Introduction

I had a patient once whose HbA1c looked beautiful, and whose actual day-to-day sugars were a mess, and it turned out the mismatch was not the diabetes at all. It was undiagnosed anaemia quietly skewing the number underneath it. HbA1c is genuinely one of our better tests, but it leans entirely on your red blood cells behaving normally, and when something disturbs those cells, the number can drift away from the truth in either direction.

When you can trust it without a second thought

For routine, ordinary follow-up, HbA1c is dependable. That means no condition interfering with how long your red blood cells live, and having been fairly stable on your current diet and treatment for the last couple of months rather than mid-change. Most people, most of the time, fall into exactly this category, which is why it remains the standard test it is.

The situations that can throw it off

Anaemia is the big one, whether from low iron or from red cells breaking down too fast, and so is anything involving recent blood loss or a transfusion. Dialysis and serious liver disease can distort it too, along with high-dose vitamin C and a small number of specific medicines, dapsone among them. Pregnancy is its own category entirely, and an oral glucose tolerance test is generally trusted more than HbA1c during it. A transfusion specifically from someone else with diabetes, or sickle-cell disease, tends to push the reading falsely high rather than low.

Reading a confusing result properly

When the numbers do not add up, there is usually a pattern worth chasing. A normal HbA1c sitting next to a fasting sugar that is a little high can point toward early insulin resistance, and a HOMA-IR test is a sensible next step to check that properly. A high HbA1c with a completely normal fasting reading often means the real problem is happening after meals, not overnight. And a suspiciously low HbA1c in someone who is clearly unwell with classic symptoms is one of the situations where I go looking for haemolytic anaemia or unrecognised blood loss before I take the number at face value. Treat it as one piece of the picture, not the whole verdict, and lean on your own doctor to read it alongside everything else.