With over 20 million patients with diabetes in the United States, many of these patients must rely on monitoring their blood glucose at home to ensure that they are staying within their target goals. Unfortunately, too often patients receive inaccurate glucose readings due to a variety of factors. This article will focus on some common causes of inaccuracies with glucometer results, including environmental factors, hematocrit issues, sampling errors, and testing interferences.
Test strips that are stored in vials, as opposed to strips that are individually wrapped in foil, are susceptible to the effects of oxygen and moisture. Several small studies that reviewed the effects of glucose test strips left open in their vial for times ranging from 2 to 18 hours showed variances in blood sugar results from 9% higher to 60% lower than the actual results. A study in Diabetes by Lilavivat et al, in 2002, reported varying results for one patient between 94 and 307 mg/dL over the course of a few weeks. It was determined that the patient's test strips had been exposed to moisture. These factors must not be overlooked, given that many patients will keep their testing supplies in a bathroom or kitchen environment. Make sure that your patients understand to always keep their test strip vial closed when not in use.
Heat exposure can also be a factor for vials of test strips as well as the individually foil-wrapped test strips. This is particularly a consideration for patients who use mail order as well as patients living in warmer, humid climates who may not use a source of air conditioning in their home. Counsel patients to make sure that they do not leave their test strips in the mailing box for extended periods of time if they order by mail and to use insulated cool packs if needed when in the home or when traveling. If they notice any acute changes in their glycemic control, such as erratic ranges, consider erroneous readings as a possible cause.
Although most glucometers now convert blood samples to plasma measurements, the actual measurement is made with whole blood samples. Therefore abnormal hematocrit (Hct) values lead to inaccurate readings. Recall that low Hct levels occur commonly in pregnant or elderly patients or those with anemia, renal disease, and some types of cancer. High Hct levels can occur during dehydration, for example. The errors that occur with glucometer testing will usually be a falsely low reading with a high Hct sample and a falsely high reading with a low Hct sample. It is sometimes hard to determine the Hct range at which a particular glucometer will still be accurate without reading the fine print. For those patients who have abnormal Hct issues, however, help them find a meter that will read accurately within a wide range of Hct (ie, 15%-65%).
Errors in Blood Sampling
One common cause of erroneous blood glucose readings can be short sampling, in other words, inadequately covering or filling the test strip with blood. While some glucometers are designed to give an error message should this occur, others may give false low or high readings with inadequate blood samples. Remind patients that while many meters are designed to "beep" when the testing starts, this is only an indication that the testing has begun, not necessarily that enough blood was applied to the test strip. They always need to look at the test window or pad to confirm that there is sufficient blood applied. If a patient continues to have a problem with short sampling, verify that the testing technique is appropriate. If so, then consider a larger-gauge lancet or a meter that requires a smaller sample size.
Elevated levels of certain medications or biological components can cause inaccuracies in blood glucose testing. Some common medications include acetaminophen, ascorbic acid, or salicylic acid. Biological components include elevated uric acid, bilirubin, or triglycerides.
In addition to educating patients on the possible causes of erroneous blood glucose readings mentioned above, pharmacists can perform a quick 4-step check with a patient's glucometer and test strips:
Dr. Brian is a clinical specialist with Cornerstone Health Care, High Point, NC.
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