Diagnosing diabetes involves several blood tests that measure your blood sugar levels. Your doctor will likely use one or more of the following tests:
Fasting Plasma Glucose (FPG) Test: This is a common test done after an overnight fast (at least 8 hours).
A fasting blood sugar level of 126 mg/dL or higher on two separate occasions indicates diabetes.
Levels between 100 and 125 mg/dL suggest prediabetes.
A1c Test: This blood test provides an average of your blood sugar levels over the past 2-3 months. It measures the percentage of hemoglobin (a protein in red blood cells) that has glucose attached to it.
An A1c level of 6.5% or higher indicates diabetes.
Levels between 5.7% and 6.4% suggest prediabetes.
Random Plasma Glucose Test: This test measures your blood sugar at any time of day, without regard to when you last ate.
A random blood sugar level of 200 mg/dL or higher, along with diabetes symptoms, suggests diabetes. This test is usually used for initial screening or when fasting isn't possible.
Oral Glucose Tolerance Test (OGTT): This test involves fasting overnight, then drinking a sugary liquid. Blood sugar levels are checked periodically over the next two hours. It's often used to diagnose gestational diabetes (diabetes during pregnancy) but can also be used for other types of diabetes.
A blood sugar level of 200 mg/dL or higher two hours after drinking the sugary liquid indicates diabetes.
Your doctor will consider your symptoms, medical history, and the results of these tests to make a diagnosis. Often, more than one test is needed to confirm diabetes.