Comparing Meter to Lab

Author: Judy Kohn, RN, BSN, CDE

Last Updated: Friday, April 11, 2008

Q: "I was told that you shouldn’t compare your meter to another meter, and that the best way to check your meter is with the lab. Please explain why and how to do that."

A: Your question is a common one, and there are many misunderstandings about meter-to-meter or meter-to-lab comparisons. First I'll explain some of the potential variables, and then I'll explain how to compare your meter to the lab.

Common misconceptions or mistakes:

• Comparing a meter to another meter (such as comparing your meter to the meter at your doctor's office) is not recommended because neither is considered a reference source, although I realize this is a common and quick way to get an idea if your readings are similar. Technically only a laboratory test is considered the official reference standard.
  • You might compound this problem if you were comparing your FreeStyle® system or Precision® system, which are plasma calibrated, to another meter that is whole blood calibrated.¹ Because laboratories provide plasma glucose results, most meters currently on the market are also plasma calibrated, so that they will be similar to the laboratory reading.
  • Unlike the FreeStyle and Precision systems which are minimally affected by interfering substances, some other meter systems are affected by other interferences, which could add another variable if you tried to compare your meter to another meter.
• Allowing too much time to elapse between your meter test and the laboratory test:
  • Even though you might not have eaten, your glucose can change due to stress, activity, and especially due to the early morning rise in blood glucose, called the dawn phenomenon. In fact, frequently asked questions are not only: “Why is my glucose higher in the morning than when I went to bed, even though I haven't eaten?”-but also: “I checked my glucose at home and it was 150. An hour later at the lab, it was 200-how can that be?”
  • Your glucose could rise further if you are rushing to get to the lab, stuck in traffic, or feeling stressed as you wait in line to have your blood drawn at the lab.
• Eating shortly before having your blood drawn at the lab: Studies show that after a meal, the glucose in the fingertips can be 20-70 mg higher than the glucose drawn from the vein. Because of the way glucose circulates throughout the body, it is recommended that when you compare your meter to the lab, you should either be fasting in the morning, or do not eat for at least 4 hours before the lab test. Also perform a meter test right after the lab test, and no more than 15 minutes later.
• Assuming that your meter result should be exactly the same as the lab result: It is normal for blood glucose test results to vary by as much as 20% when comparing a meter reading with the laboratory. That is not to say that 20% variance is routine, but rather that it may occur and is considered acceptable by industry standard. Certain factors can contribute to an expected variation in test results.
  • The cleanliness of the test site-If you have touched any food, the glucose in the food could affect your meter result.
  • Using an alternative site: It is important to remember to rub the test site vigorously just before lancing, to minimize any possible differences between the alternative site and the finger and/or lab result.
  • Insuring that the Code set in the meter matches the Code printed on the vial of test strips if applicable, (not all meter systems require coding).
  • Making sure not to apply blood to both sides of the test strip as opposed to one side of the test strip.
  • Proper storage of the test strips: Always store the test strips in the vial with the cap firmly in place and make sure they are used before the expiration date.
  • Strip variability: Strips vary slightly in such factors as their internal blood volume, the rate at which they fill with blood, and the amount of reagent (enzyme and mediator) deposited inside the strip. Each factor can have a small effect on the glucose reading.
  • Lab variation: Although small, there can be a variation between different laboratories-approximately 5-10%.

So how can you verify that your meter is working correctly?

2 ways-using control solution, or getting a laboratory blood glucose

Control Solution:

Follow the instructions in your meter's owner's booklet.

Comparing your meter to the laboratory:

I recommend that each time you have a lab appointment for your routine diabetes tests (usually 2 to 4 times a year), check your meter at the same time. Here is the suggested procedure:

1. First make sure that your doctor ordered a blood glucose test on the lab slip. The lab cannot automatically perform a test that you request unless your doctor ordered it.
2. Don't eat for at least 4 hours before the lab test.
3. Be sure that both samples are done at the same time (no more than 10 minutes between these two tests).
4. Circle your glucose reading in your logbook to remind you of this result, since you will not be able to get an immediate result from the laboratory.
5. Follow up with your diabetes team to obtain the laboratory glucose result and compare it to your meter result. The two results should be within 20% of each other.
6. Should you have any questions, remember that the Abbott Diabetes Care customer service department is available 24-hours a day, 7-days a week at: 1-888-522-5226.

¹ The FreeStyle® system is plasma calibrated for the U.S. market. In some countries, the FreeStyle system is whole blood calibrated. If you purchased your FreeStyle system internationally, you should refer to your owner's manual for meter specification on whether your FreeStyle system is whole blood or plasma calibrated.

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DOC09465-Rev-B 04/08