A glucose test is a blood test that measures how much sugar is in your bloodstream. It’s the primary tool used to diagnose diabetes, prediabetes, and gestational diabetes, and it’s also used to monitor blood sugar in people already living with these conditions. There are several types of glucose tests, each measuring blood sugar in a slightly different way, with different preparation requirements and diagnostic cutoffs.
Types of Glucose Tests
Four main glucose tests are used in clinical practice, and which one your doctor orders depends on what they’re looking for and your circumstances.
The fasting plasma glucose test is the most straightforward. It measures your blood sugar at a single point in time after you’ve gone without food for at least 8 hours (water is fine). A normal result is 99 mg/dL or below. A reading between 100 and 125 mg/dL falls into the prediabetes range, and 126 mg/dL or above indicates diabetes.
The A1C test works differently. Rather than capturing a snapshot of your blood sugar right now, it reflects your average blood sugar over the previous two to three months. It does this by measuring the percentage of your red blood cells that have sugar attached to them. Because it looks at a longer window, you don’t need to fast beforehand. A normal A1C is below 5.7%, prediabetes falls between 5.7% and 6.4%, and 6.5% or higher points to diabetes.
The oral glucose tolerance test checks how well your body handles a large dose of sugar. You fast for at least 8 hours, then drink a syrupy solution containing 75 grams of glucose. Your blood is drawn before you drink it and again two hours later. A two-hour reading of 140 mg/dL or below is normal, 140 to 199 mg/dL suggests prediabetes, and 200 mg/dL or above indicates diabetes. This test is particularly useful for catching prediabetes that a fasting test might miss.
The random plasma glucose test can be done at any time of day without fasting. It’s typically used when someone already has symptoms of diabetes, like excessive thirst, frequent urination, or unexplained weight loss, and the doctor doesn’t want to wait for a scheduled fasting test. A result of 200 mg/dL or above, combined with symptoms, is enough to diagnose diabetes.
Glucose Testing During Pregnancy
Pregnant women go through a separate screening process for gestational diabetes, usually between weeks 24 and 28. This typically happens in two steps.
The first step is a one-hour glucose screening. No fasting is required. You drink a solution containing 50 grams of glucose, and your blood is drawn one hour later. A result of 140 mg/dL or below is generally considered normal. If your number comes back higher, it doesn’t mean you have gestational diabetes. It means you need the second, more thorough test.
That second test is the three-hour glucose tolerance test. This one does require fasting for 8 to 14 hours beforehand. You drink a stronger solution with 100 grams of glucose, and your blood is drawn four times: once before drinking and then at one, two, and three hours after. Each draw has its own threshold. A fasting level of 95 mg/dL or above, a one-hour reading of 180 mg/dL or above, a two-hour reading of 155 mg/dL or above, or a three-hour reading of 140 mg/dL or above are all considered abnormal. You typically need two or more abnormal values for a gestational diabetes diagnosis.
Some providers use a single-step alternative: a two-hour test with 75 grams of glucose, similar to the standard oral glucose tolerance test for non-pregnant adults.
How to Prepare for a Glucose Test
If your test requires fasting, you’ll need to go without food or drink for 8 to 12 hours before your blood draw. Plain water is the only thing allowed. Coffee, juice, soda, and even flavored or lemon-infused sparkling water can affect your results and should be avoided. Most people schedule fasting tests first thing in the morning so most of the fast happens while they sleep.
For the A1C test and the one-hour pregnancy screening, no preparation is needed. You can eat and drink normally beforehand.
What the Results Mean in Practice
A single abnormal result doesn’t always lock in a diagnosis. Doctors often repeat the test on a different day to confirm, especially if you’re in the prediabetes range or the result is borderline. Two tests showing diabetic-level results, whether the same test repeated or two different test types, are typically needed for a definitive diagnosis.
Prediabetes results are worth paying close attention to. A fasting glucose between 100 and 125 mg/dL or an A1C between 5.7% and 6.4% means your blood sugar is higher than normal but not yet in the diabetes range. At this stage, changes to diet, physical activity, and weight can often prevent or delay progression to type 2 diabetes.
If you’re already diagnosed with diabetes, your doctor will likely use the A1C test regularly to track how well your blood sugar has been controlled over time, while daily finger-prick tests or continuous monitors give you real-time numbers throughout the day. These tools serve different purposes: the A1C shows the big picture, while daily monitoring captures the day-to-day swings.
Factors That Can Affect Accuracy
Certain conditions can throw off glucose test results. Dehydration and anemia (a low red blood cell count) can both reduce accuracy, particularly for finger-prick monitors and the A1C test. The A1C specifically relies on measuring sugar attached to red blood cells, so anything that changes how long those cells survive in your body, like sickle cell disease or significant blood loss, can skew the number up or down.
Stress, illness, and certain medications can temporarily raise blood sugar, which may affect a fasting or random glucose reading taken on a bad day. This is one reason doctors confirm abnormal results with a second test rather than diagnosing from a single draw.