Blood glucose is measured in several ways, from a simple finger-prick test that takes seconds to lab-based blood draws that diagnose diabetes. The method depends on the goal: a quick snapshot of your current level, a multi-hour challenge test, or a long-term average. Each approach relies on different chemistry and gives you different information.
Finger-Prick Meters: The Most Common Method
A standard blood glucose meter (glucometer) works by placing a small drop of blood on a disposable test strip, which contains an enzyme that reacts with glucose. The two main enzymes used are glucose oxidase and glucose dehydrogenase. When glucose in your blood contacts the enzyme, a chemical reaction produces a tiny electrical current. The meter reads that current and converts it into a number, displayed in mg/dL or mmol/L, within about five seconds.
The accuracy standard for these meters, set by ISO 15197, requires that at least 95% of readings fall within 15 mg/dL of a laboratory reference value when blood sugar is below 100 mg/dL, and within 15% when blood sugar is at or above 100 mg/dL. That’s good enough for day-to-day management, but it means two readings taken moments apart can differ slightly. Strips made with glucose dehydrogenase can cross-react with maltose, which matters for people on peritoneal dialysis who have maltose in their blood from the dialysis fluid.
Continuous Glucose Monitors
Continuous glucose monitors (CGMs) use a hair-thin sensor inserted just under the skin, typically on the abdomen or the back of the upper arm. Instead of measuring blood directly, the sensor sits in the interstitial fluid, the thin layer of liquid surrounding your cells. Glucose moves from your bloodstream into this fluid, where the sensor detects it electrochemically, similar to how a test strip works but on a continuous basis.
Because glucose has to travel from blood into interstitial fluid before the sensor can detect it, there’s a built-in time lag. Studies report this delay ranges from 5 to 25 minutes, depending on the individual and conditions. The lag matters most when blood sugar is changing rapidly, like after a meal or during exercise. A CGM might show a lower number than a finger-prick meter during a fast rise, or a higher number during a rapid drop. Modern CGMs use algorithms to partially correct for this delay, but it’s worth understanding that the two measurements aren’t sampling the same fluid.
Substances That Affect Sensor Accuracy
Several common medications and supplements can throw off CGM readings. High doses of acetaminophen (Tylenol) can falsely raise readings on Dexcom G6, G7, and Medtronic Guardian sensors. High-dose vitamin C can do the same on FreeStyle Libre sensors. Hydroxyurea, a medication used for sickle cell disease and some cancers, can cause falsely elevated readings on Dexcom devices, creating a risk of missed low blood sugar episodes.
Environmental factors also play a role. Cold temperatures slow down the enzymatic reactions in both test strips and CGM sensors. At high altitudes, lower oxygen levels tend to cause glucose oxidase-based meters to overestimate blood sugar. Glucose dehydrogenase-based meters perform somewhat better in those conditions. Of all environmental factors, cold appears to cause more measurement problems than altitude alone, though modern meters stay reasonably accurate as long as they’re kept above freezing.
The Oral Glucose Tolerance Test
An oral glucose tolerance test (OGTT) measures how efficiently your body processes a large dose of sugar. You fast overnight, then drink a syrupy glucose solution and have your blood drawn at set intervals. For screening type 2 diabetes or prediabetes, the solution contains 75 grams of sugar, and blood is drawn at one hour and two hours after drinking it.
Gestational diabetes screening during pregnancy uses a slightly different approach. The initial screening test uses a 50-gram solution with a single blood draw at one hour. If that result is elevated, a follow-up three-hour test uses 100 grams of sugar with blood draws at one, two, and three hours. A less common alternative is a single two-hour test using 75 grams.
A two-hour reading of 140 to 199 mg/dL on the 75-gram test indicates prediabetes. A result of 200 mg/dL or higher points to diabetes.
The A1C Test
Unlike the other methods, the A1C test doesn’t measure your blood sugar at a single moment. It tells you what your average blood sugar has been over the past two to three months. The test works because glucose in your bloodstream naturally sticks to hemoglobin, the oxygen-carrying protein inside red blood cells. The more glucose in your blood over time, the more hemoglobin gets coated. Since red blood cells live about three months, measuring the percentage of hemoglobin with glucose attached gives a reliable average for that window.
A1C is reported as a percentage. A normal result is below 5.7%. A reading between 5.7% and 6.4% indicates prediabetes, and 6.5% or higher indicates diabetes. Because it reflects a long stretch of time rather than a single moment, a bad night’s sleep or a stressful morning won’t significantly move the number. That stability is what makes it useful for tracking how well blood sugar is being managed over months.
Fasting and Random Plasma Glucose
A fasting plasma glucose test is a straightforward lab blood draw taken after you’ve had nothing to eat or drink (besides water) for at least eight hours. It’s one of the most common screening tools. A normal fasting level is below 100 mg/dL. Between 100 and 125 mg/dL falls in the prediabetes range, and 126 mg/dL or higher on two separate tests indicates diabetes.
A random plasma glucose test can be taken at any time of day, regardless of when you last ate. It’s typically used when someone has obvious symptoms of high blood sugar, like excessive thirst, frequent urination, and unexplained weight loss. A random reading of 200 mg/dL or higher, combined with those symptoms, is enough for a diabetes diagnosis without a fasting test.
How These Tests Work Together
No single test tells the whole story. A fasting glucose test captures one moment. An A1C captures a three-month trend. A glucose tolerance test reveals how your body handles a sugar load in real time. A CGM tracks the full pattern of rises and falls across days and weeks, including overnight dips and post-meal spikes that a single blood draw would miss entirely.
For diagnosis, the American Diabetes Association considers any one of four criteria sufficient: a fasting glucose of 126 mg/dL or higher, a two-hour OGTT result of 200 mg/dL or higher, an A1C of 6.5% or higher, or a random glucose of 200 mg/dL or higher with symptoms. In practice, an abnormal result on any single test is usually confirmed with a repeat test or a different method before a formal diagnosis is made.