How is type 2 diabetes diagnosed?

700 people in the UK are diagnosed with type 2 diabetes every single day. But how is a type 2 diabetes diagnosis made?
Napala Pratini
min read
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Quick summary

  • Type 2 diabetes can be diagnosed with a combination of clinical and laboratory test criteria, often including an HbA1C measurement
  • There are set ranges for clinical diagnosis of prediabetes and type 2 diabetes

Type 2 diabetes is an often silent disease, with 1 in 70 people in the UK living with undiagnosed type 2 diabetes. This is in part due to the fact that symptoms are often vague, or in some cases absent altogether. For some people, a diagnosis of type 2 diabetes will arise due to a serious complication, however for many others it will be detected as a side effect of an examination done for other reasons or as a result of a routine check-up. Whether type 2 diabetes has been on your mind for a while (perhaps due to a family history) or you’ve only now started thinking about your blood sugar, this article will explain everything you need to know about the diagnosis of type 2 diabetes.

How is type 2 diabetes diagnosed?

A diagnosis of type 2 diabetes is primarily based on symptoms as well as the results of blood tests. An ever-present feature of type 2 diabetes (and in all forms of diabetes mellitus) is hyperglycaemia (that is, elevated blood sugar levels); therefore, identification of hyperglycaemia is crucial to diagnosis.

Clinical diagnosis

In many cases, people with type 2 diabetes do not experience any symptoms, or their symptoms are confused with another condition. For this reason, the majority of people who suffer from the disease do not go to the doctor or do not show symptoms before being diagnosed with type 2 diabetes.

The most common symptoms which are experienced include:

  • A condition called polyuria which causes the need to urinate frequently. This, as with the other symptoms, is a result of elevated blood sugar: When the kidneys filter blood with high levels of glucose in it, some of this sugar is released into the urine
  • The sensation of an intense and sometimes unquenchable thirst, a condition defined as polydipsia that results from the loss of water in urine and a compensatory mechanism in the hypothalamus, the region of the brain that regulates primary needs including food, thirst, and body temperature.
  • Blurred vision, which results from an excess of water in the cornea, a region of the eye that allows us to see clearly only when it is completely transparent.

Diagnostic blood tests

Laboratory tests are complementary to the clinical picture and include several tests that aim to quantify the amount of sugar present in the blood, both in dissolved form in plasma and in the form that binds to the proteins that transport oxygen to tissues throughout the body.

Normal HbA1c values range from 4 to 5.6%

The most frequently used tests for diagnosis are:

  • Glycated haemoglobin test, commonly called HbA1c: This test indirectly measures the average level of sugar in the blood over the past two or three months, and is the standard test used for type 2 diagnosis in the UK. Haemoglobin is the protein mentioned above that transports oxygen around the body, but it also binds to excess sugar in the blood, so HbA1c measures the percentage of haemoglobin that’s bound to sugar… higher percentage equals higher average blood sugar. Normal HbA1c values range from 4 to 5.6%, and this test can be done at any time of day (before or after eating).
  • Fasting blood sugar test: Random tests of blood glucose aren’t sufficient to diagnose type 2 diabetes, since blood glucose levels can be impacted by whether and what you’ve had to eat and/or drink. However, fasting blood sugar gives a better picture of how well your body controls blood sugars. This test is done after fasting for at least 8 to 12 hours, so it’s typically done in the morning. A fasting blood sugar reading of less than 100 mg/dl is considered normal.
  • Random blood glucose test: This test involves drawing blood at any time of the day, regardless of when you last ate. A blood glucose level between 70 and 140 mg/dl is considered normal, however random tests are not typically used to diagnose type 2 diabetes (due to the influence that food and drink have on your blood glucose reading).
  • Oral Glucose Tolerance Test (OGTT): This test evaluates how well your body is able to handle sugar that’s been ingested, and is done by giving you a high-sugar solution to drink, typically after fasting for 8-12 hours. In healthy people, blood sugar rises but after insulin intervenes it comes back to normal values; in people suffering from diabetes, blood sugar levels drop back to normal very slowly or don’t come back to the values considered normal. Blood glucose measurements are taken at either 30 or 60 minute intervals after ingesting the solution, and your measurements are compared to set standard for normal, prediabetic, and diabetic. Being a bit laborious, this test is not commonly used for diabetes diagnosis, except in pregnant women.

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Type 2 diabetes diagnosis criteria

Now that you know how type 2 diabetes is diagnosed, we’ll dive into the criteria required to make a diagnosis.

Chart showing ranges for prediabetes and type 2 diabetes based on HbA1c, fasting blood glucose, and OGTT


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