Fasting glucose vs HbA1c: why both

Two tests, two different questions

A fasting glucose asks what is your blood sugar right now, after a night without food. HbA1c asks what has it been doing for the past three months. They are not two attempts at the same measurement, and a panel that includes both is not being padded.

The first is a photograph. The second is closer to a long exposure.

What each one is actually measuring

Fasting glucose is the concentration of sugar circulating at the moment the needle went in. It is sensitive to the last meal, to sleep, to stress, to a cold coming on, to the cortisol rise of an early morning. A single raised reading can mean a great deal or very little.

HbA1c measures the proportion of your haemoglobin that has sugar stuck to it. Red cells live about three months, so the number reflects that window — though not evenly. Roughly half of it is set by the most recent month, which is why it moves sooner after a change than people expect.

The thresholds, and where they come from

The widely used diagnostic cut‑offs are the American Diabetes Association's, and most laboratories here report against them:

Costa Rican laboratories usually report glucose in mg/dL. If you have come from Europe or Canada you are used to mmol/L, and the two look nothing like each other. Divide mg/dL by 18 to get mmol/L.

These are thresholds for diagnosis, drawn by committee from population data. They are not a line between health and illness, and a diagnosis requires a repeat on a separate day, not one abnormal result.

When they disagree

This is the useful part, and the reason for running both.

• Normal fasting glucose, raised HbA1c. Your fasting number is held in range but your sugar is spending time high after meals. Common, and invisible if only the fasting test is run.
• Raised fasting glucose, normal HbA1c. Often the dawn phenomenon — an early‑morning cortisol rise lifting the reading — or simply a bad night. Worth repeating before it means anything.
• Both raised. Two independent measures pointing the same way. That is a stronger signal than either alone.

When HbA1c lies

Because it depends on red cells, anything that changes their lifespan changes the number without your sugar changing at all:

• Anaemia — iron deficiency tends to raise it, haemolysis to lower it
• Recent blood loss or transfusion
• Pregnancy
• Some haemoglobin variants, which can interfere with the assay itself
• Chronic kidney disease

This is the quiet argument for including a full blood count alongside it. If the haemogram shows anaemia, the HbA1c needs reading differently — and you would not know to do that from the HbA1c alone.

What neither of them tells you

Neither shows what happens after you eat. A glucose tolerance test or continuous monitoring does that, and both are a different order of effort and cost.

Neither distinguishes a pancreas that is struggling to produce insulin from a body that has stopped responding to it. That is a separate question, measured separately.

The practical version

1. Fast properly — eight to twelve hours, water only. A coffee with milk invalidates the glucose.
2. Draw in the morning, and keep to the same hour when you repeat it.
3. Use the same laboratory for anything you intend to track.
4. Treat one abnormal result as a reason to repeat, not as a diagnosis.

What your particular pair of numbers means depends on your history, your medicines and what else is on the panel. That is a conversation with a physician who has all three.