Type 1 diabetes is classified as an autoimmune disease, where the pancreatic beta-cells, responsible for insulin secretion are destroyed. Elevated blood glucose levels are due to absence of insulin. Type 1 diabetes is usually diagnosed in children and young adults. Type 1 diabetes requires taking insulin daily.

Type 2 diabetes is characterized by decreased insulin sensitivity or decreased secretion of insulin. Decreased insulin sensitivity means that the cells have become resistant to insulin. The pancreas is secreting insulin, but it is not enough to keep blood glucose levels normal. Type 2 diabetes can develop at any age, even during childhood. However, it occurs most often in middle-aged and older people. Type 2 is the most common type of diabetes.

Gestational diabetes is a type of diabetes that affects pregnant women, usually during the second or third trimester. The hormones produced during pregnancy can make it difficult for the body to use insulin properly, putting one at an increased risk of insulin resistance. Because pregnancy places a heavy demand on the body, some women are less able to produce enough insulin to overcome this resistance which then manifests itself as diabetic blood glucose levels. 

Women with gestational diabetes don’t have diabetes before their pregnancy, and after giving birth it usually goes away. Women with gestational diabetes are however more at risk for developing diabetes later in life.

There are also some other more rare forms of diabetes such as different types of monogenic diabetes, cystic fibrosis-related diabetes, and diabetes caused by rare syndromes. Certain medications such as steroids and antipsychotics could lead to other types of diabetes, as well as surgery or hormonal imbalances. About 2% of all diabetic cases fall into these forms. 

Diabetes is commonly diagnosed either with fasting plasma glucose test (FPG) or an oral glucose tolerance test (OGTT). Sometimes a random plasma glucose test is also performed. 

A fasting plasma glucose test is performed by drawing blood after fasting for at least 8 hours. Typically blood is drawn in the morning after a night of fasting. An oral glucose tolerance test measures blood sugar by drawing blood after gone at least 8 hours without eating and then again two hours after you drink a glucose-containing beverage. Both tests can be used to diagnose prediabetes and diabetes. 

In a random plasma glucose test, blood sugar is checked without regard to when you ate your last meal. This test, along with an assessment of symptoms, is used to diagnose diabetes, but not prediabetes. 

Positive test results should be confirmed by repeating the same test used on a different day. Threshold values for diabetes are defined either as mmol/l or mg/dL depending on a country. The WHO criteria for diabetes are:

• a random venous plasma glucose concentration ≥ 11.1 mmol/l or

• a fasting plasma glucose concentration ≥ 7.0 mmol/l (whole blood ≥ 6.1 mmol/l) or

• two hour plasma glucose concentration ≥ 11.1 mmol/l two hours after 75g glucose in an oral glucose tolerance test.

The criteria for diagnosing gestational diabetes is different. Gestational diabetes should be diagnosed if the woman has either:

• a fasting plasma glucose level of 5.6mmol/l or above or

• a 2-hour plasma glucose level of 7.8mmol/l or above in an oral glucose tolerance test.

Sometimes the doctor might also measure glycated hemoglobin (HbA1c) levels. HbA1c is used to check the long term blood sugar balance of diabetic patients. It reflects average plasma glucose over the previous 8 to 12 weeks. 

If the plasma glucose concentration is found to be above normal, but below the defined threshold for diabetes, one might get a diagnosis for prediabetes, intermediate hyperglycemia or being told that one is at a high risk for developing diabetes in the near future. Elevated fasting plasma glucose is called impaired fasting glucose (IFG), while elevated plasma glucose during the OGTT is called impaired glucose tolerance (IGT). 

Prediabetes diagnosis should be considered as the ultimate warning sign that one is on their way to developing diabetes. It is much easier to turn things around and avoid developing type 2 diabetes at this stage compared to when the condition has already progressed much further to a stage of diabetes.