Diabetes is diagnosed by a venous blood test conducted in a laboratory. It is not unusual that a person may have already discovered high blood sugar using a meter meant for measuring blood glucose, especially if someone in their circle of friends or family has diabetes. That said, it is not enough for a diagnosis; the home result needs to be varified with a laboratory test.
The diagnosis is diabetes if...
the plasma glucose (laboratory abbreviation P-gluk) i.e. blood sugar level measured by the lab is repeatedly 7 mmol/l (millimol in litre) or above after fasting
or
the 2-hour glucose tolerance test result in laboratory taken as a venous blood sample is above 11 mmol/l
or
glycated haemoglobin HbA1c is 48 mmol/mol (millimol in mol) or above.
If the diabetes symptoms are clear, in principle, it is enough to diagnose diabetes if the blood sugar test shows hyperglycaemia of above 11 mmol/l at any time of the day.