Blood sugar is measured by detecting how much glucose is present in a small sample of blood or, increasingly, in the fluid just beneath your skin. The most common method is a finger-prick glucometer, which uses a disposable test strip to produce a reading in about five seconds. But that’s only one option in a toolkit that includes continuous monitors, lab-drawn blood tests, and a hemoglobin-based test that reveals your average glucose over two to three months.
Finger-Prick Glucometers
A home glucometer works by triggering a chemical reaction on a tiny test strip. The strip contains an enzyme, either glucose oxidase or glucose dehydrogenase, that reacts specifically with glucose in your blood. When you place a drop of blood on the strip, the enzyme breaks down glucose and releases electrons. Those electrons create a small electrical current that the meter measures and converts into a number on the screen. The higher the glucose concentration, the stronger the current.
Modern strips use synthetic compounds called electron mediators to shuttle those electrons to the electrode more reliably. Older designs depended on oxygen in the blood to drive the reaction, which made readings sensitive to differences in oxygen levels. Current strips have largely solved that problem, though accuracy can still be affected by other factors. High or low levels of red blood cells (hematocrit) interfere with nearly all meters. Vitamin C, acetaminophen (the active ingredient in Tylenol), and certain sugars like maltose and galactose can also skew results, particularly when your actual glucose is low. If you’re taking high-dose vitamin C supplements or IV medications containing maltose, your meter may read falsely high.
Capillary Blood vs. Venous Blood
A finger prick collects capillary blood, the blood flowing through the tiny vessels near your skin’s surface. A lab draw pulls venous blood from a vein in your arm. These two sources don’t give identical numbers. After eating, capillary blood typically reads higher than venous blood because your fingertip tissues are actively absorbing glucose from the bloodstream. In one large screening study, average fasting capillary glucose was about 122 mg/dL compared to 115 mg/dL for venous plasma, and two hours after a glucose load the gap widened further: 203 mg/dL capillary versus 176 mg/dL venous.
This difference matters mostly for borderline results. The diagnostic thresholds your doctor uses are based on venous plasma values, so a slightly elevated finger-prick reading doesn’t always mean the same thing as a slightly elevated lab result. For day-to-day diabetes management, though, home meters are calibrated to approximate plasma-equivalent values, and the trend matters more than any single number.
Continuous Glucose Monitors
Continuous glucose monitors, or CGMs, don’t measure blood at all. Instead, a hair-thin sensor inserted just under your skin reads glucose levels in interstitial fluid, the liquid that surrounds your cells in the fatty tissue beneath the skin’s surface. Glucose moves from your bloodstream through capillary walls and into this fluid, so interstitial readings track blood glucose closely but with a built-in delay. That lag is typically 5 to 6 minutes, based on tracer experiments in humans. During times of rapid change, like right after a meal or during intense exercise, the number on your CGM may trail your actual blood sugar by that margin.
The two most widely used systems are the Dexcom G7, which lasts 10 days per sensor (with an optional 12-hour grace period to avoid gaps when switching sensors), and the FreeStyle Libre 3, which lasts 14 days. Both send readings to your phone automatically, giving you a glucose value every few minutes without finger pricks. This stream of data reveals patterns that a few daily finger sticks could never capture: how specific foods affect you, what happens to your glucose while you sleep, and how quickly you recover after exercise.
The A1C Test
While glucometers and CGMs capture what’s happening right now, the A1C test tells you what’s been happening over the past two to three months. It works by measuring how much glucose has permanently attached to hemoglobin, the oxygen-carrying protein inside your red blood cells. Glucose in your bloodstream naturally binds to hemoglobin in a slow, irreversible chemical reaction. The higher your blood sugar has been, and the longer it stays elevated, the more hemoglobin gets coated.
Because red blood cells live for roughly 120 days, an A1C reading reflects your average blood sugar across that entire lifespan. The lab separates different forms of hemoglobin using a technique that sorts proteins by their electrical charge, then measures the proportion that has glucose attached. The result is expressed as a percentage. A single high-sugar day won’t meaningfully move your A1C, but weeks of consistently elevated glucose will.
The Oral Glucose Tolerance Test
The oral glucose tolerance test, or OGTT, measures how efficiently your body clears a known amount of sugar from the bloodstream. You fast overnight, have your blood drawn for a baseline reading, then drink a solution containing 75 grams of glucose (roughly equivalent to the sugar in two cans of soda). Your blood is drawn again two hours later. The comparison between those two numbers shows how well your insulin response is working.
This test is especially common during pregnancy, where it’s used to screen for gestational diabetes. Pregnant women may follow a two-step process: first a 50-gram glucose drink with a one-hour blood draw, and if that result is elevated, a more detailed test using a 100-gram dose with blood drawn at fasting, one hour, two hours, and three hours. For children, the dose is adjusted by weight at 1.75 grams per kilogram of body weight, up to a maximum of 75 grams.
What the Numbers Mean
The American Diabetes Association uses three main tests for diagnosis, each with its own cutoffs:
- Fasting blood glucose: Below 100 mg/dL is normal. Between 100 and 125 mg/dL indicates prediabetes. At 126 mg/dL or above, diabetes is diagnosed.
- A1C: Below 5.7% is normal. Between 5.7% and 6.4% indicates prediabetes. At 6.5% or above, diabetes is diagnosed.
- Two-hour glucose (after OGTT): Below 140 mg/dL is normal. Between 140 and 199 mg/dL indicates prediabetes. At 200 mg/dL or above, diabetes is diagnosed.
These thresholds apply to non-pregnant adults. A single abnormal result is usually confirmed with a repeat test on a different day before a formal diagnosis is made. The tests measure different things: fasting glucose is a snapshot, A1C is a long average, and the OGTT tests your body’s dynamic response. It’s possible to have a normal fasting glucose but an abnormal A1C, or vice versa, which is why doctors sometimes use more than one method.
What Affects Your Reading
Beyond the chemical interferences that can throw off a meter, several everyday factors influence what number you see. Testing right after washing your hands matters more than most people realize: residual sugar from food on your fingertip can produce a falsely high reading. Temperature affects strips too. Most are designed to work between about 50°F and 104°F, and readings outside that range may be unreliable.
For A1C, anything that changes the lifespan of your red blood cells can distort the result. Conditions like sickle cell disease, significant blood loss, or recent blood transfusions can make A1C unreliable. Iron deficiency anemia tends to falsely elevate A1C because red blood cells circulate longer and accumulate more glucose. In these situations, fasting glucose or an OGTT gives a more accurate picture.
For CGMs, the sensor’s first 12 to 24 hours often produce less accurate readings as the sensor stabilizes in the tissue. Compression can also cause false lows: if you sleep on the arm where your sensor is placed, the pressure can temporarily reduce blood flow to that area and produce a dip that doesn’t reflect your actual glucose level. Most CGM users learn to recognize these artifacts quickly.