Physicians use glucose tests to diagnose diabetes. These blood tests measure the level of glucose (blood sugar) in a person’s bloodstream.

Often when people have a physical examination they are screened for diabetes with a fasting plasma glucose test (FPG). An FPG is usually performed in the morning because this makes it easier for the patient to fast for the required eight hours.

Glucose is measured in milligrams per deciliter (mg/dL) of blood. FPG results below 100 mg/dL are normal. Glucose between 100 and 125 mg/dL is considered prediabetes. Glucose above 125 mg/dL indicates diabetes. To confirm diagnosis, another glucose test should be performed on another day, according to the National Institute of Diabetes and Digestive and Kidney Disorders.

If glucose testing determines that a patient has diabetes, additional tests may be offered to establish the type. For example:

• A C-peptide test can distinguish autoimmune from metabolic diabetes. People with type 2 diabetes have C-peptide, which is a byproduct of insulin production, but people with type 1 diabetes and latent autoimmune diabetes of adulthood do not or have a very low level.
  
  
• Autoantibody testing can reveal misguided antibodies present in autoimmune but not metabolic diabetes.
  
  
• Genetic tests can help diagnose conditions such as maturity-onset diabetes of the young and Wolfram syndrome.
  
  
• Other tests, such as thyroid blood tests, may be ordered to find the cause of secondary diabetes.

During pregnancy, usually during the 24th to the 28th week, women may be screened for gestational diabetes with a glucose challenge test, which evaluates the body’s ability to metabolize sugar. Blood is drawn an hour after the patient drinks a solution containing 50 grams of glucose. If results are abnormal, an additional, more complicated blood test called an oral glucose tolerance test (OGTT) is used to confirm diabetes.