HbA1c is shorthand for glycated hemoglobin A1c, a surrogate marker for blood glucose levels.

People with diabetes mellitus have higher levels of HbA1c. A diabetic with good glucose control has a HbA1c level that is close to the reference range. A very high HbA1c represents poor control.

The HbA1c level is proportional to average blood glucose concentration over the previous two to four weeks (it was previously thought to average over approximately three months). The mapping between HbA1c and blood glucose average is shown in the table below.

Endocrinologists currently believe that HbA1c values below 6.5% (American College of Endocrinology: The AACE System of Intensive Diabetes Self-Management — 2002 Update) are acceptable and indicate good control; normal is 3-6%. Interpreting results is somewhat difficult because (1) laboratory results can differ depending on the analytical technique and (2) biological variation between individuals can be up to 1% (i.e. two individuals with the same average blood sugar can have A1C values that differ by up to 1%).