Your Numbers, Explained

The three tests

Almost everyone gets diagnosed through one of three blood tests. They measure different things over different timeframes, which is why your doctor may order more than one.

What A1c actually measures

A1c is not a glucose measurement. It measures what glucose has been doing to your red blood cells. Glucose in the blood attaches to hemoglobin, and the more glucose there has been, the more of your hemoglobin ends up carrying it. A1c is the percentage that does.

Because red blood cells live around three months, A1c reflects roughly the last two to three months, weighted towards the most recent weeks. That is its strength: you cannot game it by eating carefully for two days before the appointment. It is also why it moves slowly. If you change something significant today, the number will not have caught up at a test in three weeks.

It has real limitations. Anything that changes the lifespan of your red blood cells distorts it. Anemia, recent blood loss, pregnancy, kidney disease, certain hemoglobin variants and some medications can all push A1c up or down independently of your actual glucose. If your A1c and your day-to-day readings tell obviously different stories, that mismatch is worth raising with your doctor rather than assuming one of them is lying.

Fasting versus after a meal

A fasting reading, taken after roughly eight hours without food, tells you what your body settles to when nothing is coming in. It largely reflects what your liver is doing overnight and how well insulin is holding that in check.

A reading after a meal tells you something different: how well your body handled a specific load of carbohydrate. This is usually the first thing to change and the last thing anyone measures. Fasting glucose often looks unremarkable for years while post-meal responses have already become large and prolonged.

This is the practical argument for paired testing, covered properly in chapter 7. One reading before a meal and one after it tells you more about that meal than either number does alone, because what matters is the size of the rise, not the height of a single point.

What an average hides

Here is the thing that A1c genuinely cannot show you. It is an average, and two very different weeks can average to exactly the same number.

One person spends the week fairly steady. Another spends it climbing hard after every meal and dropping hard afterwards. Their A1c can match. Their experience of the week does not match at all, and increasingly the view is that the variability itself matters, not only the average.

You feel the variability. It is the energy crash, the fog, the craving at 3pm. So if your labs came back unremarkable and you still feel like something is off, that is not necessarily you imagining it. Averages are good at hiding swings.

What to ask at your next appointment

• Which test was I diagnosed on, and was it confirmed on a second test?
• What is my A1c trend over the last few results, rather than just today's number?
• Is there anything about my blood that would make A1c less reliable for me?
• Should I be testing at home, and if so, when specifically?
• What would you want to see change by my next appointment?

That last question is worth asking out loud. It converts a number you are handed into a target your clinician has actually set with you, which is a considerably easier thing to work towards.