How is diabetes diagnosed?

Diabetes is diagnosed if blood sugar readings are above certain internationally agreed-upon levels (“thresholds”). Because blood sugar levels tend to go up and back down again after a meal, for example, the threshold varies depending on how long ago you last ate.

It is important to note that the is usually only confirmed if at least two different blood sugar readings are too high. But if you also have typical symptoms (like extreme thirst), one clearly too-high reading is already enough.

Fasting blood sugar test

This test is often done in the morning before breakfast. Diabetes is diagnosed if the fasting blood sugar level is over 126 milligrams per deciliter (mg/dL). That’s 7 millimoles per liter (mmol/L).


Sugar molecules can bind to the red pigment in your blood known as hemoglobin. Your HbA1c value shows how much of your hemoglobin has been changed in this way. It is a measure of your average blood sugar level over the last 8 to 12 weeks. People with an HbA1c over 6.5% are considered to have diabetes.

Random glucose test

Blood sugar levels are also often tested when blood is taken for another reason. In these random tests, levels above 200 mg/dL (11.1 mmol/L) are a sign that you may have diabetes. Then doctors usually recommend that you do a fasting blood sugar test or a glucose tolerance test to find out for sure.

Glucose tolerance test

The oral glucose tolerance test (OGTT) measures how well your body can process sugar. It is done if the results of other tests aren’t clear or if you have a high risk of developing diabetes. First, you drink a glass of water with 75 grams of sugar (glucose) dissolved in it. This makes your blood sugar levels rise, but they should go back down quite quickly after that. If they’re still above 200 mg/dL (11.1 mmol/L) after two hours, you’re diagnosed with diabetes.

Bundesärztekammer (BÄK), Kassenärztliche Bundesvereinigung (KBV), Arbeitsgemeinschaft der Wissenschaftlichen Medizinischen Fachgesellschaften (AWMF). Nationale Versorgungsleitlinie: Therapie des Typ-2-Diabetes. S3-Leitlinie. AWMF-Registernr.: nvl-001g. 2023.

Petersmann A, Müller-Wieland D, Müller UA et al. Definition, Classification and Diagnosis of Diabetes Mellitus. Exp Clin Endocrinol Diabetes 2019; 127(S 01): S1-S7.

IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

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Updated on December 18, 2023

Next planned update: 2026


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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