The Strengths and Blind Spots of A1c

A while back, a patient came in with an A1c that looked completely unremarkable — solidly in the normal range, nothing that would raise a flag on a standard panel. But once we put a CGM on, a different picture emerged: glucose spiking well above 200 mg/dL after certain meals, several times a week, followed by crashes not long after. None of that showed up in the labs. The average simply smoothed it all out. It's a good reminder of what A1c can and can't see.

What A1c Does Well

Hemoglobin A1c measures the percentage of red blood cells carrying glucose, giving a rough average of blood sugar over the roughly three-month lifespan of a red blood cell. It's cheap, requires no fasting, and takes a single blood draw. For diagnosing established diabetes and tracking whether a treatment plan is working over months, it remains a reliable, well-validated tool.

Where It Falls Short

Red blood cell turnover distorts the number. A1c isn't measuring glucose directly — it's measuring glucose exposure accumulated over a red blood cell's life. Anything that shortens that lifespan (blood loss, hemolytic anemia, pregnancy) produces a falsely low result. Anything that extends it (iron, B12, or folate deficiency) produces a falsely high one. Hemoglobin variants like sickle cell trait or thalassemia can skew the number either direction and sometimes make it unreliable altogether.

A flat average hides highs and lows. Because A1c is an average, it can't tell the difference between someone with steady, gently fluctuating glucose and someone swinging between sharp post-meal spikes and crashes, as long as the average comes out similar. Two people with identical A1c can have very different — and very differently risky — glucose patterns underneath that number.

No real-time feedback. A1c tells you nothing about what happened today, this week, or after a specific meal. It can't show you what food, stress, sleep, or exercise did to your glucose in the moment, which makes it useless as a day-to-day tool for adjusting behavior.

It's a late-stage marker, not an early one. A1c and fasting glucose only start moving once insulin resistance has been building for a while and the pancreas can no longer fully compensate. Compared with other tests, A1c is one of the least sensitive options for catching metabolic dysfunction early — it tends to stay "normal" the longest, even as underlying risk is already accumulating.

More Sensitive Markers Worth Using

Because A1c is a late and relatively insensitive signal, we lean on other tools to catch problems earlier and see more detail:

Fasting insulin and HOMA-IR. Fasting glucose can sit comfortably under 100 mg/dL for years while fasting insulin quietly climbs to keep it there. Fasting insulin, and the HOMA-IR score calculated from it, can flag that compensatory strain well before glucose or A1c budge.

Oral glucose tolerance testing (OGTT). By measuring how the body handles a standardized glucose load over one to two hours, OGTT catches impaired glucose handling that a single fasting draw or A1c can miss entirely. It's more sensitive than either, though also more involved to administer.

Continuous glucose monitoring (CGM). Worn for one to two weeks, a CGM shows real-time glucose data — every meal, every workout, every night. It reveals the shape of the glucose curve, not just its average: post-meal spikes, time spent in higher ranges, and the kind of variability that's invisible to A1c but linked to metabolic and cardiovascular risk.

Using Them Together

No single test tells the whole story. A1c is still useful as a long-term checkpoint, but it shouldn't be the only signal we trust — especially in patients with risk factors like family history, PCOS, unexplained fatigue, or weight change, where fasting insulin, HOMA-IR, OGTT, or a CGM can surface issues years before they'd show up on standard labs. If your labs "look fine" but something feels off, that's usually exactly where these more sensitive markers earn their place.

Let's Get a Clearer Picture of Your Metabolic Health

If you or a loved one is managing diabetes, prediabetes, or unexplained symptoms that don't match up with "normal" labs, a smarter testing strategy can make all the difference. At Mission Metabolic Health, we go beyond a single number to build a full picture of your glucose and metabolic health — and a plan to actually improve it. Reach out to us to schedule a visit.