A1C: The Three-Month Report Card Your Blood Carries (And Why Your Eye Doctor Cares)

Patients sometimes blink at me when I ask about their A1C. "Doc, you're the eye doctor — why do you care about my blood sugar labs?" Fair question. Here's my answer: your A1C tells me how hard your tiny retinal blood vessels have been working for the last three months, and those vessels are the first place diabetic eye disease shows up. So yes, I care. A lot.

What the A1C actually measures

Hemoglobin is the protein in your red blood cells that carries oxygen. When sugar floats around in your bloodstream, some of it gloms onto hemoglobin — the more sugar, the more coated hemoglobin. Since a red blood cell lives about 90 days, the percentage of sugar-coated hemoglobin (your HbA1c) is essentially a three-month average of your blood sugar. One blood draw, no fasting, no drama.

A daily finger-stick tells you what your sugar is right now. The A1C tells you the movie, not the snapshot — which is why it's the gold standard for both diagnosing diabetes and tracking how well it's controlled.

Reading your number

Here's the map, plain and simple:

• Below 5.7% — Normal. Keep doing what you're doing.
• 5.7% to 6.4% — Prediabetes. The warning track. (For perspective, 5.7% works out to an average blood sugar around 117 mg/dL.)
• 6.5% or higher — Diabetes range, confirmed with a second test. (An A1C of 10% means your average has been hovering around 240 mg/dL — that's rough territory.)

Most guidelines set the target for people already diagnosed at below 7% (the American Diabetes Association) — though your endocrinologist personalizes it. Tighter isn't always better for everyone; lows carry their own risks, especially in older adults.

Why it matters for your eyes (my favorite part)

Every point your A1C climbs, your risk of diabetic retinopathy, macular edema, cataracts, and glaucoma climbs with it. I've watched patients hold their vision steady for decades with a well-managed A1C — and I've watched poorly controlled sugar quietly wreck retinas in a few short years. The vessels don't negotiate.

There's a subtler point too: when your A1C swings wildly — say you start a new medication and your average drops fast — your vision can blur temporarily as the lens inside your eye adjusts to the fluid shifts. Patients panic; I reassure them. It settles as the numbers stabilize. But it's one more reason your eye doctor and your diabetes doctor should be on the same team.

How often should you test?

• At diagnosis, to set your baseline.
• Every 3 to 6 months for routine monitoring.
• More often if your numbers are uncontrolled or your medications just changed.

A quick caveat I share with patients: a few things can skew the A1C. Anemia, recent blood loss or transfusion, kidney disease, and certain hemoglobin variants (more common in people of African, Mediterranean, or Southeast Asian ancestry) can make the number read falsely high or low. If your A1C doesn't match your daily readings, tell your doctor — there's usually an explanation, and alternative tests exist.

And please — don't just glance at the number and file it away. Bring it to your eye exam. When I know your A1C, I know how aggressively to look, how often to image your retina, and how urgently to act on anything I find.

The bottom line

The A1C is the single most useful number in diabetes care: one test, three months of truth. Keep it in range and you protect your kidneys, your nerves, your heart — and, from where I sit, your sight.

I'm Paddy Kalish, OD — an optometric physician in Brookline, MA. If you have diabetes or prediabetes, bring your latest A1C to your next eye exam and let's protect your vision together: 1296 Beacon St, book online at paddykalish.com, or call/text 857-314-4645.

Author

Paddy Kalish OD, JD and B.Arch Author and Blogger