Updated 20 February 2017

Diagnosing diabetes

Diabetes is diagnosed by measuring the blood glucose levels in the blood.


Type 1, type 2 and gestational diabetes can be diagnosed fairly easily with a blood test that measures the levels of glucose in the blood.

Type 1 and type 2 diabetes

Diabetes can be diagnosed quite simply by measuring the levels of glucose in the blood (alternatively an HbA1c – the average blood sugar over a 3-month period measured as a % can be checked).

The normal fasting plasma glucose level is less than 5.6mmol/l

The South African Endocrine and Diabetes Association of South Africa and American Diabetes Association define diabetes mellitus as a condition fulfilling one or more of the following criteria:

- Fasting plasma glucose (plasma glucose measured before breakfast) is equal to or more than 7.0 mmol/l on two separate occasions and/or

- Random plasma glucose (blood glucose measured at any time) is over 11.1 mmol/l and/or

- Two-hour plasma glucose during glucose tolerance test is over 11.1 mmol/l

Impaired glucose or pre-diabetes is diagnosed if the 2-hour plasma glucose on a glucose tolerance test is between 7.8-11.0mmol/l.

Impaired fasting glucose (IFG) is diagnosed if the fasting plasma glucose is between 5.6-6.9mmol/l. (The WHO defines IFG cut-off at 6.1mmol/l.)

Gestational diabetes

Similar glucose levels are used in diagnosing gestational diabetes as for types 1 and 2 diabetes. Your health care team will check your blood glucose level. Depending on your risk and your test results, you may be advised to have one or more of the following tests:

- Fasting blood glucose

- Random blood glucose test 

Oral glucose tolerance test. A glucose drink is given and blood glucose levels are taken one, two and three hours later. 

Read more:

Causes of diabetes

Treating type 1 diabetes

Treating type 2 diabetes 

Reviewed by Dr Hilton Kaplan, MB BCH (Rand), FCP(SA), MMed(UCT), Specialist Endocrinologist and Physician (March 2016)


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Dr. May currently works as a fulltime endocrinologist and has been in private practice since 2004. He has a variety of interests, predominantly obesity and diabetes, but also sees patients with osteoporosis, thyroid disorders, men's health disorders, pituitary and adrenal disorders, polycystic ovaries, and disorders of growth. He is a leading member of several obesity and diabetes societies and runs a trial centre for new drugs.

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