When person does not have diabetes their fasting blood sugar is normally 4.0–6.0 mmol/l (millimol in litre). After a meal, the blood sugar level of a healthy person is below 7.8 mmol/l in a laboratory venous blood sample and below 8.9 mmol/l when taken from the fingertip.
The blood sugar level of a person with diabetes can become very high if untreated or in relation to a severe infection, for example between 20–30 mmol/l. High blood sugar is called hyperglycaemia.
According to the international classification, hyperglycaemia is divided in the following way:
high blood sugar: 10.0–13.9 mmol/l
very high blood sugar: > 13.9 mmol/l
In practice, it is important to tell the difference between a temporary rise in blood sugar and a recurring, trend-like, increase in the blood sugar level.
A temporarily high level will either correct itself or it can be corrected with an additional dose of rapid-acting insulin, if person uses insulin. If blood sugar increases repeatedly at a certain time of the day or in connection with a specific meal or activity, the possible reasons should be investigated and treatment adjusted accordingly.
Blood sugar that is high for a longer period of time attaches itself to the structures of the blood vessel walls and connective tissue. This results in damage to the small and large blood vessels, nerves and the structure of connective tissue. Over the years, it causes various complications related to diabetes.