Diabetes Blood Tests

Diabetes and prediabetes are diagnosed with blood tests, not symptoms alone. Several different tests can be used, and doctors often choose based on convenience, cost, and individual circumstances.

Fasting Blood Glucose

Measures blood sugar after at least 8 hours without food. Simple, requires only one blood draw, and is one of the most commonly used screening tests. See our normal fasting blood sugar guide for the specific ranges.

HbA1c

Reflects average blood sugar over roughly the past two to three months, doesn't require fasting, and can be drawn at any time of day. See our HbA1c guide for the exact ranges and what can affect its accuracy.

Oral Glucose Tolerance Test (OGTT)

Involves a fasting blood draw, followed by drinking a standardized glucose solution, then a second blood draw two hours later. More involved than the other tests, but considered the most sensitive for detecting prediabetes and gestational diabetes specifically — it's the standard test used during pregnancy screening. Pregnancy screening for gestational diabetes follows its own distinct protocol and diagnostic thresholds, different from the general OGTT described above — see our gestational diabetes guide for what that involves. Pregnancy screening for gestational diabetes follows its own distinct protocol and diagnostic thresholds, different from the general OGTT described above — see our gestational diabetes guide for what that involves. Pregnancy screening for gestational diabetes follows its own distinct protocol and diagnostic thresholds, different from the general OGTT described above — see our gestational diabetes guide for what that involves.

Random Plasma Glucose

A single reading taken at any time, regardless of when you last ate. A result of 200 mg/dL or higher along with classic symptoms (increased thirst, frequent urination, unexplained weight loss) can support a diabetes diagnosis, though it's typically confirmed with one of the other tests.

Which Test Is Right for You

There's no single "best" test for everyone — a doctor typically chooses based on your symptoms, risk factors, whether you're pregnant, and practical considerations like whether fasting is realistic for you. A single abnormal result on any of these tests is usually confirmed with a repeat test before a diagnosis is finalized.