What Diagnostic Tests Are Used to Confirm Diabetes?
There are three primary blood tests used to confirm a diagnosis of diabetes or prediabetes: the Hemoglobin A1c (A1C) test, the Fasting Plasma Glucose (FPG) test, and the Oral Glucose Tolerance Test (OGTT).
Each test measures blood glucose levels in a different way, and sometimes a doctor may use a combination of tests or repeat a test on a different day to confirm the diagnosis, ensuring accuracy before initiating a treatment plan.
- Hemoglobin A1c (A1C) Test:
This is a common and convenient blood test that provides an estimate of your average blood glucose levels over the preceding two to three months. It measures the percentage of hemoglobin, the protein in red blood cells that carries oxygen, that is coated with sugar (glycated). A higher A1C percentage corresponds to higher average blood sugar levels.
For diagnosis, an A1C level of 6.5% or higher indicates diabetes. A level between 5.7% and 6.4% suggests prediabetes, while a result below 5.7% is considered normal. This test does not require fasting.
- Fasting Plasma Glucose (FPG) Test:
This test measures your blood glucose level at a single point in time after you have fasted (not eaten or drunk anything except water) for at least eight hours. It is typically performed in the morning.
A fasting blood glucose level of 126 milligrams per deciliter (mg/dL) or higher on two separate occasions indicates diabetes. A result between 100 and 125 mg/dL is classified as prediabetes (also known as impaired fasting glucose), and a level of 99 mg/dL or below is normal.
- Oral Glucose Tolerance Test (OGTT):
The OGTT is a more sensitive test that assesses how your body processes glucose over time. It requires you to fast overnight and then have your blood drawn to establish a baseline fasting glucose level. Afterward, you drink a standardized sugary liquid, and your blood glucose level is tested again two hours later. A blood glucose level of 200 mg/dL or higher after two hours indicates diabetes.
A result between 140 and 199 mg/dL indicates prediabetes (also called impaired glucose tolerance), and a reading below 140 mg/dL is considered normal. This test is frequently used to diagnose gestational diabetes in pregnant women.
