuracy standards mean that glucose meter values should be within 15 percent of the lab measurement 95 percent of the time and within 20 percent of the lab measurement 99 percent of the time.

What this means is that 19 times out of 20, glucose meters should be accurate within 15 percent of the lab value and within 20 percent of the lab value 99 out of 100 times. This should help you feel more confident about the accuracy of your meter if it's one that was made after 2016 and it has been cleared by the FDA. The recommendations don't apply to older meters.

Sources:

Joslin Diabetes Center. Plasma Glucose Meters and Whole Blood Meters. 2017.

Runge A, Brown A. FDA Publishes Final Recommendations on Blood Glucose Meter Accuracy. diaTribe. Published October 31, 2016.

U.S. Food and Drug Administration (FDA). Self-Monitoring Blood Glucose Test Systems for Over-the-Counter Use: Guidance for Industry and Food and Drug Administration Staff. Published October 11, 2016.