Diabetes Diagnosis

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By Ananya Mandal, MDReviewed by April Cashin-Garbutt, MA (Cantab)

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.