Introduction

People often walk in braced for this number like it is a school report card, and I try to talk them down from that before we even look at it. HbA1c is genuinely one of the more useful tests we have, but reading it well means understanding what it actually measures, and it is not quite what most people assume.

It is an average, not a snapshot

HbA1c measures the share of the haemoglobin inside your red blood cells that has glucose stuck to it. Because a red blood cell lives for around 120 days, that measurement ends up reflecting your average blood sugar over roughly the last two to three months, not how you have been doing this particular week. That distinction matters enormously. A single rough fortnight will barely move it, and a single very good one will not fix it overnight either.

What the ranges actually mean

Below 5.7 percent is considered normal, which roughly lines up with a fasting glucose under 100. Between 5.7 and 6.4 percent sits prediabetes, the early stage where glucose tolerance is starting to slip, roughly a fasting reading of 100 to 125. At 6.5 percent or above, that is diagnostic for diabetes, corresponding to a fasting glucose of 126 or higher. If you are already being treated, we tend to think of 6.5 percent or under as good control, 6.6 up to 7.5 as moderate control that probably needs a small tweak, and anything above 7.5 as a level that raises the risk of complications down the line.

Turning the percentage into a number you recognise

There is a rough formula that links the two, estimated average glucose in mg per deciliter equals 28.7 times your HbA1c minus 46.7, which sounds more complicated written out than it is in practice. The version I actually use with patients is simpler. Each full percentage point your HbA1c rises corresponds to roughly a 22 to 25 point rise in your average day-to-day blood sugar. It is not exact, but it is close enough to give you a real feel for the number.

Key points

  • HbA1c is a two to three month average, not a snapshot, because it is anchored to how long a red blood cell actually lives, around 120 days.
  • Under 5.7 percent is normal, 5.7 to 6.4 is prediabetes, and 6.5 percent or above is diagnostic for diabetes.
  • Roughly speaking, every 1 percent rise in HbA1c corresponds to about a 22 to 25 point rise in average blood sugar.