Variations in blood sugar readings from a home glucose meter are common and can be confusing for anyone managing their health. A glucometer is a small, portable device designed to measure the amount of glucose in a small capillary blood sample, which is essential for daily diabetes management. While these devices are manufactured for reliability, they are not perfect laboratory instruments, and various factors can cause readings to fluctuate. Understanding the expected range of accuracy, the impact of testing methods, and internal body conditions can help explain why your meter gives different results.
Understanding Expected Variability
Home glucose meters are not expected to deliver the exact same result as a sophisticated laboratory test. They are designed to provide a result that is sufficiently accurate for making daily treatment decisions. The International Organization for Standardization (ISO) sets the benchmark for accuracy that meters must meet to be sold. This standard requires that 95% of readings must fall within a specific range of a lab reference value.
For glucose levels at or above 100 mg/dL, the meter reading must be within 15% of the lab result. For example, if a lab test shows 150 mg/dL, a home meter reading between 127.5 mg/dL and 172.5 mg/dL is considered accurate. At levels below 100 mg/dL, the standard requires the reading to be within 15 mg/dL of the lab value. Therefore, two sequential tests on the same finger, or tests using two different compliant meters, can expectedly show a difference of 10 to 20 points and still be functioning correctly.
Errors Related to Testing Technique
The way a blood sample is collected is a major source of variability that the user can control. Contaminants on the skin are a frequent issue, as residue from fruit, lotions, or hand sanitizer can mix with the blood sample and falsely elevate the reading. Therefore, washing hands thoroughly with soap and water and drying them completely before lancing the finger is an effective measure to ensure accuracy.
The quality and size of the blood drop also significantly influence the result. Squeezing the finger too hard after the prick can dilute the blood sample with interstitial fluid, leading to an artificially lowered glucose reading. Instead, a large, naturally formed drop of blood should be allowed to wick onto the test strip without smearing or reapplication. Readings taken from alternate sites, such as the forearm or palm, may not be as accurate as a fingertip sample, especially when blood sugar is rapidly changing, because glucose concentration takes longer to equalize in those areas.
Equipment and Test Strip Malfunctions
The consumables and the meter itself can introduce errors separate from the user’s technique. Test strips contain enzymes that react with the glucose in the blood to generate a reading, and these are sensitive to environmental conditions. Exposure to heat, humidity, or moisture can degrade the enzymes, causing the strip to provide an inaccurate result.
Test strips must be stored in their original, tightly closed vial away from temperature extremes and never used past their expiration date. Another equipment error relates to the meter’s calibration, which some models still require the user to ensure the code on the meter matches the code on the new strip vial. A mismatch will cause systematic errors across all readings. If doubt remains about the meter or strips, a liquid control solution can be applied to the strip to verify that the system is operating within the manufacturer’s accepted range.
Biological and Physiological Factors
Sometimes, a genuine discrepancy in two readings is not an error, but a reflection of a true physiological change within the body. Blood sugar levels can change rapidly after a meal, during exercise, or in response to stress, meaning a 10- or 15-minute gap between tests can show a significant, real difference.
The concentration of red blood cells in the blood, known as hematocrit, also affects meter accuracy. Extreme low or high hematocrit levels can interfere with the electrochemical reaction on the test strip, potentially causing a falsely high reading if hematocrit is low or a falsely low reading if it is high. Modern meters are designed to compensate for some of this variability, but significant changes can still affect the result.
Severe dehydration concentrates the blood, which can lead to an artificially elevated glucose reading even if technique is flawless. Certain medications can also interfere with the chemical sensors used in some test strips. High doses of common substances like Vitamin C (ascorbic acid) or acetaminophen can interact with the strip’s enzyme and produce an inaccurate result. For example, some older meters using a specific enzyme type may report a falsely low glucose reading in the presence of high-dose Vitamin C.