A glucose test is a blood test that measures how much sugar is in your bloodstream. It’s one of the most common lab tests in medicine, used primarily to screen for and diagnose diabetes and prediabetes. There are several types, each measuring blood sugar in a slightly different way, and the one your doctor orders depends on what they’re looking for.
Types of Glucose Tests
Three main glucose tests are used to screen for diabetes and prediabetes. They can be used individually or in combination, and each has different requirements for preparation.
The fasting plasma glucose (FPG) test is the most straightforward. You fast overnight, get your blood drawn, and the lab measures your blood sugar at that single point in time. It’s quick and widely available, which makes it the most commonly ordered version.
The oral glucose tolerance test (OGTT) checks how your body handles a sugar load. After a fasting blood draw, you drink a solution containing 75 grams of glucose, then have your blood drawn again two hours later. This test reveals how efficiently your body clears sugar from the bloodstream, which can catch problems the fasting test misses.
The A1C test takes a different approach entirely. Instead of measuring blood sugar at one moment, it reflects your average blood sugar over the past three months. It works because glucose naturally attaches to hemoglobin, the protein in red blood cells that carries oxygen. Since red blood cells live about three months, the percentage of hemoglobin coated with glucose tells your doctor what your blood sugar has been doing over that entire period. No fasting is required.
What the Numbers Mean
Each test has its own scale, but all three divide results into three categories: normal, prediabetes, and diabetes. The American Diabetes Association sets these thresholds:
For a fasting plasma glucose test, a result below 100 mg/dL is normal. Between 100 and 125 mg/dL falls in the prediabetes range. A reading of 126 mg/dL or higher indicates diabetes.
For the oral glucose tolerance test, the two-hour reading is what matters. Below 140 mg/dL is normal. Between 140 and 199 mg/dL signals prediabetes. At 200 mg/dL or above, it’s diabetes.
For the A1C test, below 5.7% is normal. Between 5.7% and 6.4% is prediabetes. At 6.5% or higher, the result indicates diabetes.
One important detail: a single abnormal result usually isn’t enough for a diabetes diagnosis on its own. Unless your blood sugar is very clearly elevated with obvious symptoms, doctors typically require two abnormal results, either from two different tests done at the same time or the same test repeated on a different day.
How to Prepare
If your test requires fasting, you’ll need to go without food or drinks (other than plain water) for 8 to 12 hours before your blood draw. Most people schedule their test first thing in the morning so the fasting period overlaps with sleep.
During the fast, avoid flavored water, carbonated drinks, gum, smoking, and exercise, as all of these can influence your results. Plain water is fine and actually encouraged, since dehydration can affect accuracy. Ask your doctor whether to continue taking your regular medications or supplements beforehand, as some can shift your numbers. Don’t stop any prescription medication unless you’re specifically told to.
The A1C test requires no preparation at all. You can eat and drink normally before it.
Glucose Testing During Pregnancy
Pregnant women get a specialized version of glucose testing to screen for gestational diabetes, typically between 24 and 28 weeks. The most common approach in the U.S. uses two steps.
The first step is a glucose challenge test. You drink a smaller sugar solution (50 grams) without fasting, and your blood is drawn one hour later. If the result comes back at or above the screening threshold (usually 130 to 140 mg/dL, depending on the practice), you move to the second step.
The second step is a longer, more detailed oral glucose tolerance test. This time you fast overnight, then drink a 100-gram glucose solution. Your blood is drawn at fasting, one hour, two hours, and three hours. A diagnosis of gestational diabetes is made if at least two of those four readings are elevated: fasting at or above 95 mg/dL, one hour at or above 180 mg/dL, two hours at or above 155 mg/dL, or three hours at or above 140 mg/dL.
Some providers use a one-step method instead, which involves a 75-gram glucose drink with blood draws at fasting, one hour, and two hours. Your doctor will let you know which approach they prefer.
What Can Affect Your Results
Several factors beyond actual blood sugar levels can throw off glucose test accuracy. Dehydration is one of the most common culprits, as it concentrates your blood and can push readings higher than they truly are. Anemia, or a low red blood cell count, can also skew results.
If you’re using a home glucose meter with fingerstick testing, clean hands matter more than you might think. Residue from food, lotion, or hand sanitizer on your fingertips can contaminate the sample. If you clean the site with an alcohol wipe, let it dry completely before pricking. Washing your hands with soap and water is the most reliable preparation.
It’s also worth knowing that fingerstick readings from capillaries tend to run about 15 mg/dL higher than venous blood draws from your arm. That gap widens when blood sugar is low, reaching differences of around 22 mg/dL. So a fingerstick number and a lab number from the same moment won’t match exactly, and that’s expected.
Continuous Glucose Monitors
Continuous glucose monitors, or CGMs, are wearable sensors that sit just under the skin and measure glucose in the fluid between your cells (not in blood directly). They take a reading every 1 to 5 minutes and send results to a phone or receiver, giving you a real-time picture of how your blood sugar moves throughout the day.
Because CGMs measure a different fluid than traditional blood tests, their readings don’t perfectly align with fingerstick or lab values. There’s typically a time lag of several minutes, and accuracy tends to be lowest when blood sugar is dropping quickly or sitting in the low range. Studies have found CGM accuracy decreases most noticeably during hypoglycemia, where error rates can roughly double compared to normal blood sugar ranges. CGMs are powerful tools for spotting trends and patterns, but individual readings are less precise than a lab blood draw.
Low Blood Sugar Readings
Glucose tests can also detect blood sugar that’s too low, a condition called hypoglycemia. This is most relevant for people already taking insulin or certain diabetes medications, though it occasionally occurs in other situations.
A reading at or below 70 mg/dL is considered a warning level, where you may notice shakiness, sweating, or hunger. Below 54 mg/dL is clinically significant and requires prompt treatment with fast-acting sugar. Severe hypoglycemia is defined not by a specific number but by needing someone else’s help to recover, regardless of what the meter reads.