Encyclopedia entry

HbA1c

HbA1c (glycated haemoglobin, sometimes A1C) measures the percentage of haemoglobin molecules in the bloodstream that have glucose attached. Because red blood cells live approximately 120 days, HbA1c reflects average blood glucose over the previous 8 to 12 weeks. The standard NHS test for type 2 diabetes diagnosis, control, and pre-diabetes screening.

UK reference ranges

• · Normal: under 42 mmol/mol (under 6.0 percent).
• · Pre-diabetes: 42 to 47 mmol/mol (6.0 to 6.4 percent).
• · Type 2 diabetes diagnosis: 48 mmol/mol or above (6.5 percent or above), confirmed on a second test.
• · Good diabetes control: typically under 53 mmol/mol (under 7.0 percent), adjusted by NICE NG28 based on individual factors.

When NHS tests HbA1c

• · NHS Health Check (40 to 74-year-olds, every 5 years).
• · Diabetes diagnosis confirmation.
• · Diabetes monitoring (typically 6-monthly).
• · Pre-conception planning where diabetes risk exists.
• · Targeted screening based on BMI, family history, or ethnicity-related risk.

What GLP-1 medications do to HbA1c

Both semaglutide (Wegovy, Ozempic) and tirzepatide (Mounjaro) reduce HbA1c substantially. In type 2 diabetes trials, average reductions of 1.5 to 2.5 percentage points (16 to 27 mmol/mol) are typical at maintenance doses. In non-diabetic patients on GLP-1 for weight management, HbA1c moves from upper-normal or pre-diabetic ranges into solidly normal ranges within 6 to 12 months in most patients.

Caveats and limitations

HbA1c is affected by anything that changes red blood cell lifespan: anaemia, recent blood loss, haemoglobinopathies (sickle cell trait, thalassaemia), kidney disease. Iron deficiency anaemia tends to elevate HbA1c falsely; haemolytic conditions lower it falsely. The test is also a lagging indicator; rapid recent glucose change is not reflected until 4 to 8 weeks later.

Related: CGM · GLP-1 mechanism.