Diabetes mellitus is a group of conditions defined by persistently raised blood glucose (hyperglycemia). It is diagnosed from specific laboratory test results, not from symptoms alone.
Why is diagnosis a work-up and not one test?
The American Diabetes Association (ADA) criteria name the tests that can be used: plasma glucose measured fasting, plasma glucose at a random time, plasma glucose after an oral glucose tolerance test (OGTT), and glycated hemoglobin (HbA1c). The criteria also set the threshold that makes a result abnormal and the conditions under which results are enough to confirm the diagnosis.
The tests are not interchangeable. They reflect different aspects of glucose metabolism, so they answer different questions. The clinical context decides which test to use and how many abnormal results are needed. A single number is therefore not the whole story, and the route goes through four questions: who should be tested, what a result means, which type of diabetes it represents, and how pregnancy changes the rules.
Which note answers which question?
- Screening for Diabetes Mellitus and Prediabetes answers who to test. It covers 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 answers what a result means. It gives the fasting plasma glucose, random plasma glucose, 2-hour 75-gram OGTT and HbA1c thresholds, how the thresholds separate prediabetes from diabetes, how a diagnosis is confirmed, and when HbA1c should not be used.
- Classification of Diabetes Mellitus answers which type it is. It explains how the type is assigned once hyperglycemia is confirmed and how type 1 and type 2 diabetes are told apart.
- Diagnosing Gestational Diabetes Mellitus answers how pregnancy changes the rules. It gives the separate thresholds used in pregnancy, the one-step 75-gram OGTT and the less used two-step test.
To find who is tested, start with screening; to interpret a result already in hand, start with the diagnostic criteria. The first question is how someone with raised glucose can be found when nothing feels wrong.