Diabetes is diagnosed by performing a blood test. The test usually reveals high blood glucose. Steps in diagnosis includes:

• If a patient presents with symptoms of diabetes a blood test for blood glucose is ordered. In most cases of type 2 diabetes mellitus there may be little or no symptoms. This means high blood sugar may be detected on a routine blood tests for example one taken before a surgery. It is important to make the diagnosis early since uncontrolled high blood glucose for a long duration leads to long term damage to blood vessels and other complications.
• Diabetes is diagnosed on the basis of a single abnormal plasma glucose reading. When taken randomly at any time of the day the levels are significant if they are above 11.1 mmol/L and when taken after an overnight fast, the numbers are significant if above 7 mmol/L (126 mg/dL). This is considered positive for diabetes when there is presence of diabetic symptoms such as thirst, increased urination, recurrent infections, weight loss, drowsiness etc.
• In people who have no symptoms an abnormal random plasma glucose followed by two more abnormal fasting blood glucose readings (over 7 mmol/L) is significant. Patients with fasting glucose levels from 100 to 125 mg/dL (6.1 and 7.0 mmol/L) are considered to have impaired fasting glucose.
• Once an abnormal fasting blood glucose is obtained, blood glucose is tested again two hours after a full meal. This usually means after 75 g anhydrous glucose when an oral glucose tolerance test (OGTT) is performed. Readings over 11.1 mol/L is significant for diabetes. Patients with plasma glucose at or above 140 mg/dL or 7.8 mmol/L, but not over 200, two hours after a 75 g oral glucose load are considered to have impaired glucose tolerance. This raises the risk of acquiring diabetes in near future if uncontrolled.
• The World Health Organization (WHO) now recommends that glycated haemoglobin (HbA1c) can be used as a diagnostic test for diabetes. This indicates the blood sugar control in an individual over the last three months. Even a single episode of uncontrolled blood sugar during this period can be detected as a high HbA1c. An HbA1c of 48 mmol/mol (6.5%) is the cut-off point for diagnosing diabetes. A value less than 6.5% does not however exclude diabetes diagnosed using blood glucose tests.
• Diagnosis also involves assessment of damage to kidneys, eyes and other organs due to long standing diabetes.