The American Diabetes Association (ADA) recommends that diabetes screening for most adults begin at age 35. The ADA advises diabetes screening before age 35 if you're overweight and have additional risk factors for prediabetes or type 2 diabetes.
If you've had gestational diabetes, your health care provider will likely check your blood sugar levels at least once every three years.
There are several blood tests for prediabetes.
Glycated hemoglobin (A1C) test
This test indicates your average blood sugar level for the past 2 to 3 months.
In general:
Below 5.7% is normal
Between 5.7% and 6.4% is diagnosed as prediabetes
6.5% or higher on two separate tests indicates diabetes
Certain conditions can make the A1C test inaccurate — such as if you're pregnant or have an uncommon form of hemoglobin.
Fasting blood sugar test
A blood sample is taken after you haven't eaten for at least eight hours or overnight (fast).
Blood sugar values are expressed in milligrams of sugar per deciliter (mg/dL) or millimoles of sugar per liter (mmol/L) of blood. In general:
Less than 100 mg/dL (5.6 mmol/L) is normal
100 to 125 mg/dL (5.6 to 6.9 mmol/L) is diagnosed as prediabetes
126 mg/dL (7.0 mmol/L) or higher on two separate tests is diagnosed as diabetes
Source: https://www.mayoclinic.org/diseases-conditions/prediabetes/diagnosis-treatment/drc-20355284
