A false high glucose reading occurs when a blood glucose meter or continuous glucose monitor (CGM) suggests an elevated sugar level (hyperglycemia), while the person’s actual blood sugar is normal or low. This discrepancy is concerning because it can lead to inappropriate medical decisions, such as taking an unnecessary dose of insulin. Numerous factors unrelated to the body’s metabolic state can artificially inflate these readings. Understanding these causes is necessary for accurate self-monitoring and effective diabetes management.
Errors Related to Testing Equipment and Supplies
The physical components used for testing are a frequent source of falsely elevated glucose results. Test strips contain specialized enzymes that are sensitive to environmental conditions. If the test strip vial is not closed properly, the enzymes can degrade prematurely due to exposure to air and moisture, often resulting in an inaccurate measurement. Using strips past their expiration date also risks enzyme degradation, which can lead to unreliable, artificially high results.
Storing the meter or strips in extreme ambient temperatures, such as a hot car or near a heater, can damage the chemical structure of the strips, causing them to malfunction. While many modern meters are designed to be self-calibrating, older devices may require a manual code that must match the strip batch. An incorrect code can introduce a significant percentage of error into the final glucose value.
Improper Sample Collection Techniques
Procedural errors during the collection of the blood sample are among the most common causes of false high readings. The most frequent mistake is failing to thoroughly wash and dry hands before pricking the finger. Trace amounts of residual sugar on the skin from handling food, such as fruit or sugary drinks, can mix with the blood droplet and directly contaminate the sample, leading to a drastically inflated result.
Using an alcohol wipe or hand sanitizer immediately before testing can also skew results if the site is not allowed to dry completely. The residue or moisture can interfere with the chemical reaction on the test strip, though contamination with external sugar remains the greater risk for a false high reading. Forcefully squeezing or “milking” the finger to produce a larger blood drop can pull interstitial fluid—the fluid surrounding the cells—into the sample. This dilution of the capillary blood skews the reading, making the result unreliable.
Medications and External Chemical Interference
Certain substances, whether ingested or circulating in the body, can chemically interfere with the glucose meter’s measurement process, distinct from raising the body’s actual glucose level. Most meters use an electrochemical method where glucose reacts with enzymes on the strip to generate an electrical current. High doses of Vitamin C (ascorbic acid) are a known culprit because this compound can react similarly to glucose in the meter’s sensor. This chemical cross-reactivity causes the meter to register an artificially higher current, which it incorrectly interprets as a high glucose concentration.
Acetaminophen can also cause falsely elevated readings in some glucose meters and most continuous glucose monitors (CGMs). Other less common medications, such as the cancer drug hydroxyurea or high doses of dopamine, have also been documented to interfere with the chemical measurement, leading to overestimations of true blood sugar.
Temporary Physiological States
Internal body conditions that temporarily alter the blood’s concentration or composition can lead to inaccurate readings. Severe dehydration concentrates the blood, meaning the proportion of sugar relative to the overall plasma volume can appear higher than expected. This leads to a less accurate measurement that is higher than the true value.
The blood’s hematocrit level, which is the volume percentage of red blood cells, also affects the meter’s electrical current measurement. In some meters, a low hematocrit level, often associated with anemia, can cause the device to overestimate the glucose concentration in the sample, resulting in a false high reading.
A state of acute physical or emotional stress triggers the release of hormones like cortisol and adrenaline. These hormones are designed to raise actual blood sugar levels for a “fight or flight” response. This physiological change contributes to a genuinely higher reading that is temporary and not related to chronic hyperglycemia.