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Accuracy of Blood Glucose Meters

Accuracy of Blood Glucose Meters

The accuracy of blood glucose testing depends on many factors including:
  • the quality of your meter.
  • the quality of your test strips.
  • how well you do the test.
  • your hematocrit (the amount of red blood cells in the blood). If you have a high hematocrit, you may test low for blood glucose. Or, if you have a low hematocrit, you may test high for glucose. If you know your hematocrit is low or high, discuss with your health care provider how it may affect your glucose testing.
  • interfering substances (some substances, such as Vitamin C and uric acid, may interfere with your glucose testing). Check the package insert for your meter and test strips to find out what substances may affect the testing accuracy.
  • Altitude, temperature, and humidity (high altitude, low and high temperatures, and humidity can cause unpredictable effects on glucose results). Check the meter and test strip package inserts for more information. Store and handle the meter and strips according to instructions.

Choosing a Glucose Meter

You can purchase more than 25 different types of meters. They differ in several ways including:
  • amount of blood needed for each test
  • how easy it is to use
  • pain associated with using the product
  • accuracy
  • testing speed
  • overall size
  • ability to store test results in memory
  • cost of the meter
  • cost of the test strips used
  • doctor's recommendation
  • technical support provided by the manufacturer
  • special features such as automatic timing, error codes, large display screen, or spoken instructions or results
Talk to your health care practitioner about glucose meters and how to use them. If you have insurance, you can usually get the glucose meter for free from your doctor. Also, you may want to call your insurance provider to find out which meters they support.

You can get information about your meter and test strips from several different sources including the toll free number in the user manual or the manufacturer's web site. If you have an urgent problem, always contact your healthcare provider or a local emergency room for advice.

How do you compare your home test glucose values with the laboratory values? Most home blood glucose meters in the U.S. measure glucose in whole blood. Most lab tests, in contrast, measure glucose in plasma. Plasma is blood without the cells.

A lab test of your blood glucose will be about 10-15% higher than the value given by your meter. Look at the instructions for your meter to find out if it gives its results as "whole blood" or "plasma equivalent." Many meters now sold give values that are "plasma equivalent," which means they can be compared more directly to lab test values.

How accurate are the home blood glucose meters? Although meters are a lot more accurate today, they may not be as accurate as you think for the following reasons:
1) The meter itself has an accuracy of about +/- 10 points
2) The control liquid is not designed to one specific number, so any particular bottle of solution may fit anywhere within that range.
3) The control liquid has an expiration date. If the bottle is close to that date, the number will be slightly different. If the bottle is expired, then the reading could be totally wrong.
4) Since all control liquids are not the same, use only the brand of control liquid sold by the manufacturer of the meter. Do not mix manufacturers.
5) The test strips have expiration dates. if the test strips are out of date, their readings will be wrong, too.

In addition, different site testing will also cause readings to vary. For example, arm testing tends to provide lower readings than your finger.

There so many variables between meters that your "method" of testing with several different meters would be judged a wrong thing to do.

Get one meter, and stick with it. If you use another meter (for alternate site testing), do not try to compare the readings between meters!

Watch for inconsistent results. If you get poor results, try strips made or recommended by the maker of your meter until you again get consistent results.

Use electronic checks. Every time you turn on your meter, it does an electronic check. If it detects a problem it will give you an error code. Look in your owner's manual to see what the error codes mean and how to fix the problem.

Compare your meter with a laboratory meter. Take your meter with you to your next appointment with your health care provider. Ask your provider to watch your technique to make sure you are using the meter correctly. Ask your healthcare provider have your blood tested with a routine laboratory method. If the values you obtain on your glucose meter match the laboratory values, then your meter is working well and you are using good technique.

What should you do if your meter malfunctions? If your meter malfunctions, you should tell your health care professional and the company that made your meter and strips.

Can you test blood glucose from sites other than your fingers? Some new meters allow you to test blood from the base of your thumb, upper arm, forearm, thigh, or calf. If your glucose changes rapidly, these other sites may not give you accurate results. You should probably use your fingers for consistent testing.

Useful Tips to Increase Accuracy and Reduce Errors in Test Results from Glucose Meters

Have you ever wondered why you got a bad glucose meter test result when there is nothing obvious wrong with your meter, your test strips are new, and you’ve been running glucose tests for years? The simple answer is that glucose meters are not perfect, and neither are the people who use them! This chart lists some tips to help you get the most accurate results from your glucose meter.
Make sure you...
  • Follow the user instructions about sample size. Repeat the test if you have any doubt that enough blood was added.
If there is insufficient blood on the test strip, the meter may not be able to read the glucose level accurately. Although many meters are designed to alert you when the sample size is too small, some meters detect only large errors. There have been cases where meters have displayed glucose levels that were less than half the actual levels without displaying error messages.
  • Insert the test strip completely into the meter guides.
When a test strip is not fully inserted into the meter, the meter cannot read the entire strip area. Many meters are designed to detect strip placement errors and will not provide a result. But, just as described above, many meters detect only large problems. There have been cases where meters have displayed glucose levels that were significantly higher or lower than the actual levels when there was only a small error in strip placement.
  • Keep the meter clean.
Even small amounts of blood, grease, or dirt on a meter’s lens can alter the reading.
  • Check the test strip package to make sure the strips are compatible with your meter.
Test strips are not always interchangeable, and meters cannot always detect incompatible strips. Test strips that look alike may have different chemical coatings. Small variations in strip dimensions can also affect results.
  • Check the expiration date on the test strips.
As a test strip ages, its chemical coating breaks down. If the strip is used after this time, it may give inaccurate results.
  • Enter the correct calibration code from the outside of the strip bottle each time you run a test (if applicable).
Results can vary significantly between manufactured lots of reagent strips; the calibration codes help the meter compensate for these variations.
  • Run quality control as directed.
Running quality control is typically the only way to know when test strips have gone bad. Test strips do not always last until the expiration date on the bottle. This may be because the manufacturer has over-estimated the dating or because the cap was not replaced promptly after use.
  • Check the results from your meter against laboratory results as often as possible.
Over time, test systems can drift apart. Since results from either test system maybe used to treat your patients, it is important for the systems to remain synchronized.
  • Question results that are not consistent with physical symptoms. If a test result seems wrong, have a blood sample tested by the main laboratory.
There may be many reasons why a test result is incorrect. In addition to the items above, some physiological conditions such as dehydration, hyperosmolarity, high hematocrit, or shock may significantly affect test results.

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Bottom line: Test, don't guess.


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