Curriculum · Metabolic and Endocrine and Diabetes Mellitus
HbA1c: what it measures and what alters it
What it is
Some of the glucose in the blood binds to haemoglobin, the protein that carries oxygen in the red blood cells, and that combination is haemoglobin A1c. The amount of HbA1c formed is directly related to the average concentration of glucose in the bloodstream. Red blood cells live for 2-3 months, and because of this the amount of HbA1c reflects the average level of glucose during the last 2-3 months. That is why it is the best investigation for long control of diabetes: if control is poor, the blood glucose levels are high and the HbA1c is higher with them.
How it is diagnosed
In pregnancy the numbers are set out. An HbA1c above 5.7 is one of the reasons to screen early for gestational diabetes. Overt diabetes in pregnancy is diagnosed by RBG at or above 11.1 mmol/l, or FBS at or above 7 mmol/L, or HbA1c at or above 6.5%. The desirable HbA1c value in pregnancy is below 6.5%, and below 6% if the woman is not a known diabetic. A high HbA1c in the first trimester carries a risk of miscarriage.