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The diagnosis of type 2 diabetes

Scientific support: Dr. Yanislava Karusheva

In people with type 2 diabetes, the blood glucose level is elevated. There are several ways of diagnosing diabetes. The German and American Diabetes Society name 4 methods for diagnosis:

  • HbA1c value (long-term blood glucose value)
  • Fasting blood glucose level
  • 2-hour blood glucose level after an oral glucose tolerance test with 75 grams of glucose (OGTT)
  • Random blood glucose

HbA1c value

The HbA1c value, also known as the long-term blood sugar value, describes the average blood glucose concentration over the last 8 to 12 weeks. This value can be measured with a blood sample. Patients do not have to do this test on an empty stomach.

If the HbA1c value is 6.5 percent or higher (48 mmol/mol), the patient has diabetes mellitus.

If the HbA1c value is under 5.7 percent (39 mmol/mol), the disease can be ruled out. If the HbA1c value is between these limits, doctors carry out further tests of the fasting blood glucose and the OGTT 2-hour blood glucose value (OGTT = oral glucose tolerance test).


Fasting blood glucose level

Fasting blood glucose is taken in the morning from venous plasma in a blood sample after a fasting period of at least 8 hours.

If the fasting blood glucose level is 126 mg/dl (7.0 mmol/l) or higher, the patient has diabetes mellitus. A reading between 100 mg/dl (5.6 mmol/l) and 125 mg/dl (6.9 mmol/l) means an impaired fasting glucose tolerance, or also prediabetes. To check the levels, doctors usually do an oral glucose tolerance test.

Good to know:

Impaired fasting glucose tolerance, also known as prediabetes is potentially a precursor to diabetes.


Many other diabetes societies and the World Health Organization (WHO) set their level for impaired fasting glucose tolerance or prediabetes somewhat higher – at 110 mg/dl (6.1 mmol/l).

Oral glucose tolerance test (OGTT)

The oral glucose tolerance test (OGTT) is used to check the blood glucose level and is carried out if diabetes mellitus is suspected. It shows how well sugar in food can be absorbed by the cells in the body. With a normal metabolism, eating carbohydrates leads to an increase in blood glucose concentration, which triggers the release of insulin from the pancreas. The insulin causes the cells in the body to absorb glucose, which then promptly reduces the blood glucose level. With type 2 diabetes, the pancreas releases too little insulin, or the released insulin does not act properly in the cells of the body. This leads to a higher and more sustained increase in the blood glucose level.

  • The OGTT is done with the patient sitting or lying down. The patient may not eat or smoke for 8 to 12 hours beforehand.
  • Three days before the OGTT, a high carbohydrate diet should be followed (at least 150 grams of carbohydrate at day).
  • To begin the test (time 0), the patient drinks the premixed glucose solution with 75 grams of glucose within 5 minutes. During the 2-hour measuring period, blood is taken at 0 minutes and at 120 minutes to determine the blood glucose level.

A normal glucose level is defined as a fasting glucose level (time 0) of less than 100 mg/dl (less than 5.6 mmol/l) and an 2-hour OGTT blood glucose level of less than 140 mg/dl (less than 7.8 mmol/l), measured in venous plasma. Fasting blood glucose levels of 126 mg/dl (7.0 mmol/l) or higher or 2-hour OGTT values of 200 mg/dl (11.1 mmol/l) or higher means that the patient has diabetes mellitus.

People with a blood glucose level above the normal glucose tolerance, but below the diabetes diagnosis have a higher risk of developing diabetes. This transition range is called prediabetes and includes impaired fasting glucose (fasting blood glucose of 100 mg/dl or higher (5.6 mmol/l or higher) and less than 126 mg/dl (less than 7.0 mmol/l)) and impaired glucose tolerance (2-hour OGTT of 140 mg/dl or higher (7.8 mmol/l or higher) and less than 200 mg/dl (less than 11.1 mmol/l)).


Random blood glucose

If patients already feel that they have symptoms of diabetes, such as increased urination or increased thirst and dizziness, a random blood glucose test in venous plasma may be a criterion by which diabetes is diagnosed.

A fasting blood glucose level of 200 mg/dl (11.1 mmol/l) or higher indicates that the patient has diabetes. The diagnosis should also be checked with a fasting blood glucose test or with an OGTT.

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American Diabetes Association: Standards of Medical Care in Diabetes - 2019. In: Diabetes Care, 2019, 42: S1-S193
Nauck, M. et al.: Definition, Klassifikation und Diagnostik des Diabetes mellitus. In: Diabetologie, 2017, 12: S94-S100
As of: 04.12.2019