HbA1c %

The older DCCT percentage unit, still used in the USA and on some private reports; 4.0 to 5.9% corresponds to the UK range of 20 to 41 mmol/mol. The same cautions about red cell lifespan apply as for HbA1c in mmol/mol.

This is the same test as HbA1c, glucose-coated haemoglobin averaged over the previous two to three months, reported in the older percentage units rather than the mmol/mol used by UK laboratories since 2011. The two scales measure exactly the same thing; only the numbers differ. For reference, 6.5% is the same as 48 mmol/mol.

It is requested for the same reasons as HbA1c: to look at how the body has handled sugar over the previous two to three months. It appears on reports from US laboratories and some private services.

• Diabetes and pre-diabetes
• Steroid medicines
• Iron or vitamin B12 deficiency, because red cells live longer
• Kidney disease
• Older age, a little, in the general population

• Better sugar control with diet, exercise, weight loss and diabetes medicines
• Anything that shortens the life of red cells, such as blood loss, haemolysis or a recent transfusion
• Pregnancy, in the second and third trimesters
• Some inherited haemoglobin variants, depending on the laboratory method

• No fasting is needed and a recent meal does not affect it
• Anything that changes how long red cells live changes the figure without any change in blood sugar
• Different laboratory methods handle haemoglobin variants differently

No fasting needed and the time of day does not matter. It reflects the previous two to three months. To compare with a UK result in mmol/mol, use the IFCC conversion rather than reading the two numbers side by side.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.