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How Accurate Is Your Glucose Meter?

Understanding the difference between whole blood glucose and plasma glucose

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Updated June 04, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Woman testing for diabetes
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If you are like most people with type 1 diabetes you 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.

Your test results are not exact measures of your glucose

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% of what a lab test would indicate. For example, if your glucose meter result was 100 mg/dL, it could vary on the down side to 80 mg/dL or on the up side to 120 mg/dL and still be considered clinically accurate.

Your glucose meter measures your blood differently than the lab does

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

Your meter is calibrated to either 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 must 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 that number 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 is 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 and glucose meter result.

Whether your glucose meter is calibrated for whole blood or plasma you must still factor in the 20% 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 glucose meter 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 do not 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 check your blood immediately after you have a lab glucose test. Once 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 process your blood sample within 30 minutes of drawing your blood.

Sources:

Joslin Diabetes Center. "Plasma Glucose Meters and Whole Blood Meters.”

Lifescan, Inc."Is My Blood Glucose Meter Accurate? "

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