It is a puzzle that confuses many diabetes patients: two people with similar day-to-day blood sugar readings can end up with very different HbA1c results. The explanation lies in what the HbA1c test actually measures, and in the surprising number of factors that can nudge it up or down.

What HbA1c Actually Measures

HbA1c, or glycated haemoglobin, reflects average blood sugar over the previous two to three months. Sugar in the bloodstream attaches itself to haemoglobin inside red blood cells, and since these cells live for roughly 120 days, the test offers a long-term picture that daily finger-prick readings cannot.

But that 120-day lifespan is an average, not a rule. Anything that shortens or lengthens the life of red blood cells changes how much sugar gets a chance to attach, and therefore changes the HbA1c reading, even when actual blood sugar levels are identical.

Why Readings Can Differ

Conditions that accelerate red blood cell turnover, such as haemolytic anaemia or recent blood loss, can falsely lower HbA1c, because younger cells have had less time to accumulate sugar. Conversely, iron deficiency anaemia can falsely raise it. Haemoglobin variants, kidney disease, liver disease and even pregnancy can also skew results.

This is why doctors caution against reading HbA1c in isolation. For people with anaemia, haemoglobin disorders or kidney problems, alternative markers like fructosamine or continuous glucose monitoring may give a truer picture. The takeaway is reassuring: a surprising HbA1c does not always mean your sugar control has slipped. Sometimes, it means the test needs a second opinion.

Patients who notice a mismatch between their HbA1c and their home readings should discuss it with their doctor rather than adjusting medication on their own. A simple blood workup can reveal anaemia or other conditions affecting the result, and the treatment plan can then be fine-tuned accordingly. In diabetes care, understanding the test is just as important as taking it.