Glucose testing measures the amount of sugar in your blood and is the primary way to screen for diabetes and prediabetes. The most common methods are a fasting blood sugar test, an A1C test, an oral glucose tolerance test, and at-home monitoring with a glucometer or continuous sensor. Which test you get depends on whether you’re screening for a new diagnosis, managing an existing one, or monitoring during pregnancy.
Fasting Blood Sugar Test
This is the most straightforward lab test for glucose. You fast overnight (no food or caloric drinks for 8 to 12 hours), then have blood drawn. Plain water is fine during the fast, but coffee, juice, soda, and flavored water are not, since they can affect results.
The results break down into three ranges:
- Normal: 99 mg/dL or below
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or above
A single high reading doesn’t automatically mean diabetes. Your provider will typically repeat the test on a different day to confirm. If your result falls in the prediabetes range, it signals that your body is starting to struggle with blood sugar regulation, but lifestyle changes at this stage can often prevent progression.
The A1C Test
Unlike a fasting test that captures a single moment, the A1C (also called HbA1c) reflects your average blood sugar over the previous two to three months. It works by measuring the percentage of hemoglobin in your red blood cells that has glucose attached to it. No fasting is required, which makes it convenient for routine screening.
The diagnostic thresholds from the CDC are:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or above
One important caveat: certain conditions can throw off A1C accuracy. Iron deficiency anemia, sickle cell trait, and other hemoglobin variants can produce misleadingly high or low readings. Pregnancy also affects A1C reliability because of changes in red blood cell turnover. If you have any of these conditions, your provider may rely more heavily on direct blood sugar measurements instead.
Oral Glucose Tolerance Test
This test measures how efficiently your body processes sugar in real time. You fast overnight, get your blood drawn for a baseline reading, then drink a syrupy solution containing 75 grams of glucose. Your blood is drawn again at the one-hour and two-hour marks.
A two-hour result of 140 mg/dL or below is normal. Between 140 and 199 mg/dL indicates prediabetes, and 200 mg/dL or higher points to diabetes. This test is more sensitive than a fasting test alone because it reveals problems with how your body responds to a sugar load, which can show up before fasting levels become abnormal.
Glucose Testing During Pregnancy
Screening for gestational diabetes follows a slightly different process. Most pregnant people first take a one-hour screening where they drink a solution with 50 grams of glucose (no fasting required). If that result is elevated, a follow-up diagnostic test is scheduled. The most common version uses 100 grams of glucose, with blood drawn at one, two, and three hours after drinking. A less common two-hour version uses 75 grams, where a reading of 153 mg/dL or higher at two hours indicates gestational diabetes.
Home Blood Glucose Monitors
If you’ve already been diagnosed with diabetes or prediabetes, you’ll likely use a home glucometer for day-to-day monitoring. These devices use a small lancet to prick your finger, and a test strip analyzes the drop of blood. Results appear in seconds.
Home monitors are reasonably accurate but not perfect. The international accuracy standard (ISO 15197) requires that at least 95% of readings fall within 15 mg/dL of the true value when blood sugar is below 100 mg/dL, and within 15% when blood sugar is 100 mg/dL or above. In practical terms, if your actual blood sugar is 150 mg/dL, your meter could read anywhere from about 128 to 173 mg/dL and still meet the standard. This is precise enough for daily management decisions, but it’s why lab tests remain the gold standard for diagnosis.
Continuous Glucose Monitors
Continuous glucose monitors (CGMs) use a tiny sensor inserted just under the skin, typically on the upper arm or abdomen, to track glucose levels around the clock. Rather than measuring blood directly, the sensor reads glucose in the fluid between your cells (interstitial fluid). This gives you a near-continuous stream of data, updated every few minutes, along with trend arrows showing whether your levels are rising or falling.
The trade-off is a built-in time lag. Because glucose has to move from your bloodstream into the surrounding tissue before the sensor detects it, CGM readings trail actual blood sugar by several minutes. During rapid changes, like right after a meal, the lag can create errors of 6% or more. For this reason, some situations still call for a fingerstick to confirm a CGM reading, particularly if the number doesn’t match how you’re feeling or if you’re about to make a dosing decision.
Urine Glucose Testing
Urine test strips can detect glucose, but they’re far less useful than blood-based methods. Your kidneys only spill glucose into urine once blood sugar climbs above roughly 180 mg/dL (about 10 mmol/L), though this threshold varies between individuals. That means urine testing completely misses prediabetes and even moderately elevated blood sugar. It also can’t tell you your current level, only that at some point since you last emptied your bladder, your blood sugar was high enough to overflow into your urine. Urine strips are still sometimes used as a quick, inexpensive check in resource-limited settings, but they’ve been largely replaced by blood-based options for routine monitoring and diagnosis.
Which Test Is Right for You
If you’re getting screened for the first time, your provider will likely order either a fasting blood sugar test or an A1C. Both are simple blood draws, and the choice often comes down to convenience. The A1C doesn’t require fasting, which makes it easier to schedule. The fasting test is cheaper and widely available. If results from either test are borderline or conflicting, an oral glucose tolerance test may be added for a more detailed picture.
For ongoing management of diagnosed diabetes, home glucometers and CGMs are the everyday tools. Your provider will still check your A1C every few months (typically every three to six months) to see how well your overall blood sugar control is trending over time. That combination of daily spot checks and periodic A1C readings gives the most complete view of how your body is handling glucose.