Diagnosis of Diabetes
Symptoms of type 1 diabetes often appear suddenly and are often the reason for checking blood sugar levels. Because symptoms of other types of diabetes and prediabetes come more gradually or may not be evident, the American Diabetes Association (ADA) has recommended screening guidelines.7 The ADA recommends that the following people be screened for diabetes:
Anyone with a body mass index higher than 25 (23 for Asian-Americans), regardless of age, who has additional risk factors such as high blood pressure, abnormal cholesterol levels, a sedentary lifestyle, a history of polycystic ovary syndrome or heart disease, and having a close relative with diabetes.7
Anyone older than age 45 is advised to receive an initial blood sugar screening, and then, if the results are normal, to be screened every three years thereafter.7
Any woman who has had gestational diabetes, is advised to be screened for diabetes every three years.7
Anyone who has been diagnosed with prediabetes is advised to be tested every year.7
Tests for type 1 and type 2 diabetes and prediabetes7
Glycated hemoglobin (A1C) test
This blood test, which doesn't require fasting, indicates your average blood sugar level for the past two to three months. It measures the percentage of blood sugar attached to hemoglobin, the oxygen-carrying protein in red blood cells.7
The higher your blood sugar levels, the more hemoglobin you'll have with sugar attached. An A1C level of 6.5 percent or higher on two separate tests indicates that you have diabetes. An A1C between 5.7 and 6.4 percent indicates prediabetes. Below 5.7 is considered normal.5,7
If the A1C test results aren't consistent, the test isn't available, or you have certain conditions that can make the A1C test inaccurate — such as if you're pregnant or have an uncommon form of hemoglobin (known as a hemoglobin variant) — your doctor may use the following tests to diagnose diabetes:5,7
Random blood sugar test
A blood sample will be taken at a random time. Regardless of when you last ate, a random blood sugar level of 200 milligrams per deciliter (mg/dL) — 11.1 millimoles per liter (mmol/L) — or higher suggests diabetes.
Fasting blood sugar test
A blood sample will be taken after an overnight fast. A fasting blood sugar level less than 100 mg/dL (5.6 mmol/L) is normal. A fasting blood sugar level from 100 to 125 mg/dL (5.6 to 6.9 mmol/L) is considered prediabetes. If it's 126 mg/dL (7 mmol/L) or higher on two separate tests, you have diabetes.5,7
Oral glucose tolerance test
For this test, you fast overnight, and the fasting blood sugar level is measured. Then you drink a sugary liquid, and blood sugar levels are tested periodically for the next two hours.5,7
A blood sugar level less than 140 mg/dL (7.8 mmol/L) is normal. A reading of more than 200 mg/dL (11.1 mmol/L) after two hours indicates diabetes. A reading between 140 and 199 mg/dL (7.8 mmol/L and 11.0 mmol/L) indicates prediabetes.5,7
5. Diagnostic Criteria and Classification of Hyperglycaemia First Detected in Pregnancy. World Health Organization. Geneva, WHO Press. 2013: 20-23.
7. Zheng Y, et al. Global aetiology and epidemiology of type 2 diabetes mellitus and its complications. Nat Rev Endocrinol 2018: 14(2): 88-98.