Since your readings determine your insulin doses, diet choices, and other health factors, having an accurate blood glucose meter is crucial for managing your diabetes. How Blood Glucose Accuracy is Determined: Blood glucose is measured primarily in two ways:
- Laboratory tests performed by professional healthcare providers.
- Personal blood glucose meters.
How are these tests different?
Personal meters measure your blood glucose using a sample from your fingertip (or alternate site), and then analyze the whole blood, including red blood cells and all. In a lab test performed by clinical staff, the sample is typically drawn from a vein in your arm and processed to remove red blood cells, so only the plasma is tested. Virtually all home glucose meters factor in the difference of using a whole blood sample while providing a test result that is considered plasma equivalent.
Which is more accurate?
Lab tests generally demonstrate a higher level of accuracy than personal blood glucose meters, but even these machines can have a slight margin of error. Blood glucose meter test results are not exact measures, but they’re designed to give you accurate readings for daily treatment decisions when you’re not at a doctor’s office. Without going into detail on how your circulatory system works, there are other factors that may cause the two readings to vary slightly, but typically a result from a meter that’s within 15-20% of your lab reading is considered accurate. So, if your lab result says that your blood glucose is 100 mg/dL, your meter might report your results as 85 mg/dL or 115 mg/dL and still be considered accurate.
Who determines accuracy?
ISO, also known as the International Organization for Standardization, is responsible for developing and publishing specifications for products and services. The standards set by the organization are intended to ensure quality, safety, and efficiency, so that you can be confident in your purchases. Blood glucose meters are one of the many products with ISO standards. For a system to meet the latest ISO 15197:2013 accuracy standard, 95% of blood glucose results must be within 15% (for values equal to or greater than 100 mg/dL) and 15 mg/dL (for values below 100 mg/dL). It also requires 99% of results to fall within zones A or B of the Consensus Error Grid.
Factors That Can Affect Glucose Meter Accuracy
Test Strips
When you use a test strip, the glucose in your blood interacts with enzymes on the strip. This releases electrons, creating an electrical current. The meter then converts that into a concentration of glucose to give you a reading. Because this process is so complicated, your test strips can be very sensitive to variables. Inaccurate results can come from test strips that:
- Are damaged or expired.
- Are affected by heat or moisture.
- Do not have enough blood applied.
- Are not designed for your meter.
- Have contaminants (oil, lotion, food) on the sample area.
Accuracy Tip: You should always keep your test strips in their container, stored at the temperature specified by the manufacturer. Test strip designs may vary, so it’s important to understand where the blood sample should be applied to avoid testing errors or inaccurate results. Also make sure that you’re using tests strips that have not expired and are made specifically for your meter model.
Environmental Conditions
The chemistry of your test strips, as well as your own blood flow and composition, can be affected by the conditions of your environment. An active component in the test strip chemistry, called an enzyme, can be sensitive to temperature and humidity conditions. Temperature and humidity extremes can cause the enzyme activity to decrease and affect the accuracy of results. Your body also reacts differently at different temperatures. High temperatures can make it easier for you to become dehydrated and concentrate (elevate) glucose levels, while low temperatures may affect circulation and lower blood flow to testing sites. Some studies have shown devices to over or under estimate glucose when testing at high altitudes. If this is of concern, confirm the altitude rating for your device.
Accuracy Tip: Keep test strips at room temperature and keep the container closed so your strips are protected from the elements. This means taking them with you wherever you go to avoid leaving them in a hot or cold car. If you have been exposed to colder temperatures and your hands do not feel warm, wash and hold them under warm water before taking a test to improve blood flow and reduce meter errors.
Substances on Your Skin
Anything you touch or put on your skin can end up in your blood sample and then on your test strip. Since many current meters utilize a micro-sized blood sample, just touching something that contains sugar and not washing your hands can affect your readings. If you recently washed your hands but didn’t dry them completely, the water left on your fingers can have a diluting effect on the sample, as well.
Accuracy Tip: Always wash and thoroughly dry your hands before every test.
Amount of Blood in Your Test Strip
It’s possible to have too much or not enough blood in the test strip sample area, which can lead to inaccurate results or an error reading.
Accuracy Tip: Your meter should state the required blood sample size, like 0.5 microliters, so you have an understanding of how much blood is needed for testing. Avoid squeezing directly around the lanced area when forming a blood drop. You can also experiment with different size lancets to find one that produces the amount you need.
Testing Site Location
Because of the physiological differences in the circulation of certain areas of your body, test results from alternate sites (like your forearms, calf, or thigh) are often less accurate than testing with your fingertip. This is due to the frequency with which fresh blood is circulated to each part of the body. If you have recently eaten, exercised, or taken medication that could lower your glucose, these changes may not be detected as quickly in alternate sites as in the fingertip.
Accuracy Tip: Check the instructions for your meter to understand how alternate site testing may affect your results.
Hematocrit Variations
Variations in your hematocrit — the proportion of red blood cells in your blood — can affect the accuracy of your blood glucose meter. These variances in the amount of red blood cells can interfere with the calculation of glucose. The ISO 15197:2013 standard requires performance criteria that limits the effect of varying hematocrit levels on a blood glucose meter. If the meter you’re using does not meet this criteria, you may receive inaccurate results.
Accuracy Tip: Many meters with the latest technology can detect and correct for hematocrit variations. Check the guide that came with your meter or contact the manufacturer for more details.
Meter Variances
The calibration process, enzymes, and computer programs (algorithms) in every meter on the market are different. Each brand’s manufacturing, chemistry, and calibration process can differ, even across product lines from the same company. If you’re using one meter brand/model in the morning and another at night, your readings can have significant variations.
Accuracy Tip: Stick with one meter to make it easier to interpret your glucose levels and the effect of treatment decisions.
Incorrect Coding
A meter that requires coding means you’ll need to input a code printed on each test strip vial in order to calibrate it before use. If the code doesn’t match correctly, it could lead to inaccurate readings.
Accuracy Tip: Many “No Code” meters are available now, which lets you avoid this step. But if you have a meter that still requires coding, make sure you input the code correctly to avoid inaccuracies.
How to Choose an Accurate Meter
Independent testing, like the study done by the Diabetes Technology Society, is helpful to compare the performance of top commercially available glucose meters. This study tested meters and test strips which were independently purchased from stores across the country, and representative of what a consumer would purchase. After testing the products over 300 times at three different clinical sites, the results showed that over 66% of the top meters on the market today didn’t pass the rigorous study. A surprising number of name brand meters did not pass all studies.
(Author is a Microbiologist Certified infection control Auditor, Kidney Hospital Srinagar [email protected])
