A1C is one of the most commonly ordered labs in primary care, and one of the most commonly misunderstood. It measures your average blood glucose over roughly three months — specifically, what percentage of your hemoglobin has glucose attached to it. That average is useful. But it hides a lot.
Two patients can have identical A1C values with completely different underlying glucose patterns. One might have steady, modestly elevated glucose all day. Another might swing between hypoglycemia and significant postprandial spikes that average out to the same number. Their cardiovascular and metabolic risk profiles are not the same, even though the lab result looks identical.
What A1C tells you
It tells you the average. That’s genuinely useful for tracking long-term trends, assessing response to an intervention, and screening for diabetes and prediabetes at a population level. For most patients, it’s the right test to order and the right thing to follow over time.
What it doesn’t tell you
It doesn’t tell you the shape of your glucose — the peaks after meals, the overnight pattern, how quickly your body clears a glucose load. For patients who are metabolically at risk but not yet diabetic, that shape is often where the early signal lives.
How I use it
I use A1C as a starting point, not a destination. If someone’s A1C is trending up even within the normal range, I want to understand why. I’m also increasingly interested in fasting insulin alongside fasting glucose — the ratio tells you something about insulin resistance that A1C can’t, especially earlier in the disease course.
The goal isn’t to chase a number. It’s to understand what the number is trying to say.