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1. The diagnosis of diabetes requires a fasting plasma glucose of 7mmol/L (126mg/dL) or more.

2. Diabetes can also be diagnosed when the 2-hour post prandial plasma glucose is 11.1mmol/L (200mg/dL) or more.

3. Impaired glucose tolerance is diagnosed when postprandial plasma glucose values are higher than normal but not high enough for the diagnosis of diabetes. Impaired glucose tolerance is said to be present when the 2- hour postprandial plasma glucose is between 7.8mmol/L (140mg/dL) and 11mmol/L (198mg/dL).

4. According to the World Health Organisation guidelines, impaired fasting glucose is present when the fasting plasma glucose is between 6.1mmol/L and 6.9mmol/L. According to the American Diabetic Association, impaired fasting glucose is present when the fasting plasma glucose is between 5.6mmol/L and 6.9mmol/L. This difference in views is because of differences in opinion regarding what constitutes normal fasting plasma glucose.

Two views about what constitutes "normal" fasting plasma glucose

There are two views about what constitutes a normal fasting plasma glucose. One view is from the World Health Organization (WHO) and the other view is that of the American Diabetic Association (ADA). According to the WHO guidelines (2006), a fasting plasma glucose of 6mmol/L (108mg/dL) or lower is normal. Guidelines (2003) by the American Diabetic Association have suggested that the normal fasting plasma glucose should be lower than 5.6mmol/L (100mg/dL). The reason for these two differing guidelines lies in the significance attached to these target values of fasting plasma glucose. WHO has set 6mmol/L (108mg/dL) as the upper limit for normal fasting plasma glucose because of the increased risk of developing diabetes in the future for those whose fasting sugars are more than this value. A study in Mauritius had shown that the 5-yr incidence of diabetes was in the order of 15% for a fasting plasma glucose of 5.5–5.7mmol/l (99mg to 103mg/dL) compared with 30% for a fasting plasma glucose of 6.1–6.9mmol/l (110mg to 124mg/dL). However the ADA looks at the risk of developing diabetes-related cardiovascular complications and not at the risk of future diabetes. A meta-analysis in JAMA in 2005 had shown that cardiovascular risk increases with fasting plasma glucose levels above 5.5mmol/L (99mg/dL).

5. The diagnostic criteria are different for gestational diabetes because in pregnancy there are only normal and gestational diabetes while in the non-pregnant state there is normal, impaired glucose tolerance /impaired fasting glucose, and diabetes. In other words, the term gestational diabetes also includes glucose intolerance.

The diagnostic criteria for gestational diabetes mellitus using the 75gram oral glucose tolerance test is often stated like this:
Fasting - more than 5.3mmol/L
One hour - more than 10mmol/L
Two hours - more than 8.6mmol/L
Three hours - more than 7.8mmol/L

The 2007 clinical practice guidelines by the American Association of Clinical Endocrinologists on the oral glucose tolerance test for the diagnosis of gestational diabetes tell us that only fasting, one-hour and two-hour samples are needed. Estimation in the third hour is not needed.


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