Diabetes mellitus is a group of conditions defined by persistently raised blood glucose (hyperglycemia). It is diagnosed from the results of specific laboratory tests, not from symptoms alone. The American Diabetes Association (ADA) criteria define which tests are used (plasma glucose measured while fasting, at a random time or after an oral glucose tolerance test, OGTT, and glycated hemoglobin, HbA1c), the threshold that makes a result abnormal, and the conditions under which the results are enough to confirm the diagnosis. Because different tests answer different questions, the clinical context decides which one to use and how many abnormal results are needed.
Choose a route through this family
The diagnostic work-up runs through four questions: who should be tested, what a result means, which type of diabetes it represents, and how pregnancy changes the rules. If the question is who to test, start with screening. If the question is how to interpret a result, start with the diagnostic criteria.
- Screening for Diabetes Mellitus and Prediabetes: the symptoms that prompt testing, and the ADA recommendations on who to screen and how often, including people with cystic fibrosis or HIV.
- Diagnostic Criteria for Diabetes and Prediabetes: the fasting plasma glucose, random plasma glucose, 2-hour 75-gram OGTT and HbA1c thresholds that separate prediabetes from diabetes, how a diagnosis is confirmed, and when HbA1c should not be used.
- Classification of Diabetes Mellitus: how the type is assigned once hyperglycemia is confirmed, and how type 1 and type 2 diabetes are told apart.
- Diagnosing Gestational Diabetes Mellitus: the separate thresholds used in pregnancy, through the one-step 75-gram OGTT and the less used two-step test.