People whose doctors test their plasma glucose levels in the afternoon may have normal values and therefore not be diagnosed as having diabetes when in fact, they do. This was the conclusion of researchers from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). They compared the test results of over 6,400 people who were tested in the morning following 13.5 hours of fasting with those from about 6,400 people who were tested in the afternoon, after fasting for 7 hours. They found that the fasting plasma glucose levels were consistently higher in the morning group than the afternoon group, with an overall difference of 5 mg/dl.

They recommended that the diagnostic standard of glucose levels be changed when tests are conducted in the afternoon, to 114 mg/dl or higher instead of the current standard of 126 mg/dl. They also advised doctors to repeat tests on people with these levels on a different day, in the morning, to confirm the diagnosis.