How is diabetes diagnosed, and what does an HbA1c test really measure?

Diabetes is diagnosed with a blood test for glucose, by showing any one of several results. One is a fasting plasma glucose of at least 7.0 mmol/L (126 mg/dL), measured after fasting overnight or at least 8 hours. Another is a plasma glucose of at least 11.1 mmol/L (200 mg/dL) two hours after a 75 gram oral glucose load, in what is called a glucose tolerance test. A third is a glycated hemoglobin, or HbA1c, of at least 48 mmol/mol (6.5%). A fourth is symptoms of high blood sugar together with a plasma glucose of at least 11.1 mmol/L (200 mg/dL), fasting or not. A positive result without unequivocal high blood sugar should be confirmed by repeating a test on a different day.

The HbA1c test works differently from the others, and cleverly. When blood glucose is high, glucose molecules attach to the hemoglobin in red blood cells, and the longer hyperglycemia lasts, the more glucose binds. Once a hemoglobin molecule is glycated, it stays that way, so the build-up reflects the average glucose the cell was exposed to during its life. Because the average lifespan of a red blood cell is three to four months, the test is treated as a three-month average.

The American Diabetes Association added HbA1c as a diagnostic criterion for diabetes in 2010.