Blood sugar is measured by detecting how much glucose is present in a small sample of blood, interstitial fluid, or sometimes other body fluids. The most common methods are fingerstick meters for daily monitoring, lab-drawn blood tests for diagnosis, and continuous glucose monitors that track levels around the clock. Each method works differently, measures slightly different things, and serves a different purpose.
Fingerstick Meters and How They Work
A standard glucose meter (glucometer) uses a disposable test strip and a tiny drop of blood from your fingertip. The strip contains an enzyme called glucose oxidase that reacts specifically with glucose in your blood. When glucose hits the enzyme, the reaction produces a small electrical current. The meter reads the strength of that current and converts it into a number, displayed in either mg/dL (used in the United States) or mmol/L (used in most other countries). The whole process takes about five seconds.
Older versions of this technology worked by color change rather than electricity. The enzyme reaction would produce a visible color on the strip, and the meter would read it using reflected light. Some of the earliest strips, developed in the 1960s, didn’t even need a meter. You compared the color on the strip to a printed chart. Modern meters are almost exclusively electrochemical because it’s faster and more precise.
A few things can throw off the accuracy of a fingerstick reading. Dehydration and anemia both alter the concentration of red blood cells in your sample, which can skew results. Extreme temperatures affect the chemistry on the test strip, so you should store your meter and strips at room temperature. Residue on your fingers, especially from food, can contaminate the sample. Washing your hands before testing is one of the simplest ways to get a reliable number.
Lab Tests for Diagnosing Diabetes
Fasting Plasma Glucose
A fasting plasma glucose test measures blood sugar from a vein after you haven’t eaten for at least eight hours, typically first thing in the morning. Because the blood is drawn by a professional and processed in a lab, it’s more precise than a home meter. The American Diabetes Association uses this test as one of its primary diagnostic tools, with clear cutoffs: below 100 mg/dL is normal, 100 to 125 mg/dL indicates prediabetes, and 126 mg/dL or higher points to diabetes. A diagnosis usually requires a second confirmatory test on a different day.
Hemoglobin A1C
The A1C test works on a completely different principle than any other blood sugar measurement. Instead of capturing what your glucose is right now, it reveals your average blood sugar over the past two to three months. It does this by measuring how much glucose has attached to hemoglobin, the oxygen-carrying protein inside red blood cells. Glucose in your bloodstream naturally sticks to hemoglobin, and the higher your blood sugar runs day after day, the more hemoglobin gets coated. Since red blood cells live about three months, the A1C provides a rolling average of that entire lifespan.
An A1C below 5.7% is normal. Between 5.7% and 6.4% falls in the prediabetes range, and 6.5% or higher indicates diabetes. This test doesn’t require fasting, which makes it convenient for both patients and clinicians. However, conditions that affect red blood cell turnover, such as sickle cell disease or significant blood loss, can make the result unreliable.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) measures how efficiently your body clears sugar from the bloodstream. After fasting overnight, you drink a solution containing 75 grams of sugar, roughly equivalent to drinking two cans of soda at once. Your blood is drawn one hour later and again at two hours. The test reveals not just your baseline blood sugar but how your body responds to a large glucose load, making it especially useful for catching problems that a fasting test might miss.
A three-hour version is used specifically to diagnose gestational diabetes during pregnancy. That version uses a 100-gram glucose solution, with blood drawn at one, two, and three hours after drinking it.
Continuous Glucose Monitors
Continuous glucose monitors (CGMs) use a tiny sensor filament inserted just under the skin, usually on the arm or abdomen. Unlike a fingerstick meter that measures blood directly, a CGM measures glucose in the interstitial fluid, the thin layer of liquid that surrounds your cells beneath the skin. Glucose moves from your blood into this fluid naturally, so interstitial readings closely track blood glucose, but with a delay. That lag can be up to 15 minutes, though it’s typically shorter. The gap matters most when blood sugar is rising or falling quickly, such as right after a meal or during exercise.
Most CGMs take a reading every one to five minutes and transmit the data wirelessly to a phone or dedicated receiver. This produces a continuous trend line rather than isolated snapshots, making it much easier to spot patterns. You can see how specific meals, activities, or sleep habits affect your blood sugar across an entire day. Some systems also send alerts when glucose is trending too high or too low, which is particularly valuable for people on insulin who are at risk of dangerous drops overnight.
Units and How to Convert Them
Blood sugar is reported in two different units depending on where you live. The United States uses milligrams per deciliter (mg/dL). Most of Europe, Canada, Australia, and much of the rest of the world uses millimoles per liter (mmol/L). To convert between them, divide mg/dL by 18 to get mmol/L, or multiply mmol/L by 18 to get mg/dL. So a reading of 108 mg/dL is the same as 6.0 mmol/L. If you’re reading research or health information from another country, this conversion lets you compare numbers directly.
What the Numbers Mean
For someone without diabetes, fasting blood sugar typically stays below 100 mg/dL (5.6 mmol/L). After eating, it might rise to 120 or 140 mg/dL and return to baseline within a couple of hours. The diagnostic thresholds set by the American Diabetes Association serve as the standard reference points:
- Normal: Fasting glucose below 100 mg/dL, A1C below 5.7%
- Prediabetes: Fasting glucose 100 to 125 mg/dL, A1C 5.7% to 6.4%
- Diabetes: Fasting glucose 126 mg/dL or higher, A1C 6.5% or higher
A single high reading doesn’t equal a diagnosis. Stress, illness, medications, and even a poor night of sleep can temporarily push blood sugar up. That’s why diagnostic tests are repeated before any clinical conclusion is drawn.
How Often Blood Sugar Gets Checked
How frequently you need to measure depends entirely on how your diabetes is managed, or whether you have diabetes at all. People who control their blood sugar through diet and exercise, or take medications that don’t carry a risk of causing low blood sugar, may not need routine daily monitoring at all. Their A1C test every few months provides enough information.
For people taking insulin, sulfonylureas, or other medications that can cause blood sugar to drop dangerously low, regular fingerstick checks or a CGM become much more important. Testing before meals, after meals, before driving, and before bed helps catch lows before they become dangerous. Structured monitoring, where you check at specific times and learn to recognize patterns of highs and lows, tends to be the most useful approach rather than testing randomly throughout the day.
Non-Invasive Monitoring on the Horizon
Several companies are developing devices that aim to measure blood sugar without any needles or skin punctures at all. These use technologies like near-infrared light, ultrasound, or electromagnetic waves to estimate glucose levels through the skin. One device, the egm1000 from Evia Medical Technologies, clips onto the earlobe and combines ultrasound, radio frequency waves, and heat sensing. It has received European CE certification and is currently available for purchase in some markets. Another device, Wizmi, uses near-infrared light on the wrist and showed promising accuracy in a clinical trial of 32 pregnant women. A miniaturized sensor from NovioSense sits in the lower eyelid and measures glucose in tear fluid, reporting accuracy comparable to existing CGMs in phase II trials.
None of these non-invasive technologies have yet matched the reliability of traditional blood-based or interstitial fluid-based methods across large, diverse populations. Accuracy tends to vary more widely in people with diabetes compared to healthy individuals, with error margins ranging from about 8% to 17% in some devices tested on diabetic populations. For now, they represent a promising but still-maturing category.