The HbA1c blind spot: Conditions that distort your results

Your doctor orders an HbA1c test. You get the results back. Everything looks fine. Your blood sugar is under control. You're doing great. Except you're not. Your blood sugar is actually all over the place. You just can't see it in that number sitting in front of you.
This happens more often than people realize, and it's one of the most dangerous blind spots in diabetes care. The HbA1c test, that marker doctors rely on to check your average blood glucose over the past two to three months, isn't actually as reliable as we've been taught to believe. There's a whole category of conditions that can make it lie to you. And when your test lies, everything downstream gets compromised.

Dr. Santosh Kumar Agrawal, Director of Internal Medicine at Yatharth Super Speciality Hospital in Faridabad, has seen this problem firsthand. "There's a wide range of conditions that could cause artificially elevated HbA1c levels despite blood sugar levels being either within or slightly above the normal range," he explains. "HbA1c is just one of several tests used to diagnose diabetes." That distinction matters more than most people understand.

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The fundamental issue comes down to red blood cells and how long they live. HbA1c measures glucose that's bonded to hemoglobin, the protein inside red blood cells that carries oxygen. The longer your red blood cells stick around, the more glucose they accumulate. So if your red blood cells are living longer than normal, your HbA1c climbs even if your actual blood sugar is fine.

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Iron-deficiency anemia is a classic culprit. When you're low on iron, your remaining red blood cells tend to survive longer. That means more time for glucose to bind to hemoglobin, which means a higher HbA1c reading. Same thing happens with vitamin B12 or folate deficiency—the red blood cells live longer, and the test gets distorted. You think your diabetes is worse than it is. You might end up on medication you don't actually need. Or worse, you delay changing your lifestyle because the numbers seem to already be under control when they really aren't.

Dr. Agrawal points out another crucial factor: "Chronic kidney disease can both alter the way the body metabolizes glucose, as well as create an anemic state." So you're dealing with a double problem. Your kidneys aren't processing glucose correctly, and you're probably anemic because your kidneys aren't making enough erythropoietin, the hormone that tells your bone marrow to produce red blood cells.
This is where the research gets sobering. A study on diabetes and chronic kidney disease found that over half their study population demonstrated CKD progression, and more than 40% developed end-stage kidney disease, noting that falsely low HbA1c values could partly explain these high renal complication rates by potentially misleading patients and providers to inaccurate glycemic targets. Patients thought they were controlling their blood sugar. Turns out, they weren't. And their kidneys paid the price.

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Then there are hemoglobin variants—genetic differences in the hemoglobin itself. Some people have hemoglobin variants that cause hemolytic anemia, meaning their red blood cells break down faster. The classic case involves a patient with hemoglobin Leiden, a rare variant that causes mild hemolytic anemia and an enlarged spleen. This patient had genuinely high blood sugar—his glucose was elevated—but his HbA1c stayed normal because his red blood cells weren't living long enough to accumulate much glucose. He went undiagnosed for years. By the time he saw a specialist, he had microalbuminuria, an early sign of kidney damage that probably could have been prevented with earlier treatment.

These cases sound rare, and some are. But Dr. Agrawal emphasizes that "HbA1c tests may be inaccurate in patients who have certain types of hemoglobin." It's not just rare variants either. People who've had their spleen removed can have falsely elevated HbA1c because their red blood cells turn over more slowly without the spleen filtering out the older ones. Patients with reduced turnover rates of red blood cells for any reason end up with numbers that don't match reality.

The core problem is that doctors have treated HbA1c as the gold standard when it's really just one tool among several. "Although HbA1c provides trends as well as long-term trends, it cannot detect the short-term fluctuations in blood glucose levels caused by eating or being hypoglycemic," Dr. Agrawal explains. "It can also be impacted by specific conditions, including, but not limited to, anemia, renal failure, pregnancy, and various types of hemoglobinopathies."
What gets less attention is that the interpretation of HbA1c results assumes something that isn't always true: that your red blood cells are living their normal lifespan and turning over at normal rates. The moment that assumption breaks down, and it breaks down a lot, the test becomes unreliable.

The solution isn't to abandon HbA1c. It's to stop relying on it alone. "HbA1c should always be evaluated in combination with either fasting blood sugars, postprandial blood sugars or continuous glucose monitoring in order to provide additional context," Dr. Agrawal says. That combination approach catches what HbA1c misses.
If your HbA1c result seems off compared to your other glucose readings, say something. If you have anemia, kidney disease, or any condition that affects red blood cell survival, make sure your doctor knows to interpret the test with that context. If you're on dialysis or have a hemoglobin variant, continuous glucose monitoring might serve you better than relying on HbA1c at all.
The test has been useful. But it's not infallible. And sometimes, that blind spot is the most important thing to see.