HbA1c (glycated haemoglobin)

HbA1c is the proportion of the red blood pigment haemoglobin that has sugar attached to it – the higher your blood sugar has been over the weeks, the larger this proportion. Because red blood cells live for about 120 days, the value reflects your average blood sugar over the past 8 to 12 weeks. That's why it's also called “long-term blood sugar” or your “blood sugar memory”.

It's given as a percentage or – internationally – in mmol/mol. The thresholds according to the German Diabetes Society (DDG): below 5.7% (39 mmol/mol) is normal, 5.7 to 6.4% means an increased risk (prediabetes) that should be investigated further, and from 6.5% (48 mmol/mol) it's diabetes. When an existing diabetes is being treated, individual targets are agreed, often between 6.5 and 7.5%; for older people or those at high risk of hypoglycaemia, they are deliberately set higher.

The big advantage over a single blood sugar reading: HbA1c doesn't depend on your last meal, you don't need to fast, and a one-off outlier barely distorts it. As a rule of thumb, each additional percentage point of HbA1c corresponds to an average blood sugar about 30 mg/dl higher: 6% stands for around 125 mg/dl, 7% for around 155 mg/dl.

But it has blind spots. Anything that changes the lifespan of red blood cells skews it: iron deficiency makes it look too high, while blood loss, anaemia with high red blood cell production or pregnancy make it look too low; certain haemoglobin variants and kidney disease can also distort it. Nor does it show big swings between high and low readings – that's what continuous glucose monitors are for.

If you change your habits, HbA1c shows it with a delay: about half of the value comes from the last four weeks, so the first changes appear after a few weeks, and the effect can be reliably assessed after about three months. That's a good rhythm – instead of staring at numbers every day, think in quarters.