HbA1c

A fasting glucose can be talked out of trouble. Eat carefully the day before, sleep well, draw the blood at the right moment, and the number cooperates. HbA1c does not work that way. It is the one metabolic test you cannot prepare for, because it is not measuring a moment. It is measuring a memory.

HbA1c reflects your average blood sugar over the past two to three months, written into your blood and impossible to wash off before a draw. A single good week barely moves it; a season of high blood sugar shows clearly. This is what makes it the anchor of metabolic testing: where fasting glucose reports a snapshot and can be misled, HbA1c reports the conditions your body has actually lived in, day and night, fed and fasted.

It is also more than a glucose average. The very thing it measures, sugar bonding permanently to a protein, is the same process that quietly damages blood vessels, nerves, and organs when blood sugar runs high. So the number is doing two jobs at once: estimating your typical blood sugar, and reading, directly, how much of that sugar-driven damage is accumulating.

Like fasting glucose, it tends to move late, so it is best read alongside the markers that shift earlier. But of the numbers that confirm where your metabolism truly sits, this is the most honest one on the panel.

HbA1c forms through glycation, a slow chemical reaction in which glucose binds to hemoglobin without any enzyme directing it. The higher the average glucose in the blood, and the longer the exposure, the greater the share of hemoglobin that ends up glycated. Because the bond is effectively permanent for the life of the red cell, the percentage measured reflects a weighted average of blood sugar over the preceding two to three months, with the most recent weeks counting most.

This is the same fundamental reaction that produces advanced glycation end products throughout the body, the molecular wear-and-tear that stiffens arteries and damages fine tissue when blood sugar stays high ^[1]. Measuring it on hemoglobin is simply convenient: red blood cells are easy to sample, and their predictable lifespan turns the coating into a clean timeline.

That dependence on red blood cells is also the test's main blind spot. HbA1c assumes a normal cell lifespan, so anything that changes it distorts the result. Iron deficiency lengthens the average life of a red cell and pushes HbA1c falsely upward, while blood loss, hemolysis, recent transfusion, and certain hemoglobin variants pull it the other way. When an HbA1c does not match the glucose picture around it, the red cells themselves are the first thing to question.

The diagnostic lines are defined sharply: an HbA1c below 5.7% is labeled normal, 5.7 to 6.4% is prediabetes, and 6.5% or higher confirms diabetes ^[2]. As with fasting glucose, those thresholds describe a smooth slope rather than a true edge.

The case for caring about the upper-normal range is strong. In a large study of adults without diabetes, HbA1c predicted cardiovascular events and all-cause mortality continuously, with risk rising steadily from the bottom of the normal range upward and no safe plateau below the diabetic cutoff ^[3]. Broader analyses confirm the same continuous relationship with cardiovascular disease ^[4]. A 5.6% is technically normal and still carries more risk than a 5.2%.

There is an important exception at the very bottom. Looking across whole populations, the relationship between HbA1c and total mortality is U-shaped: very low values associate with higher mortality too ^[5]. This low-end risk is largely a matter of reverse causation, since anemia, liver disease, and frailty can all lower HbA1c, rather than a sign that a low number earned through healthy living is dangerous. The practical reading: aim for the lower part of the normal range, but treat an unexplained low value as a reason to look for a cause rather than a trophy.