Why Your Doctor's A1c Reading Might Be Wrong (And What to Do About It)
A1c tests aren't perfect. Learn about the medical conditions, medications, and genetic factors that can throw off your results—and what alternatives exist.
Your fasting glucose is consistently 95 mg/dL. Your post-meal readings rarely go above 130. But your A1c just came back at 6.8%—well into prediabetic range. Your doctor is concerned. You're confused. How is this possible?
Here's what nobody tells you: A1c tests can be wrong. Not because of lab error (though that happens too), but because certain medical conditions, medications, and genetic factors systematically skew the results. Let's talk about when your A1c might be lying to you—and what to do about it.
How A1c Tests Can Be Inaccurate
The A1c test measures how much glucose is stuck to your hemoglobin—the protein in red blood cells that carries oxygen. The test assumes your red blood cells live about 120 days. But here's the problem: anything that affects red blood cell lifespan or hemoglobin structure throws off the calculation.
Your A1c can read falsely high or falsely low depending on what's happening in your body. Let's break down both scenarios.
When Your A1c Reads Falsely High
If your A1c seems too high compared to your actual glucose readings, one of these conditions might be the culprit:
Iron Deficiency Anemia
When you're iron-deficient, your red blood cells live longer than normal. More time in circulation = more time for glucose to attach = artificially elevated A1c. Studies show iron-deficient patients can have A1c values 0.5-1.5% higher than their actual glycemic control warrants.
What to do: Get your ferritin and iron levels checked. Treat the iron deficiency and retest A1c in 2-3 months.
Vitamin B12 or Folate Deficiency
These deficiencies also extend red blood cell lifespan. Common in vegetarians/vegans, older adults, and people taking metformin long-term (ironically, a diabetes medication can cause this).
What to do: Test B12 and folate levels. Supplement if low. Metformin users should monitor B12 annually.
Kidney Disease
Chronic kidney disease can falsely elevate A1c by altering red blood cell metabolism and reducing hemoglobin breakdown. The worse your kidney function, the less reliable your A1c becomes.
What to do: Use fructosamine or glycated albumin instead—both are more accurate with kidney disease.
Hypothyroidism
Low thyroid function slows red blood cell turnover, leading to falsely elevated A1c readings. If you're tired, cold, and gaining weight with a high A1c, get your thyroid checked.
What to do: Test TSH, free T4. Treat hypothyroidism and recheck A1c.
When Your A1c Reads Falsely Low
This is the dangerous scenario—your A1c looks great, but your actual glucose control is worse than the number suggests.
Hemolytic Anemia or Blood Loss
If your red blood cells are being destroyed faster than normal (hemolysis) or you've lost significant blood recently, your A1c will read falsely low. The cells don't live long enough to accumulate much glucose.
What to do: Blood transfusion, recent surgery, or chronic internal bleeding? Your A1c is unreliable for 2-3 months. Use alternative tests.
Chronic Liver Disease
Liver cirrhosis and chronic liver disease reduce red blood cell lifespan and can falsely lower A1c. Your glucose control might be worse than your A1c suggests.
What to do: If you have known liver disease, request fructosamine testing instead.
Recent Pregnancy
Pregnancy increases red blood cell turnover, which can lower A1c values. This is why gestational diabetes is diagnosed using glucose tolerance tests, not A1c.
What to do: If you're pregnant or recently postpartum, rely on fasting and post-meal glucose readings instead.
Hemoglobin Variants
Genetic hemoglobin variants (like sickle cell trait, hemoglobin C, or thalassemia) can interfere with A1c testing. This is more common in African, Mediterranean, and Southeast Asian populations. Some lab methods can't measure A1c accurately in these cases.
What to do: Ask your doctor if you have a hemoglobin variant. Use alternative tests like fructosamine or continuous glucose monitoring.
Check What Your A1c Means
Convert your A1c to average glucose and see if it matches your actual readings.
Calculate Your A1c →Better Alternatives When A1c Fails You
If you fall into any of the categories above, here are more accurate alternatives:
Fructosamine Test
Measures glycated albumin (protein), not hemoglobin. Reflects average glucose over the past 2-3 weeks instead of 2-3 months. Not affected by red blood cell lifespan. Best for kidney disease, anemia, or hemoglobin variants.
Glycated Albumin (GA)
Similar to fructosamine but newer and more standardized. Reflects 2-3 weeks of glucose control. Particularly useful in pregnancy, liver disease, and rapid glucose changes.
Continuous Glucose Monitoring (CGM)
Tracks glucose every 5-15 minutes for weeks at a time. Provides time-in-range, glucose variability, and real-time trends. Gold standard for understanding actual glucose control when A1c is unreliable. Available over-the-counter (Stelo, Lingo) or by prescription (Dexcom, Freestyle Libre).
Self-Monitored Blood Glucose (SMBG)
Old school, but reliable. Test fasting glucose and 2-hour post-meal readings several times a week. Calculate your own average. Labor-intensive but not affected by any of the conditions listed above.
What to Tell Your Doctor
If your A1c doesn't match your glucose readings, don't just accept it. Bring this up with your doctor:
What to Say:
"I've been tracking my glucose at home, and my readings average around [X] mg/dL, which should give me an A1c of about [Y]%. But my lab result was [Z]%. Could I have iron deficiency, kidney issues, or another condition affecting the accuracy of my A1c? Should we consider testing fructosamine or using a CGM instead?"
This shows you've done your homework and opens the door to alternative testing.
The Bottom Line
A1c is the gold standard for diabetes diagnosis and monitoring—but it's not infallible. If you have anemia, kidney disease, liver disease, hemoglobin variants, or recent blood loss, your A1c might be systematically wrong.
The good news? You have options. Fructosamine, glycated albumin, and CGM all provide accurate glucose assessment when A1c fails. Don't let a flawed test dictate your treatment. Advocate for yourself, get the right tests, and make sure your diabetes management is based on accurate data.
Your glucose control is too important to trust a test that might be lying to you.
References
- Gallagher, E. J., et al. (2009). Review of hemoglobin A1c in the management of diabetes. Journal of Diabetes, 1(1), 9-17.
- English, E., et al. (2015). The effect of anaemia and abnormalities of erythrocyte indices on HbA1c analysis: a systematic review. Diabetologia, 58(7), 1409-1421.
- Radin, M. S. (2014). Pitfalls in hemoglobin A1c measurement: when results may be misleading. Journal of General Internal Medicine, 29(2), 388-394.
- American Diabetes Association. (2024). Classification and Diagnosis of Diabetes: Standards of Medical Care in Diabetes—2024. Diabetes Care, 47(Supplement_1), S20-S42.
Medical Disclaimer
This article is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare provider before making changes to your diabetes management plan. Do not stop or change medications without medical supervision.