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March 12, 2010
Two things landed on my desk recently. One was a newspaper article saying that the Food and Drug Administration has asked the international body that sets standards for home blood glucose meters to tighten its accuracy requirements. (A public meeting on the topic is scheduled for March 16 and 17.) The other was a letter from a Diabetes Self-Management subscriber who described how she had attempted to compare two home meters with a plasma glucose measurement done in a laboratory — and how she was unnerved when she got three different readings.
Stricter standards for meter accuracy would be a welcome development: Currently, most available blood glucose meters are accurate only to within 10% to 15% of the actual blood glucose level. But it will likely be a while before standards are changed and more accurate meters become the norm.
In the meantime, there are steps you can take to get the best possible results from your meter. One of the most important is to use only strips made for your meter and to use them correctly. This includes making sure the strips have not passed their expiration date, coding your meter for each new batch of strips (if you use a meter that requires coding), and being careful to store your strips in their original container, away from sources of heat, cold, or humidity. Improperly used or stored strips are one of the biggest sources of error in home blood glucose monitoring. To check whether a batch of strips is OK, use a drop of the control solution (unexpired!) that’s compatible with your meter on one of the strips in the batch.
If you still want to check the accuracy of your meter by comparing it to a laboratory measurement, there is a way to do that. But getting meaningful results requires doing it right — both on your end and on the laboratory’s.
To explain how to do it right, I asked diabetes nurse specialist and DSM Editorial Board member Virginia Peragallo-Dittko to update some instructions she’d written for the magazine back in 2000, and here are the results:
Comparing Your Meter With the Lab
The best way to determine whether your meter readings are accurate is to test them against the results you get from a laboratory. The advanced equipment in a lab allows technicians to make exact measurements. Don’t bother comparing your meter to another home meter; neither system is considered a laboratory standard. If your self-monitoring results match the ones you get from the lab, you’ll know they’re accurate. If not, consult your diabetes educator, who can help you figure out what the problem is.
To compare meter and lab results, the two tests must be done at the same time. In addition, you must do the comparison with a fasting blood glucose sample. After-meal readings will differ between capillary blood (used by your meter) and venous blood (used in the lab).
Before any blood is drawn, if you use a meter that needs to be coded to the numbered lot of strips, double-check that the code is correct. In addition, use a drop of control solution on your meter’s test strip so you are sure that your test strips are not damaged.
Once you and the technician are ready, perform a fingerstick check just prior to having blood drawn from your arm. (Doing the fingerstick after the blood is drawn from your arm is impractical since you may have to hold gauze to your arm after the venipuncture.) Doing a fingerstick at home either before or after the venipuncture allows too much time between the readings for a valid comparison.
Use a blood sample from your fingertip only for your meter reading. Do not use blood from an alternative site such as your forearm, thigh, palm, or upper arm. Meter readings from these sites are not always identical to fingertip readings. Similarly, if the lab technician offers to place a drop of blood from the venipuncture needle onto your meter strip, don’t accept the offer. Some strips are designed for capillary blood only and will give false results if the venous blood is used.
For the purposes of comparison, the venous blood that was collected into a tube must be spun by a centrifuge machine to separate the red blood cells from the plasma within 30 minutes after the time the blood sample was taken. If it is not, the glucose in the blood will begin to break down, and the results will not accurately reflect the blood glucose levels at the time the sample was collected. (Note, however, that even if the sample is not centrifuged within 30 minutes, the results are still good enough to diagnose diabetes or to alert your health-care provider to a problem with your diabetes control.)
Although you have no direct control over when the specimen is centrifuged, you can ask the lab technician about the time frame and request that it be done within 30 minutes. Explain that you are trying to assess the accuracy of your meter and that centrifuging within 30 minutes is a critical component of making that comparison. If the lab can assure you that centrifuging will be done within 30 minutes of the blood draw, you will have a valid comparison. If it cannot, you should not bother to compare the two results.
Assuming you get the necessary assurance from the lab, be sure to write down your meter reading and flag it somehow so you know which meter reading to compare to your lab result when you get it. Under current standards, your blood glucose meter is considered accurate if its results fall within 20% of the laboratory results.
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