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Diagnosis of diabetes

A diabetes diagnosis often follows a person seeking medical advice after experiencing symptoms of diabetes, such as feeling thirsty, frequent urination, fatigue and unexpected weight loss.

A doctor may arrange blood and urine tests to check for high glucose levels which indicate diabetes.

How are diabetes and pre-diabetes diagnosed?

The following tests are used for the diagnosis of diabetes:

  • HbA1c blood test. An HbA1c reading of 48 mmol/mol (6.5%) is the recommended cut off point for diagnosing diabetes. A value below this does not exclude diabetes diagnosed using glucose tests.
  • A fasting plasma glucose test measures your blood glucose after you have gone at least eight hours without eating or drinking anything but plain water. This test is used to detect diabetes or pre-diabetes.
  • An oral glucose tolerance test measures your blood sugar after you have gone at least eight hours without eating and drinking and two hours after you drink a glucose-containing beverage. This test can be used to diagnose diabetes or pre-diabetes.
  • In a random plasma glucose test, your doctor checks your blood glucose without regard to when you ate your last meal. This test, along with an assessment of symptoms, is used to diagnose diabetes but not pre-diabetes.

Positive test results should be confirmed by repeating the fasting plasma glucose test or the oral glucose tolerance test on a different day.

Fasting plasma glucose (FPG) test

The FPG is the preferred test for diagnosing diabetes and is most reliable when done in the morning. Results and their meaning are shown in table 1. If your fasting glucose level is more than 6 but less than 7mmol/l you have impaired fasting glucose (IFG), meaning that you are more likely to develop type 2 diabetes but do not have it yet. A level of 7mmol/l or above, confirmed by repeating the test on another day, means that you have diabetes.

Table 1. Fasting Plasma Glucose Test

Plasma Glucose Result (mmol/l)

Diagnosis

6mmol/l and below

Normal

Between 6 and below 7mmol/l

Pre-diabetes
(impaired fasting glucose)

7mmol/l and above

Diabetes*

*Confirmed by repeating the test on a different day.

Oral glucose tolerance test (OGTT)

Research has shown that the OGTT is more sensitive than the FPG test for diagnosing pre-diabetes, but it is less convenient to administer. The OGTT requires you to fast for at least eight hours before the test. Your plasma glucose is measured immediately before and two hours after you drink a liquid containing 75 grams of glucose dissolved in water. Results and what they mean are shown in table 2. If your blood glucose level is 7.8mmol/l or more, but less than 11.1mmol/l two hours after drinking the liquid, you have impaired glucose tolerance or IGT, meaning that you are more likely to develop type 2 diabetes but do not have it yet. A two-hour glucose level of 11.1mmol/l or above, confirmed by repeating the test on another day, means that you have diabetes.

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