Diabetes Screening: A1C and Glucose Testing
Last updated 2026-10-01
Three ways to look at blood sugar
Diabetes and prediabetes are found with blood tests that measure sugar or its long-term imprint. The main options are the A1C, the fasting glucose, and the oral glucose tolerance test. They are not interchangeable, and each captures a different slice of time.
The A1C
A1C reflects average blood sugar over roughly the past two to three months, because it measures sugar attached to hemoglobin in red blood cells. It requires no fasting and can be drawn at any time of day, which makes it convenient for screening. Conditions that affect red blood cells, such as some anemias or hemoglobin variants, can make the result less reliable.
Fasting glucose
This test measures sugar after you have not eaten for at least eight hours. It shows your baseline level rather than the response to food. It is simple, but a single reading can miss patterns that the A1C would catch.
The oral glucose tolerance test
You drink a measured sugar solution and have blood drawn afterward, often more than once. This test shows how your body handles a sugar load. It is used most often in pregnancy and in specific diagnostic situations.
How results are grouped
Guidelines from the CDC and major diabetes organizations group results into normal, prediabetes, and diabetes ranges. Commonly cited A1C cutoffs place below 5.7 percent as normal, 5.7 to 6.4 percent as prediabetes, and 6.5 percent or higher as diabetes. Fasting glucose is grouped in a similar way, with prediabetes starting around 100 and diabetes around 126 milligrams per deciliter. These ranges are general guides, and a clinician confirms any diagnosis with context and often a repeat test.
Who should be screened
Screening is usually recommended for adults with risk factors such as excess weight, family history, high blood pressure, or a history of gestational diabetes. Age also enters the decision. Your clinician can tell you how often to repeat screening based on your results and risks.
Acting on a result
Prediabetes is a signal, not a sentence. Lifestyle changes can lower the chance of progressing. For a diabetes-range result, follow-up matters and treatment options are well established. Bring your report to a clinician who can confirm and plan the next step.
How to order these tests
Blood sugar tests are simple draws. The A1C needs no fasting and can be done at any time, which makes it convenient for screening. A fasting glucose requires at least eight hours without food. The glucose tolerance test takes longer because it involves a drink and repeated draws over a set period.
Preparing for the draw
Confirm whether fasting is required before you go. For the A1C, no special preparation is needed. For a fasting test, water is fine and helps the draw. If you take diabetes medication, ask how it should be timed around the appointment rather than changing it on your own.
Following up on a result
A single abnormal result is not a diagnosis. Clinicians often repeat the test or add a second method to confirm a pattern before acting. Bring the report to a professional who can interpret it with your history, since the same number can mean different things for different people.
This article is general information, not medical advice; talk with a clinician about your results.