Forgot to Fast?

The importance on plasma glucose values

We have recently shown that by using a casual plasma glucose value of >5.5 mmol/l as a cut point for screening, the yield of newly diagnosed diabetic subjects and those with impaired glucose tolerance (IGT) is greatly enhanced in comparison to using 6.5 or 7.5 mmol/l as cut-points for diagnostic assessment using the 1999 World Health Organization (WHO) criteria (1–3). In lowering this screening cut point for casual measurement, factors such as the time of eating before this test on the result may be relevant to its utility and warrant investigation.

A total of 4,876 high-risk subjects, identified from 50,859 individuals participating in the Australian Diabetes Screening Study (1), provided fasting plasma glucose (FPG) and 2-h plasma glucose (2hPG) test results to confirm diabetes and IGT status. High risk was defined as having either two or more symptoms and/or two or more diabetes risk factors, casual plasma glucose values of >5.5 mmol/l, and no known diabetes (2). The time between when the subjects last ate and the casual plasma glucose test was calculated in minutes and grouped into hour blocks (0–360 min). Subjects were diagnosed as having diabetes or IGT using the 1999 WHO criteria (3).

A casual plasma glucose of >5.5 mmol/l yielded a positive diagnosis of diabetes in 557 subjects (20%) and of IGT in 776 subjects (28%) >2 h after eating. Within 0–2 h of eating, the diagnostic yield of diabetes was less (316 subjects, −15%) but IGT rate similar (541 subjects, −26%). In subjects with risk factors for diabetes, a casual plasma glucose of >5.5 mmol/l generates a similar proportion of IGT cases irrespective of time since eating.

Eating within 2 h of a casual glucose test in comparison to after 2 h resulted in a significantly higher level (7.09 ± 1.66 vs. 6.6 ± 1.38 mmol/l, respectively). However, if the screening cut point was raised for subjects who had consumed food within 2 h to 6.5 mmol/l, this would have yielded 249 (23%) diabetic subjects and 295 (25%) subjects with IGT. Raising the casual plasma glucose threshold to 7.5 mmol/l would yield 187 (33%) diabetic subjects and 157 (28%) subjects with IGT. Despite the fact that consuming food within 2 h of a casual glucose test results in significantly increased values, the 5.5 mmol/l cut point identified an additional 67 subjects (27%) with frank diabetes. It is also important to note that the 5.5 mmol/l cut point almost doubled the number of subjects with IGT in comparison to the 6.5 mmol/l cut-point, and the number tripled if 7.5 mmol/l was chosen as a cutoff.

Thus the 5.5 mmol/l cut point seems to be valuable irrespective of the time since eating, as it results in early identification of subjects with IGT, which may aid in the prevention of the micro- and macrovascular complications associated with diabetes.